Supporting Information. The Dehydropolymerization of H 3 B NMeH 2 to form Polyaminoboranes using [Rh(Xantphos alkyl)] Catalysts

Transcription

1 Supporting Information The Dehydropolymerization of H 3 B NMeH 2 to form Polyaminoboranes using [Rh(Xantphos alkyl)] Catalysts Gemma M. Adams, a Annie L. Colebatch, a Joseph T. Skornia, a Alasdair I. McKay, a Heather C. Johnson, a Guy C. Lloyd Jones, b Stuart A. Macgregor, c Nicholas A. Beattie, c Andrew S. Weller* a a Chemistry Research Laboratories, Mansfield Road, Oxford, OX1 3TA, UK b School of Chemistry, University of Edinburgh, Edinburgh, EH9 3FJ, UK c Institute of Chemical Sciences, Heriot Watt University, Edinburgh, EH14 4AS, UK S1

2 Table of Contents Experimental...S4 Synthesis of [Rh(NBD)(κ 2 P,P Xantphos Et)][BAr F 4] (3)...S4 Synthesis of [Rh(NBD)(κ 2 P,P Xantphos i Pr)][BAr F 4] (4)...S5 Synthesis of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos Et)][BAr F 4] (5)...S5 Synthesis of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (6)...S5 Synthesis of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr Cl 4] (6-[BAr Cl 4])...S6 Synthesis of [RhCl(κ 2 P,P Xantphos Et)] 2 (8)...S6 Synthesis of [RhH 2 Cl(κ 3 P,O,P Xantphos Et)] (9)...S7 Synthesis of [Rh(C 6 H 3 F 2 )(κ 3 P,O,P Xantphos i Pr)] (12)...S7 Synthesis of [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4] (14)...S7 Synthesis of [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr Cl 4] (14-[BAr Cl 4])...S8 Synthesis of [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (15)...S10 Synthesis of [{RhH 2 (κ 3 P,O,P Xantphos i Pr)} 2 BH 4 ][BAr F 4] (16)...S10 Synthesis of [Rh 2 (µ-h)(µ-h 2 BNMeH)(κ 2 P,P Xantphos Et) 2 ][BAr F 4] (17)...S11 Spectroscopic data for [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos Et)][BAr F 4]...S11 Synthesis of [RhH 2 (H 3 B NMeHBH 2 NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4]...S11 Synthesis of [Rh(PPh 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4]...S11 Synthesis of [BH 2 (NMeH 2 ) 2 ][BAr F 4]...S12 Catalytic Dehydropolymerization of H 3 B NMeH 2...S12 General Procedure for Dehydropolymerization (Open Conditions)...S12 General Procedure for Gas Evolution Measurements...S12 Dehydropolymerization under Closed Conditions...S13 Dehydropolymerization in the Presence of Cyclohexene...S13 Recycling Experiment...S13 Tests for Homogeneity...S13 Mercury Poisoning...S13 Fractional Poisoning...S14 Experimental Kinetic Data for 6...S15 Experimental and Simulated Kinetic Data for 11...S15 Experimental Kinetic Data for 6 with 10 equivalents [H 2 B(NMeH 2 ) 2 ][BAr F 4]...S16 Representative 11 B NMR Spectra...S17 Eyring Analyses of 3 and 4...S22 Eyring Analysis of 6...S23 Catalyst Speciation of 6 with H 3 B NMeH 2 at 10 Mol% Loading...S24 Reaction Composition Plot for Formation of 17...S26 S2

3 GC-MS of 1,2 F 2 C 6 H 4...S28 Fitting of GPC Data for Polymer Arising From 6...S28 Crystallography...S29 Computational Details...S33 Geometry Optimizations...S33 QTAIM Results...S35 NBO Results...S36 Localized Molecular Orbitals...S37 References...S40 S3

4 Experimental All manipulations, unless otherwise stated, were performed under an argon atmosphere using standard Schlenk line and glovebox techniques. Glassware was oven dried at 130 C overnight and flame dried under vacuum prior to use. Pentane, hexane and CH 2 Cl 2 were dried using a Grubbs type solvent purification system (MBraun SPS-800) and degassed by three successive freeze-pump-thaw cycles. 1 THF was dried over sodium/benzophenone, distilled and degassed by three successive freeze-pump-thaw cycles. 1,2 F 2 C 6 H 4 (pretreated with alumina), FC 6 H 5 (pretreated with alumina) and CD 2 Cl 2 were dried over CaH 2, vacuum distilled and stored over 3 Å molecular sieves. C 6 D 6 was dried over Na, vacuum distilled and stored over 3 Å molecular sieves. H 3 B NMe 3 was purchased from Sigma-Aldrich and sublimed prior to use. H 3 B NMeH 2 was purchased from Boron Specialities and recrystallized from Et 2 O at 18 C. Cyclohexene was purchased from Sigma-Aldrich, dried over sodium, vacuum distilled and stored over 3 Å molecular sieves. Hg ( %) was purchased from Sigma-Aldrich, washed with 1,2 F 2 C 6 H 4 and dried in vacuo prior to use. Xantphos t Bu was purchased from Alfa Aesar. H 3 B THF (1M in THF) was purchased from Fisher Scientific. [N n Bu 4 ][BH 4 ] was purchased from Sigma-Aldrich. [RhH(κ 3 P,O,P Xantphos t Bu)], 2 Xantphos Et, 3 Xantphos i Pr, 4 [RhH 2 Cl(κ 3 P,O,P Xantphos i Pr)], 5 [RhCl(COE) 2 ] 2, 6 Na[BAr F 4], 7 Na[BAr Cl 4], 8 H 3 B NMeHBH 2 NMeH 2, 9 and BH 2 (NMeH 2 ) 2 Cl 10 were prepared by literature methods. [RhH(κ 3 P,O,P Xantphos i Pr)] was prepared according to the literature procedure for [RhH(κ 3 P,O,P Xantphos t Bu)]. 2 [Rh(NBD) 2 ][BAr F 4] was prepared according to the literature procedure for [Rh(COD) 2 ][BAr F 4]. 11 NMR spectra were recorded on a Bruker AVIIIHD 500 or Bruker AVIIIHD 400 nanobay spectrometer at room temperature, unless otherwise stated. For NMR samples in FC 6 H 5 and 1,2 F 2 C 6 H 4, 1 H NMR spectra were prelocked to a sample of C 6 D 6 (25%) and 1,2 F 2 C 6 H 4 (75%) and referenced to the centre of the downfield solvent multiplet, δ = 7.11 and 7.07, respectively. 31 P and 11 B NMR spectra were referenced against 85% H 3 PO 4 (external) and BF 3 OEt 2 (external) respectively. Chemical shifts (δ) are quoted in ppm and coupling constants (J) in Hz. ESI-MS were recorded on a Bruker MicrOTOF instrument interfaced with a glove-box. Microanalyses were performed by Stephen Boyer at London Metropolitan University. ICP-MS were performed by Philip Holdship at the Department of Earth Sciences, University of Oxford. GC-MS were performed by Colin Sparrow at the Department of Chemistry, University of Oxford. Gel permeation chromatography (GPC) was performed on a Malvern Viscotek GPCmax chromatograph fitted with an RI detector. The columns were contained within an oven (35 C) and consisted of a porous styrene divinylbenzene copolymer with a maximum pore size of 1,500 Å. THF containing 0.1% w/w [N n Bu 4 ]Br was used as the eluent at a flow rate of 1.0 ml min 1. Samples were dissolved in the eluent (2 mg ml 1 ), filtered (0.2 μm pore size) and run immediately. The calibration was conducted using a series of monodisperse polystyrene standards obtained from Sigma-Aldrich. Synthesis of [Rh(NBD)(κ 2 P,P Xantphos Et)][BAr F 4] (3) A solution of Xantphos Et (17.1 mg, 44.2 µmol) in FC 6 H 5 (2 ml) was added dropwise to a solution of [Rh(NBD) 2 ][BAr F 4] (50.0 mg, 43.5 µmol) in FC 6 H 5 (2 ml) over 30 minutes. The resulting orange solution was concentrated to ca. 2 ml under vacuum. Hexane (20 ml) was added and the mixture sonicated to give an orange powder, which was isolated by filtration, washed with hexane (2 ml 3) and dried under vacuum. Yield 47.5 mg (32.9 µmol, 76%). 1 H NMR (400 MHz, CD 2 Cl 2, 298 K): δ 7.72 (s, br, 8 H, [BAr F 4]-ortho-CH), 7.63 (d, 2 H, 2 J HH 7.8, aryl CH para to P), 7.56 (s, br, 4 H, [BAr F 4]-para-CH), 7.33 (dd, 2 H, 2 J HH 7.6, 2 J HH 7.8, aryl CH meta to P), 7.30 (dd, 2 H, 2 J HH 7.7, 2 J PH 6.5, aryl CH ortho to P), 4.23 (s, br, 4 H, NBD-HC=CH), 3.58 (s, br, 2 H, NBD-CH), 2.21 (s, br, 4 H, Et-CH 2 ), 1.76 (s, br, 4 H, Et-CH 2 ), 1.69 (br, 6 H, Xantphos backbone C(CH 3 ) 2 ), 1.46 (s, 2 H, NBD-CH 2 ), 1.15 (m, br, 12 H, Et-CH 3 ). 31 P{ 1 H} (162 MHz, S4

5 CD 2 Cl 2, 298 K): 6.2 (d, 1 J RhP 152.3). ESI-MS (1,2 F 2 C 6 H 4, 60 C, 4.5 kv): m/z (calculated for [Rh(NBD)(κ 2 P,P Xantphos Et)] + fragment, showing the correct isotope pattern). Microanalysis: Calc. (C 62 H 52 BF 24 OP 2 Rh): C, 51.54; H, Found: C, 51.44; H, Synthesis of [Rh(NBD)(κ 2 P,P Xantphos i Pr)][BAr F 4] (4) A solution of Xantphos i Pr (90 µl, µmol) in CH 2 Cl 2 (3.6 ml) was added dropwise to a solution of [Rh(NBD) 2 ][BAr F 4] (225 mg, µmol) in CH 2 Cl 2 (2 ml) over 10 minutes. The resulting light orange solution was concentrated to ca. 1 ml under vacuum, and pentane (30 ml) was added with stirring to afford an orange powder. Following removal of the supernatant by filtration, the orange solid was washed with pentane (5 ml x 3) and dried in vacuo. Yield: 200 mg (133.3 µmol, 68%). Dissolving the isolated solid in CH 2 Cl 2 (1 ml) and layering with pentane yielded orange crystals suitable for single crystal X-ray diffraction. 1 H NMR (500 MHz, CD 2 Cl 2, 298 K): δ 7.72 (s, br, 8 H, [BAr F 4]-ortho-CH), 7.64 (d, 2 H, 2 J HH 8, aryl CH para to P), 7.56 (s, br, 4 H, [BAr F 4]-para-CH), 7.37 (app t, 2 H, 2 J HH 8, aryl CH meta to P), 7.30 (dd, 2 H, 2 J HH 8, 2 J PH 6, aryl CH ortho to P), 4.00 (s, br, 4 H, NBD-HC=CH), 3.32 (s, br, 2 H, NBD-CH), 2.51 (s, 4 H, ipr-ch), 1.69 (s, 6 H, Xantphos backbone C(CH 3 ) 2 ), 1.59 (m, 12 H, ipr-ch 3 ), 1.36 (m, 12 H, ipr-ch 3 ), 1.32 (s, 2 H, NBD-CH 2 ). Upon decoupling to 31 P, the resonances at δ 7.30, 1.59 and 1.36 each sharpened into doublets, revealing 2 J HH coupling constants of 8, 7 and 7, respectively. 31 P{ 1 H} NMR (202 MHz, CD 2 Cl 2, 298 K): δ 26.3 (d, 1 J RhP 145.0). ESI-MS (1,2 F 2 C 6 H 4, 60 C, 4.5 kv): m/z (calculated for [Rh(NBD)(κ 2 P,P Xantphos i Pr)] + fragment, showing the correct isotope pattern). Microanalysis: Calc. (C 66 H 60 BF 24 OP 2 Rh): C, 52.82; H, Found: C, 52.85; H, Synthesis of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos Et)][BAr F 4] (5) [Rh(NBD)(κ 2 P,P Xantphos Et)][BAr F 4] (100 mg, 69.2 µmol) and H 3 B NMe 3 (5.0 mg, 69 µmol) were dissolved in 1,2 F 2 C 6 H 4 (4 ml). The resulting orange solution was immediately frozen. The headspace was evacuated and the flask refilled with one atm H 2. The flask was sealed and allowed to warm to room temperature. Upon shaking the orange solution turned pale yellow. The solution was stirred for 20 minutes, then hexane (30 ml) was added to give an off-white precipitate which was isolated by filtration, washed with hexane (5 ml 3) and dried under vacuum. Yield 74.0 mg (51.8 µmol, 75%). 1 H NMR (400 MHz, CD 2 Cl 2, 298 K): δ 7.72 (s, br, 8 H, [BAr F 4]-ortho-CH), 7.65 (d, 2 H, 2 J HH 7.6, aryl CH para to P), 7.56 (s, br, 4 H, [BAr F 4]-para-CH), 7.52 (m, 2 H, aryl CH meta to P), 7.39 (dd, 2 H, 2 J HH 7.6, 2 J PH 7.6, aryl CH ortho to P), 2.59 (s, 9 H, NMe 3 ), 2.30 (s, br, 2 H, Et-CH 2 ), 2.20 (s, br, 4 H, Et-CH 2 ), 2.06 (s, br, 2 H, Et-CH 2 ), 1.86 (br, 6 H, Xantphos backbone C(CH 3 ) 2 ), 1.34 (s, br, 6 H, Et-CH 3 ), 1.02 (s, br, 6 H, Et-CH 3 ), 0.65 (s, br, 3 H, RhH 3 B), (s, br, 1 H, RhH), (s, br, 1 H, RhH). 11 B (128 MHz, CD 2 Cl 2, 298 K): 6.6 (s, [BArF 4 ]), 10.2 (br s, RhH 3 B). 11 B{ 1 H} (128 MHz, CD 2 Cl 2, 298 K): 6.6 (s, [BArF 4 ]), 10.4 (br s, RhH 3 B). 31 P{ 1 H} (162 MHz, CD 2 Cl 2, 298 K): 42.1 (d, 1 J RhP 107.7). ESI- MS (1,2 F 2 C 6 H 4, 60 C, 4.5 kv): m/z (calculated for [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos Et)] + fragment, showing the correct isotope pattern), (calculated for [Rh(H 3 B NMe 3 )(κ 3 P,O,P Xantphos Et)] + fragment, showing the correct isotope pattern), (calculated for [RhH 2 (κ 3 P,O,P Xantphos Et)] + fragment, showing the correct isotope pattern). Microanalysis: Calc. (C 58 H 58 B 2 F 24 NOP 2 Rh): C, 48.80; H, 4.10; N, Found: C, 48.68; H, 4.13; N, Synthesis of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (6) Route A: [Rh(NBD)(κ 2 P,P Xantphos i Pr)][BAr F 4] (100 mg, 66.7 µmol) and H 3 B NMe 3 (4.9 mg, 66.7 µmol) were dissolved in 1,2 F 2 C 6 H 4 (1 ml) in a J. Young flask. The resulting orange solution was immediately frozen. The headspace was evacuated and the J. Young flask refilled with one atm H 2. The flask was sealed, resulting in an internal H 2 pressure of ca. 4 atm upon warming to S5

6 298 K. Upon thawing and shaking the orange solution turned pale yellow. The solution was shaken for 5 minutes, then layered with H 2 -saturated pentane (30 ml) and stored at 5 C, which yielded pale yellow crystals suitable for single crystal X-ray diffraction. The crystals were isolated by filtration, washed with pentane (5 ml 3) and dried under vacuum. Yield 75.0 mg (50.6 µmol, 76%). Route B: RhH 2 Cl(κ 3 P,O,P Xantphos i Pr) (79.9 mg, µmol), Na[BAr F 4] (121.5 mg, µmol) and H 3 B NMe 3 (10.0 mg, µmol) were dissolved in 1,2 F 2 C 6 H 4 (1 ml) in a J. Young flask. The pale yellow solution was stirred for 30 minutes, filtered, then layered with pentane and stored at 5 C. The pale yellow crystals were isolated by filtration, washed with pentane (5 ml x 3) and dried under vacuum. Yield mg (108.0 µmol, 79%). 1 H NMR (500 MHz, CD 2 Cl 2, 298 K): δ 7.72 (s, br, 8 H, [BAr F 4]-ortho-CH), 7.64 (d, 2 H, 2 J HH 8, aryl CH para to P), 7.56 (s, br, 4 H, [BAr F 4]-para-CH), 7.54 (m, obscured by [BAr F 4] peak, 2 H, aryl CH meta to P), 7.39 (dd, 2 H, 2 J HH 8, 2 J PH 8, aryl CH ortho to P), 2.64 (m, 4 H, i Pr-CH), 2.61 (s, 9 H, NMe 3 ), 1.62 (s, br, 6 H, Xantphos backbone C(CH 3 ) 2 ), 1.36 (m, 12 H, i Pr-CH 3 ), 1.08 (m, 12 H, ipr-ch 3 ), 0.08 (s, br, 3 H, RhH 3 B), (s, br, 2 H, RhH). Upon decoupling to 11 B, the resonance at δ 0.08 sharpens. 1 H NMR (500 MHz, CD 2 Cl 2, 200 K): δ 7.71 (s, br, 8 H, [BAr F 4]-ortho-CH), 7.59 (d, 2 H, 2 J HH 8, aryl CH para to P), 7.53 (s, br, 4 H, [BAr F 4]-para-CH), 7.52 (m, obscured by [BAr F 4] peak, 2 H, aryl CH meta to P), 7.33 (dd, 2 H, 2 J HH 8, 2 J PH 8, aryl CH ortho to P), 2.66 (br, 2 H, i Pr-CH), 2.56 (s, 9 H, NMe 3 ), 2.52 (br, 2 H, i Pr-CH), 1.83 (s, 3 H, Xantphos backbone C(CH 3 )), 1.42 (m, 12 H, i Pr-CH 3 ), 1.22 (s, 3 H, Xantphos backbone C(CH 3 )), 1.19 (m, 6 H, ipr-ch 3 ), 0.62 (m, 6 H, ipr-ch 3 ), 0.58 (s, br, 3 H, RhH 3 B), (br, 1 H, RhH), (br, 1 H, RhH). 11 B{ 1 H} NMR (160 MHz, CD 2 Cl 2, 298 K): 6.6 (s, [BAr F 4]), 9.9 (s, br, RhH 3 B). 31 P{ 1 H} NMR (202 MHz, CD 2 Cl 2, 298 K): δ 66.5 (d, 1 J RhP 111.0). 31 P{ 1 H} NMR (202 MHz, CD 2 Cl 2, 200 K): δ 68.4 (d, 1 J RhP 111.0). ESI-MS (1,2 F 2 C 6 H 4, 60 C, 4.5 kv): m/z (calculated for [RhH 2 (κ 3 P,O,P Xantphos i Pr)] + fragment, showing the correct isotope pattern). Microanalysis: Calc. (C 62 H 66 NB 2 F 24 OP 2 Rh): C, 50.19; H, 4.48; N, Found: C, 49.99; H, 4.39; N, Synthesis of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr Cl 4] (6-[BAr Cl 4]) RhH 2 Cl(κ 3 P,O,P Xantphos i Pr) (79.9 mg, µmol), Na[BAr Cl 4] (88.9 mg, µmol) and H 3 B NMe 3 (10.0 mg, µmol) were dissolved in CH 2 Cl 2 (3 ml) in a J. Young flask. The pale yellow solution was stirred for 20 hours, filtered and the solvent removed in vacuo. The light yellow powder was then dissolved in CH 2 Cl 2 (1 ml), layered with pentane and stored at room temperature. The pale yellow crystals were isolated by filtration, washed with pentane (5 ml x 3) and dried under vacuum. Yield mg (86.4 µmol, 63%). 1 H NMR (400 MHz, CD 2 Cl 2, 298 K): 7.64 (d, 2 H, 2 J HH 8, aryl CH para to P), 7.52 (m, 2 H, aryl CH meta to P), 7.37 (dd, 2 H, 2 J HH 8, 2 J PH 8, aryl CH ortho to P), δ (m, 8 H, [BAr Cl 4]- ortho-ch), (m, 4 H, [BAr Cl 4]-para-CH), 2.62 (m, 4 H, i Pr-CH), 2.60 (s, 9 H, NMe 3 ), 1.62 (s, br, 6 H, Xantphos backbone C(CH 3 ) 2 ), 1.32 (m, 12 H, i Pr-CH 3 ), 1.04 (m, 12 H, ipr- CH 3 ), 0.04 (s, br, 3 H, RhH 3 B), (s, br, 2 H, RhH). Upon decoupling to 11 B, the resonance at δ 0.04 sharpens. 11 B{ 1 H} NMR (128 MHz, CD 2 Cl 2, 298 K): 7.0 (s, [BAr Cl 4]), 10.2 (s, br, RhH 3 B). 31 P{ 1 H} NMR (162 MHz, CD 2 Cl 2, 298 K): δ 65.8 (d, 1 J RhP 111.2). ESI-MS (1,2 F 2 C 6 H 4, 60 C, 4.5 kv): m/z (calculated for [RhH 2 (κ 3 P,O,P Xantphos i Pr)] + fragment, showing the correct isotope pattern). Microanalysis: Calc. (C 54 H 66 NB 2 Cl 8 OP 2 Rh): C, 53.37; H, 5.47; N, Found: C, 53.29; H, 5.62; N, Synthesis of [RhCl(κ 2 P,P Xantphos Et)] 2 (8) Xantphos Et (106 mg, 274 µmol) and [RhCl(COE) 2 ] 2 (102 mg, 142 µmol) were dissolved in benzene (8 ml) and the resulting dark red solution was stirred for two hours. The solution was concentrated under vacuum to ca 2 ml, then pentane (40 ml) was added to give a red S6

7 precipitate which was isolated by filtration, washed with pentane (5 ml 2) and dried under vacuum. Yield 88.0 mg (83.8 µmol, 61%). 1 H NMR (400 MHz, CD 2 Cl 2, 298 K): δ 7.05 (m, 4 H, Ar), 6.89 (m, 2 H, Ar), 2.09 (s, v br, 8 H, Et-CH 2 ), 1.40 (s, 6 H, Xantphos backbone C(CH 3 ) 2 ), 1.04 (s, v br, 12 H, Et-CH 3 ). 31 P{ 1 H} (162 MHz, CD 2 Cl 2, 298 K): 22.6 (d, br, 1 J RhP 199.1). Microanalysis: Calc. (C 46 H 64 Cl 2 O 2 P 4 Rh 2 ): C, 52.64; H, Found: C, 52.50; H, Synthesis of [RhH 2 Cl(κ 3 P,O,P Xantphos Et)] (9) [RhCl(κ 2 P,P Xantphos Et)] 2 (100 mg, 95.3 µmol) was dissolved in benzene (6 ml) in a J. Young flask. The dark red solution was degassed by three successive freeze-pump-thaw cycles and the flask was filled with H 2 (4 atm, 298 K). The reaction mixture was stirred for two hours, over which time the solution changed from dark red to yellow. Volatiles were removed under vacuum, and a colour change to dark brown-red was evident upon application of vacuum. The solid residue was washed with pentane (20 ml) and dried under vacuum. 31 P{ 1 H} NMR spectroscopy of the pink powder indicated a mixture of [RhH 2 Cl(κ 3 P,O,P Xantphos Et)] 2 (74%), [RhCl(κ 2 P,P Xantphos Et)] 2 (21%) and an unknown species at δ P 40.6 (s, 5%). [RhH 2 Cl(κ 3 P,O,P Xantphos Et)] was found to be unstable to H 2 loss under vacuum and in solution under an argon atmosphere. A C 6 D 6 solution (argon atmosphere) containing a mixture of [RhCl(κ 2 P,P Xantphos Et)] 2 and [RhH 2 Cl(κ 3 P,O,P Xantphos Et)] in a ratio of 1:6.7 was found to decrease to 1:4.2 after one day. Selected NMR data for [RhH 2 Cl(κ 3 P,O,P Xantphos Et)]: 1 H NMR (400 MHz, C 6 D 6, 298 K): δ 7.08 (m, 2 H, Ar), 7.03 (dd, 2 H, J HH 7.7, J HH 1.4, Ar), 6.89 (dd, 2 H, J 7.6, J 7.6, Ar), 3.02 (m, 2 H, Et-CH 2 ), 1.79 (m, 4 H, Et-CH 2 ), 1.59 (m, br, 2 H, Et-CH 2 ), 1.40 (app. tt, 6 H, J HH 7.7, J HH 9.5, Et-CH 3 ), 1.24 (s, 3 H, Xantphos backbone C(CH 3 ) 2 ), 1.11 (s, 3 H, Xantphos backbone C(CH 3 ) 2 ), 1.00 (app. tt, 6 H, J HH 7.6, J HH 9.1, Et- CH 3 ), (dtd, 1 H, 1 J RhH 25.5, 2 J PH 16.0, 2 J HH 9.5, RhH), (dtd, 1 H, 1 J RhH 31.3, 2 J PH 15.8, 2 J HH 9.5, RhH). 31 P{ 1 H} (162 MHz, C 6 D 6, 298 K): 44.1 (d, 1 J RhP 112.9). Synthesis of [Rh(C 6 H 3 F 2 )(κ 3 P,O,P Xantphos i Pr)] (12) Route A: A mixture of [RhH 2 Cl(κ 3 P,O,P Xantphos i Pr)] (0.200 g, mmol) and KO t Bu (0.055 g, 0.49 mmol) in 1,2 F 2 C 6 H 4 (5 ml) was stirred for 18 hours, resulting in a dark orange suspension. The mixture was filtered through a pad of Celite and the volatiles were removed under vacuum. The residue was extracted with benzene (ca 8 ml), filtered and layered with hexane (ca 12 ml). This solution was stored at 4 C, yielding orange crystals of [Rh(C 6 H 3 F 2 )(κ 3 P,O,P Xantphos i Pr)], which were collected by filtration, washed with hexane (1 ml 2) and dried under vacuum. Yield 175 mg (0.265 mmol, 77%). Route B: [RhH(κ 3 P,O,P Xantphos i Pr)] (5.1 mg, 9.3 µmol) was dissolved in 1,2 F 2 C 6 H 4 (1.3 ml) and 0.5 ml of this solution was added to an NMR tube. The NMR spectra were recorded as soon as possible (ca 2 min) and indicated quantitative formation of [Rh(C 6 H 3 F 2 )(κ 3 P,O,P Xantphos i Pr)]. 1 H NMR (400 MHz, C 6 D 6, 298 K): δ 7.75 (m, 1 H, Ar), 7.20 (m, 2 H, Ar), 7.02 (m, 2 H, Ar), 6.83 (m, 2 H, Ar), 2.32 (s, br, 4 H, i Pr-CH), (m, 30 H, i Pr-CH 3 + Xantphos backbone C(CH 3 ) P{ 1 H} (162 MHz, C 6 D 6, 298 K): 40.6 (dd, 1 J RhP 160.4, 3 J PF 3.9). 19 F{ 1 H} (377 MHz, C 6 D 6, 298 K): (ddt, C 2 -F, 3 J FF 36.2, 3 J RhF 23.5, 3 J PF 4.2), (dd, C 3 -F, 3 J FF 36.2, 4 J RhF 7.6). Microanalysis: Calc. (C 33 H 43 F 2 OP 2 Rh): C, 60.19; H, Found: C, 60.07; H, Synthesis of [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4] (14) Route A: [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (15 mg, 10.1 µmol) and H 3 B NMeH 2 (4.6 mg, µmol) were dissolved in 0.4 ml 1,2 F 2 C 6 H 4 in a J. Young NMR tube. After being S7

8 shaken for 5 minutes and agitated for a further 20 minutes, 31 P{ 1 H} NMR spectroscopy indicated that the mixture contained 82% [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4]. Route B: [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (14.8 mg, 10.0 µmol) and 0.5 ml of a 0.02 M solution of H 3 B THF (1.0 mg, 10.0 µmol) were placed in a J. Young NMR tube, initially giving a pale yellow solution. Over 40 minutes, effervescence was observed and a darkening of the solution to orange. 31 P{ 1 H} NMR spectroscopy indicated that the mixture contained 50% [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4]. After 12 hours, [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4] was the sole phosphorus-containing species in solution by 31 P{ 1 H} NMR spectroscopy. Route C: [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (11.5 mg, 7.7 µmol) and [N n Bu 4 ][BH 4 ] (1.0 mg, 3.9 µmol) were dissolved in 0.4 ml 1,2 F 2 C 6 H 4 in a J. Young NMR tube, giving a pale yellow solution. Over 30 mins, effervescence was observed and a darkening of the solution to orange. 31 P{ 1 H} NMR spectroscopy indicated that the mixture contained 80% [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4]. For all routes, attempts to isolate 14 were unsuccessful. In our hands, transferral of the above reaction mixtures to stirring pentane at 30 C gave brown/orange powders which could not be interrogated by NMR spectroscopy. Attempts to crystallize 14 gave brown oils across a range of temperatures. Selected spectroscopic data: 11 B{ 1 H} NMR (128 MHz, THF, 298 K): δ 132, (v br, Rh 2 B), 6.1 (s, [BAr F 4]). 31 P{ 1 H} NMR (162 MHz, THF, 298 K): 48.1 (dd, J RhP 174.1, J RhP 6.3). 11 B{ 1 H} NMR (160 MHz, 1,2 F 2 C 6 H 4, 298 K): 135, (v br, Rh 2 B), 6.2 (s, [BAr F 4]). 31 P{ 1 H} NMR (202 MHz, 1,2 F 2 C 6 H 4, 298 K): 47.5 (dd, J RhP 174.6, J RhP 6.0). ESI-MS (1,2 F 2 C 6 H 4, 60 C, 4.5 kv): m/z (calculated for [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B] + fragment, showing the correct isotope pattern). Synthesis of [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr Cl 4] (14-[BAr Cl 4]) [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr Cl 4] (18.9 mg, 15.5 µmol) and [N n Bu 4 ][BH 4 ] (2.0 mg, 7.8 µmol) were dissolved in 0.5 ml CH 2 Cl 2 in a J. Young flask. After 3 hours, the solution was filtered and layered with pentane to yield orange needles suitable for single crystal X-ray diffraction. Selected spectroscopic data: 1 H NMR (400 MHz, CD 2 Cl 2, 298 K): δ 7.60 (d, 2 J HH 7.6, aryl CH para to P), 7.44 (m, aryl CH meta to P), 7.28 (dd, 2 J HH 7.7, 2 J PH 7.7, aryl CH ortho to P).The aliphatic region is obscured by the presence of [N n Bu 4 ][BAr Cl 4] and pentane. There are no signals < 0 ppm. 11 B{ 1 H} NMR (128 MHz, CD 2 Cl 2, 298 K): 131, (v br, Rh 2 B), 7.0 (s, [BAr Cl 4]). 31 P{ 1 H} NMR (162 MHz, CD 2 Cl 2, 298 K): δ 47.5 (dd, J RhP 174.8, J RhP 6.8). ESI-MS (1,2 F 2 C 6 H 4, 60 C, 4.5 kv): m/z (calculated for [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B] + fragment, showing the correct isotope pattern). S8

9 Figure S1. Full experimental (top) and simulated (bottom) ESI mass spectra of [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4] (14-[BAr Cl 4]). S9

10 Figure S2. Experimental (top) and simulated (bottom) ESI mass spectra of [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4] (14-[BAr Cl 4]), showing isotope pattern of the peak. Synthesis of [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (15) RhH 2 Cl(κ 3 P,O,P Xantphos i Pr) (26.0 mg, 44.6 µmol), Na[BAr F 4] (39.4 mg, 44.6 µmol) and H 3 B NMeH 2 (2.2 mg, 49.1 µmol) were dissolved in 1,2 F 2 C 6 H 4 (0.6 ml) in a J. Young NMR tube. The mixture was shaken and immediately formed a yellow solution with a colourless precipitate. Selected spectroscopic data: 1 H NMR (400 MHz, 1,2 F 2 C 6 H 4, 298 K): δ 0.33 (s, br, 3 H, RhH 3 B), (br, 1 H, RhH), (br, 1 H, RhH). Upon decoupling to 11 B, the resonance at δ 0.33 sharpens. 11 B{ 1 H} NMR (128 MHz, 1,2 F 2 C 6 H 4, 298 K): 6.2 (s, [BAr F 4]), 19.0 (s, br, RhH 3 B). 31 P{ 1 H} NMR (202 MHz, 1,2 F 2 C 6 H 4, 298 K): δ 67.6 (d, 1 J RhP 109.7). Synthesis of [{RhH 2 (κ 3 P,O,P Xantphos i Pr)} 2 BH 4 ][BAr F 4] (16) Route A: [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (14.8 mg, 10.0 µmol) and 0.5 ml of a 0.02 M solution of H 3 B THF (1.0 mg, 10.0 µmol) were placed in a J. Young NMR tube, giving a pale S10

11 yellow solution. The sample was immediately placed into the NMR spectrometer. 31 P{ 1 H} NMR spectroscopy indicated that the mixture contained 35% [{RhH 2 (κ 3 P,O,P Xantphos i Pr)} 2 BH 4 ][BAr F 4] and 5% [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4], the rest being unidentified species with the major species proposed to be [RhH 2 (THF)(κ 3 P,O,P Xantphos i Pr)][BAr F 4]. Route B: [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (11.5 mg, 7.7 µmol) and [N n Bu 4 ][BH 4 ] (1.0 mg, 3.9 µmol) were dissolved in 0.4 ml 1,2 F 2 C 6 H 4 in a J. Young NMR tube, giving a pale yellow solution. The sample was immediately placed into the NMR spectrometer. 31 P{ 1 H} NMR spectroscopy indicated that the mixture contained 79% [{RhH 2 (κ 3 P,O,P Xantphos i Pr)} 2 BH 4 ][BAr F 4] and 21% [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4]. Selected spectroscopic data: 1 H NMR (400 MHz, 1,2 F 2 C 6 H 4, 298 K): δ 2.77 (s, br, 2 H, RhHB), (br, 1 H, RhH), (br, 1 H, RhH). Upon decoupling to 11 B, the resonance at δ 2.77 sharpens. 11 B{ 1 H} NMR (128 MHz, 1,2 F 2 C 6 H 4, 298 K): 6.2 (s, [BAr F 4]), 37.5 (s, br, Rh(H 2 BH 2 )Rh). 31 P{ 1 H} NMR (202 MHz, 1,2 F 2 C 6 H 4, 298 K): δ 67.2 (d, 1 J RhP 111.3). Synthesis of [Rh 2 (µ-h)(µ-h 2 BNMeH)(κ 2 P,P Xantphos Et) 2 ][BAr F 4] (17) A mixture of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos Et)][BAr F 4] (32.0 mg, 22.4 µmol) and H 3 B NMeH 2 (2.0 mg, 45 µmol) were suspended in 1,2 F 2 C 6 H 4 (0.5 ml) in a J. Young NMR tube. An initial NMR spectrum was recorded, which contained 18% [Rh 2 (µ-h)(µ-h 2 BNMeH)(κ 2 P,P Xantphos Et) 2 ][BAr F 4], 21% [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos Et)][BAr F 4] and 49% of a species assigned to [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos Et)][BAr F 4]. The mixture was agitated overnight to allow mixing of the undissolved H 3 B NMeH 2. After 18 hours 31 P{ 1 H} NMR spectroscopy indicated formation of [Rh 2 (µ-h)(µ-h 2 BNMeH)(κ 2 P,P Xantphos Et) 2 ][BAr F 4] (93%) alongside unreacted [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos Et)][BAr F 4] (7%). Selected spectroscopic data: 1 H NMR (400 MHz, 1,2 F 2 C 6 H 4, 298 K): 5.82 (s, br, 1 H, RhHB), 9.41 (s, br, 1 H, RhHB), (s, br, 1 H, RhHRh). 11 B (128 MHz, 1,2 F 2 C 6 H 4, 298 K): 61.0 (s, br, µ- H 2 B), 6.2 (s, [BArF 4 ]). 11 B{ 1 H} (128 MHz, 1,2 F 2 C 6 H 4, 298 K): 61.1 (s, br, µ-h 2 B), 6.2 (s, [BArF 4 ]). 31 P{ 1 H} (162 MHz, 1,2 F 2 C 6 H 4, 298 K): 38.8 (m), 11.8 (m). ESI-MS (1,2 F 2 C 6 H 4, 60 C, 4.5 kv): m/z (calculated for [Rh 2 (µ-h)(µ-h 2 BNMeH)(κ 2 P,P Xantphos Et) 2 ] + fragment, showing the correct isotope pattern). Spectroscopic data for [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos Et)][BAr F 4] Selected NMR data for [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos Et)][BAr F 4]: 1 H NMR (400 MHz, 1,2 F 2 C 6 H 4, 298 K): 0.53 (s, br, 3 H, RhH 3 B), (s, br, 1 H, RhH), (s, br, 1 H, RhH). 11 B{ 1 H} (128 MHz, 1,2 F 2 C 6 H 4, 298 K): 6.2 (s, [BArF 4 ]), 19.2 (br s, RhH 3 B). 31 P{ 1 H} (162 MHz, 1,2 F 2 C 6 H 4, 298 K): 42.4 (d, 1 J RhP 113.8). Synthesis of [RhH 2 (H 3 B NMeHBH 2 NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] RhH 2 Cl(κ 3 P,O,P Xantphos i Pr) (20.0 mg, 34.3 µmol), Na[BAr F 4] (30.4 mg, 34.3 µmol) and H 3 B NMeHBH 2 NMeH 2 (3.3 mg, 37.7 µmol) were dissolved in 1,2 F 2 C 6 H 4 (0.6 ml) in a J. Young NMR tube. The mixture was shaken and immediately formed a yellow solution with a colourless precipitate. Selected spectroscopic data: 1 H NMR (400 MHz, 1,2 F 2 C 6 H 4, 298 K): δ 0.53 (s, br, 3 H, RhH 3 B), (br, 1 H, RhH), (br, 1 H, RhH). Upon decoupling to 11 B, the resonance at δ 0.53 sharpens. 11 B{ 1 H} NMR (128 MHz, 1,2 F 2 C 6 H 4, 298 K): 4.8 (s, br, RhH 2 B), 6.2 (s, [BAr F 4]), 18.0 (s, br, RhH 3 B). 31 P{ 1 H} NMR (202 MHz, 1,2 F 2 C 6 H 4, 298 K): δ 68.1 (dd, 1 J RhP 111.2, 2 J PP 111.2). Synthesis of [Rh(PPh 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (20 mg, 13.5 µmol) and PPh 3 (8.8 mg, 33.8 µmol) were dissolved in 1,2 F 2 C 6 H 4 (0.4 ml) in a J. Young NMR tube. The resulting orange solution was left for 3 hours, before being filtered into a J. Young crystallization flask, layered S11

12 with pentane and stored at room temperature, which yielded orange crystals suitable for single crystal X-ray diffraction. The crystals were isolated by filtration, washed with pentane (5 ml x 3) and dried under vacuum. Yield 10 mg (6.0 µmol, 44%). 1 H NMR (400 MHz, CD 2 Cl 2, 298 K): δ 7.98 (m, 6 H, PPh 3 -CH), δ 7.72 (s, br, 8 H, [BAr F 4]-ortho- CH), 7.68 (d, 2 H, 2 J HH 7.5, aryl CH para to P), 7.56 (s, br, 4 H, [BAr F 4]-para-CH), 7.49 (m, 9 H, PPh 3 -CH), 7.39 (m, 2 H, aryl CH meta to P), 7.33 (dd, 2 H, 2 J HH 7.5, 2 J PH 7.5, aryl CH ortho to P), 1.66 (s, 6 H, ipr Xantphos backbone C(CH 3 )), 1.48 (m, br, 4 H, ipr-ch), 0.97 (m, 12 H, ipr- CH 3 ), 0.86 (m, 12 H, ipr-ch 3 ). 31 P{ 1 H} NMR (162 MHz, CD 2 Cl 2, 298 K): δ 47.4 (dt, 1 P, 1 J RhP 217.4, 2 J PP 39.2, PPh 3 ), 44.7 (dd, 2 P, 1 J RhP 131.4, 2 J PP 39.2, P i Pr 2 ). ESI-MS (1,2 F 2 C 6 H 4, 60 C, 4.5 kv): m/z (calculated for [Rh(PPh 3 )(κ 3 P,O,P Xantphos i Pr)] + fragment, showing the correct isotope pattern). Microanalysis: Calc. (C 77 H 67 BF 24 OP 3 Rh): C, 55.35; H, Found: C, 55.44; H, Synthesis of [BH 2 (NMeH 2 ) 2 ][BAr F 4] [BH 2 (NMeH 2 ) 2 ][BAr F 4] was prepared by an analogous method to that of [BH 2 (NMe 2 H) 2 ][BAr F 4] as described by López-Serrano, Conejero and co-workers. 12 BH 2 (NMeH 2 ) 2 Cl (30 mg, 27.2 µmol) and Na[BAr F 4] (240.8 mg, 27.2 µmol) were dissolved in 3 ml THF and stirred for 15 minutes to form a white precipitate (NaCl) and a colourless solution. The solution was filtered and the solvent removed in vacuo. The remaining white solid was dissolved in CH 2 Cl 2 (3 ml) and the resultant solution was filtered into a J. Young crystallization tube. The colourless solution was layered with hexane and stored at room temperature to give colourless, crystalline material. The crystals were isolated by filtration, washed with hexane (5 ml x 3) and dried under vacuum. Yield mg (22.1 µmol, 81%). 1 H NMR (500 MHz, CD 2 Cl 2, 298 K): δ 7.72 (s, br, 8 H, [BAr F 4]-ortho-CH), 7.57 (s, br, 4 H, [BAr F 4]-para-CH), 4.03 (br, 4 H, NH), 2.71 (t, 6 H, 3 J HH 6.2, NMe), 2.22 (q, br, 2 H, 1 J BH ~ 120, BH 2 ). 11 B NMR (160 MHz, CD 2 Cl 2, 298 K): 6.6 (s, [BAr F 4]), 7.4 (t, 1 J BH 116.9, [BH 2 (NMeH 2 ) 2 ]). 13 C{ 1 H} NMR (126 MHz, CD 2 Cl 2, 298 K): (q, 1 J CB 49.7, [BAr F 4]-ipso-C), (s, br, [BAr F 4]-ortho-C), (q, 2 J CF = 31.5, [BAr F 4]-meta-C), (q, 1 J CF = 272.4, [BAr F 4]-CF 3 ), (m, [BAr F 4]-para-C), 32.3 (s, NMeH 2 -C). Microanalysis: Calc. (C 34 H 24 B 2 F 24 N 2 ): C, 43.53; H, 2.58; N, Found: C, 43.40; H, 2.43; N, Catalytic Dehydropolymerization of H 3 B NMeH 2 General Procedure for Dehydropolymerization (Open Conditions) H 3 B NMeH 2 (50.0 mg, 1.11 mmol) was suspended in 1,2 F 2 C 6 H 4 (2.2 ml, M [H 3 B NMeH 2 ] unless otherwise specified) in a two-neck Schlenk flask. Double the desired amount of the Rh precatalyst was weighed into a separate flask and dissolved in 1,2 F 2 C 6 H 4 (0.6 ml). The H 3 B NMeH 2 -containing flask was connected to an external mineral oil bubbler and the argon flow was controlled to achieve a rate of bubbling of approximately 2 bubbles/sec. 0.3 ml of the precatalyst solution was added to the reaction mixture and the resultant solution was stirred for 30 minutes at 400 rpm. The solution was decanted into 20 ml of rapidly stirring hexane to give an off-white suspension which was stirred for 5 minutes to allow polymer precipitation, then isolated by filtration. The white solid (H 2 B NMeH) n was dried under vacuum overnight. Isolated yields varied from 30% to 75% for 6 and 40% to 65% for 11. NMR data for (H 2 B NMeH) n conform to those previously reported. 13,14 General Procedure for Gas Evolution Measurements H 3 B NMeH 2 (50.0 mg, 1.11 mmol) was suspended in 1,2 F 2 C 6 H 4 (e.g. 2.2 ml for M [H 3 B NMeH 2 ]) in a jacketed two-neck Schlenk flask connected to a recirculating cooler and the temperature set at 20 C. Double the desired amount of the Rh precatalyst was weighed into a separate flask and dissolved in 1,2 F 2 C 6 H 4 (0.6 ml). The H 3 B NMeH 2 -containing flask was sealed off from the Ar supply and connected to a water-filled gas burette. 0.3 ml of the S12

13 precatalyst solution was added to the reaction mixture and the resultant solution was stirred at 400 rpm. The time taken for specified volumes of gas to evolve was recorded. Upon completion of gas evolution the solution was decanted into 20 ml of rapidly stirring hexane to give an offwhite suspension which was stirred for 5 minutes to allow polymer precipitation, then isolated by filtration. The white solid (H 2 B NMeH) n was dried under vacuum overnight. Dehydropolymerization under Closed Conditions H 3 B NMeH 2 (50.0 mg, 1.11 mmol) was suspended in 1,2 F 2 C 6 H 4 (4.7 ml for 6, 2.2 ml for 11) in a J. Young flask. Double the desired amount of the Rh precatalyst was weighed into a separate flask and dissolved in 1,2 F 2 C 6 H 4 (0.6 ml). 0.3 ml of the precatalyst solution was added to the reaction mixture and the flask was sealed and stirred at 400 rpm for 30 minutes. The solution was decanted into 20 ml of rapidly stirring hexane to give an off-white suspension which was stirred for 5 minutes to allow polymer precipitation, then isolated by filtration. The white solid (H 2 B NMeH) n was dried under vacuum overnight. 6: Isolated yield 35.0 mg (0.816 mmol, 74%). M n 9,000 g mol 1, Ð : Isolated yield 24.7 mg (0.576 mmol, 52%). M n 27,700 g mol 1, Ð Dehydropolymerization in the Presence of Cyclohexene H 3 B NMeH 2 (50.0 mg, 1.11 mmol) was suspended in 1,2 F 2 C 6 H 4 (4.7 ml for 6, 2.2 ml for 11) in a two-neck Schlenk flask and cyclohexene (0.3 ml, 3 mmol) was added. Double the desired amount of the Rh precatalyst was weighed into a separate flask and dissolved in 1,2 F 2 C 6 H 4 (0.6 ml). The H 3 B NMeH 2 -containing flask was connected to an external mineral oil bubbler and the argon flow was controlled to achieve a rate of bubbling of approximately 2 bubbles/sec. 0.3 ml of the precatalyst solution was added to the reaction mixture and the resultant yellow solution was stirred for 30 minutes at 400 rpm. The solution was decanted into 20 ml of rapidly stirring hexane to give an off-white suspension which was stirred for 5 minutes to allow polymer precipitation, then isolated by filtration. The white solid (H 2 B NMeH) n was dried under vacuum overnight. 6: Isolated yield 17.5 mg (0.408 mmol, 37%). M n 12,000 g mol 1, Ð : Isolated yield 24.7 mg (0.576 mmol, 52%). M n 33,000 g mol 1, Ð Recycling Experiment Polymerization was conducted as per standard gas evolution measurement conditions. After cessation of gas evolution (20 minutes), the reaction mixture was transferred into a second jacketed, two-neck Schlenk flask connected to a recirculating cooler at 20 C containing H 3 B NMeH 2 (50.0 mg, 1.11 mmol). Gas evolution was recorded by gas burette. After cessation of gas evolution (20 minutes), the reaction mixture was transferred into a third jacketed, twoneck Schlenk flask connected to a recirculating cooler at 20 C containing H 3 B NMeH 2 (50.0 mg, 1.11 mmol). Gas evolution was recorded by gas burette. After cessation of gas evolution the yellow solution was decanted into 20 ml of rapidly stirring hexane, stirred for 5 minutes to allow polymer precipitation, then isolated by filtration. The white solid (H 2 B NMeH) n was dried under vacuum overnight. 6: Isolated yield 52.4 mg (1.22 mmol, 37 %). M n 15,000 g mol 1, Ð : Isolated yield mg (2.36 mmol, 71 %). M n 26,300 g mol 1, Ð Tests for Homogeneity Mercury Poisoning Catalysis was performed as per standard gas evolution measurement conditions. After a short time an excess of elemental mercury (ca 0.05 ml) was added. No inhibition of gas evolution was observed, consistent with homogeneous catalysis. S13

14 Figure S3. (a) Effect of excess Hg (1500 equiv.) at t = 250 s: [6] = 8.92 x 10 4 M, [H 3 B NMeH 2 ] = M. (b) Effect of excess Hg (1500 equiv.) at t = 120 s: [11] = 8.92 x 10 4 M, [H 3 B NMeH 2 ] = M. Fractional Poisoning Catalysis was performed as per standard gas evolution measurement conditions. After a short time 0.2 equivalents (relative to Rh) of PPh 3 (0.2 ml, 2.2 mm in 1,2 F 2 C 6 H 4 ) was added. A minor decrease in the rate of gas evolution was observed, consistent with homogeneous catalysis. Figure S4. (a) Effect of sub stoichiometric PPh 3 (0.2 equiv.) added at t = 250 s: [6] = 8.92 x 10 4 M, [H 3 B NMeH 2 ] = M. (b) Effect of sub stoichiometric PPh 3 (0.2 equiv.) added at t = 120 s: [11] = 8.92 x 10 4 M, [H 3 B NMeH 2 ] = M. S14

15 Experimental Kinetic Data for 6 Figure S5. Temporal data plots for polyaminoborane formation (as measured by H 2 evolution) for catalyst 6 (4.45 x 10 4 M except where stated) and H 3 B NMeH 2 ( = M, = M, = M and = M). X = 6 (8.9 x 10 4 M), H 3 B NMeH 2 (0.446 M). Experimental and Simulated Kinetic Data for 11 Figure S6. Temporal data plot for polyaminoborane formation (as measured by H 2 evolution) and simulated fit (line) for catalyst 11 (8.9 x 10 4 M) and H 3 B NMeH 2 (0.446 M). S15

16 Experimental Kinetic Data for 6 with 10 equivalents [H 2 B(NMeH 2 ) 2 ][BAr F 4] Figure S7. Temporal data plots for polyaminoborane formation (as measured by H 2 evolution) for catalyst 6 (2.23 x 10 4 M) and H 3 B NMeH 2 ( M) ( = without additional [H 2 B(NMeH 2 ) 2 ][BAr F 4], = with 2.23 x 10 3 M [H 2 B(NMeH 2 ) 2 ][BAr F 4]). S16

17 Representative 11 B NMR Spectra Figure S8. 11 B NMR spectra of the reaction mixture in 1,2-F 2 C 6 H 4 (top) and isolated polyaminoborane in CD 2 Cl 2 (bottom) from the dehydropolymerization of H 3 B NMeH 2 using catalyst 6: 0.2 mol%, M H 3 B NMeH 2. S17

18 Figure S9. 11 B NMR spectra of the reaction mixture in 1,2-F 2 C 6 H 4 (top) and isolated polyaminoborane in CD 2 Cl 2 (bottom) from the dehydropolymerization of H 3 B NMeH 2 using catalyst 11: 0.2 mol%, M H 3 B NMeH 2. S18

19 (a) (b) (c) (d) Figure S10. Representative 11 B NMR spectra of the reaction mixtures in 1,2-F 2 C 6 H 4 from the dehydropolymerization of H 3 B NMeH 2 (0.446 M) with 2.7 eq. (relative to H 3 B NMeH 2 ) of cyclohexene using catalysts 6 and 11. (a) 0.2 mol% 6. (b) 0.2 mol% 11. (c) 10 mol% 6 (the peak at δ 45.9 is assigned to Cy 2 B=NMeH). (d) 10 mol% 11. S19

20 Figure S B NMR spectra of the reaction mixture in 1,2-F 2 C 6 H 4 from the dehydropolymerization of H 3 B NMeH 2 (0.446 M) using 10 mol% 6. Figure S B NMR spectra of the reaction mixture in 1,2-F 2 C 6 H 4 from the dehydropolymerization of H 3 B NMeH 2 (0.446 M) using 10 mol% 10. S20

21 Figure S B NMR spectra of the reaction mixture in 1,2-F 2 C 6 H 4 from the dehydropolymerization of H 3 B NMeH 2 (0.446 M) using 10 mol% 11. Figure S B NMR spectra of the reaction mixture in 1,2-F 2 C 6 H 4 from the dehydropolymerization of H 3 B NMeH 2 (0.446 M) using 10 mol% 13. S21

22 Eyring Analyses of 3 and 4 The NMR data for complexes 3 and 4 both indicate a Rh(I) centre with equivalent 31 P environments, e.g [J(RhP) = 145 Hz], and a single NBD ligand. At 298 K in CD 2 Cl 2 solution the bridge methyl groups on the Xantphos ligands are equivalent, and only one alkene environment is observed. This suggests a fluxional process is occurring, as the solid state structure has only approximate C s symmetry. We suggest a ring flipping process, as reported in related Xantphos Ph complexes. 15,16 Cooling arrests this so that at 200 K two methyl and two alkene environments are observed, consistent with the structure in the solid state. Eyring plots arising from line shape analyses of the variable temperature 1 H NMR spectroscopic data reveal similar activation parameters for both 3 and 4 for this process (Figure S15), with small negative entropies of activation, suggesting a slightly more ordered transition state. Figure S15. (a) Eyring plot for [Rh(NBD)(κ 2 P,P Xantphos Et)][BAr F 4] (3). (b) Eyring plot for [Rh(NBD)(κ 2 P,P Xantphos i Pr)][BAr F 4] (4). Table S1. Activation parameters derived from Eyring analyses of complexes 3 and 4. Errors are shown in parentheses. Complex ΔH ΔS kj mol 1 J K 1 mol 1 ΔG 298K kj mol (1) 13(4) 49(2) 4 40(1) 14(3) 45(1) Although within error the activation parameters are the same between 3 and 4 (Table S1), inspection of the solid state structures for both reveals a close H H contact between two i Pr methine groups (C11, C17, see Figure S16) in 4 (1.973 Å, van der Waals radius H = 1.20 Å) 17 that are brought together in the cis 2 P,P geometry, for which the equivalent distance is longer in 3 (2.154 Å), potentially destabilizing the ground state geometry in 4. These interactions would be accentuated for the Xantphos t Bu ligand, and indeed the corresponding 2 P,P NBD adduct could not be prepared, as commented upon by Goldman and co workers. 2 Such steric effects may influence the relative preference for 2 P,P versus 3 P,O,P coordination geometries in Xantphos Et versus Xantphos i Pr complexes. The parent Xantphos Ph NBD complex 18 shows no such interactions, with the phenyl groups lying parallel and both 2 P,P and 3 P,O,P coordination geometries are well represented in the literature, 19 e.g. complexes 1 and 2. S22

23 4 3 C11 C Å Å [Rh(Xantphos Ph)(NBD)][BAr F 4] Å Figure S16. Solid-state structures of complexes 3 and 4, highlighting the close H H contacts on the alkyl phosphines. The structure of [Rh(κ 2 P,P Xantphos Ph)(NBD)][BAr F 4] 20 is shown for comparison. Eyring Analysis of 6 Solution 1 H NMR data show that 6 is fluxional at 298 K, demonstrated by a single hydride environment observed at δ 19.09, of relative integral 2 H. Progressive cooling to 200 K reveals a low temperature limiting spectrum consistent with the solid state structure that shows two hydride environments at δ and δ 19.98, each of relative integral 1 H (Figure S17). Line shape analysis of these hydride signals in complex 6 using an Eyring plot (Figure S18) gives activation parameters ΔH = 59(4) kj mol 1, ΔS = +37(15) J K 1 mol 1 and ΔG 298K = 49(8) kj mol 1 for this fluxional process. These data are consistent with a mechanism in which the H 3 B NMe 3 ligand dissociates from the metal and re coordinates on the other side, via a symmetric 16 electron intermediate [RhH 2 (κ 3 P,O,P Xantphos i Pr)] +. These activation parameters are similar to those reported for the related fluxional process in [RhH 2 (κ 3 P,O,P Xantphos i Pr)][OTf]. 5 By contrast complex 5 is not fluxional at 298 K, and displays NMR data that are very similar to those measured at low temperature for 6 (Figure S17). S23

24 Figure S17. Variable temperature 1 H NMR spectra of complexes 5 and 6, showing the hydride regions. (a) Complex 6 at 200 K. (b) Complex 6 at 298 K. (c) Complex 5 at 298 K. Figure S18. Eyring plot for [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (6). Catalyst Speciation of 6 with H 3 B NMeH 2 at 10 Mol% Loading The reaction of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (6) with 10 equivalents of H 3 B NMeH 2 was monitored by NMR spectroscopy to study the species formed during catalysis. Initially, two complexes coexist in solution by 1 H and 31 P{ 1 H} NMR spectroscopy (Figures S19a and S20a respectively). Independent in situ syntheses of [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (15) and [{RhH 2 (κ 3 P,O,P Xantphos i Pr)} 2 BH 4 ][BAr F 4] (16) and comparison of their 1 H and 31 P{ 1 H} NMR spectra (Figures S19c and S20c for 15, Figures S19d and S20d for 16) with the 10 mol% reaction mixture indicate that these are likely the first-formed species in catalysis. However spectral similarities between 15 and [RhH 2 (H 3 B NMeHBH 2 NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (Figures S19e and S20e) mean that this complex cannot be discounted. Over time, a new species becomes dominant in the 31 P{ 1 H} NMR spectrum for the 10 mol% reaction mixture (Figure S20b), which according to the corresponding 1 H NMR spectrum (Figure S19b), does not contain any hydrides. This complex is also observed in the 31 P{ 1 H} NMR spectrum of 16, as facile loss of 4 equivalents of H 2 from 16 S24

25 forms [{Rh(κ 3 P,O,P Xantphos i Pr)} 2 B][BAr F 4] (14), which can additionally be independently prepared and characterized by single-crystal X-ray crystallography (as its [BAr Cl 4] salt) and ESI- MS. Addition of 10 equivalents of H 3 B NMeH 2 to independently prepared 14 and 15 and analysis of these reaction mixtures by NMR spectroscopy also provides spectra that match Figures S19a, S19b, S20a and S20b. Hence we propose a reaction manifold in which , where 14 reforms 15 and 16 on recharge with further H 3 B NMeH 2. Figure S19. 1 H NMR spectra (hydride region) of species relevant to catalysis using 6. (a) Complex 6 plus 10 equivalents H 3 B NMeH 2 after 2 minutes. (b) Complex 6 plus 10 equivalents H 3 B NMeH 2 after 20 minutes. (c) Independent synthesis of [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (15). (d) Independent synthesis of [{RhH 2 (κ 3 P,O,P Xantphos i Pr)} 2 BH 4 ][BAr F 4] (16). (e) Independent synthesis of [RhH 2 (H 3 B NMeHBH 2 NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4]. S25

26 Figure S P{ 1 H} NMR spectra of species relevant to catalysis using 6. (a) Complex 6 plus 10 equivalents H 3 B NMeH 2 after 2 minutes. (b) Complex 6 plus 10 equivalents H 3 B NMeH 2 after 20 minutes. (c) Independent synthesis of [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4] (15). (d) Independent synthesis of [{RhH 2 (κ 3 P,O,P Xantphos i Pr)} 2 BH 4 ][BAr F 4] (16). (e) Independent synthesis of [RhH 2 (H 3 B NMeHBH 2 NMeH 2 )(κ 3 P,O,P Xantphos i Pr)][BAr F 4]. Reaction Composition Plot for Formation of 17 The reaction of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos Et)][BAr F 4] (5) with 2 equivalents of H 3 B NMeH 2 was monitored by NMR spectroscopy to track consumption of 5, formation of the product [Rh 2 (µ-h)(µ-h 2 BNMeH)(κ 2 P,P Xantphos Et) 2 ][BAr F 4] (17), and the presence of the intermediate σ-complex [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos Et)][BAr F 4] (Figure S21). Initially, all three complexes coexist in solution, with no species dominant. However, the steady consumption of [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos Et)][BAr F 4] and formation of 17 ensues over the following 400 minutes. Binding of H 3 B NMe 3 and H 3 B NMeH 2 to [RhH 2 (κ 3 P,O,P Xantphos Et)] + appears to be competitive, and thus as H 3 B NMeH 2 is consumed (via formation of 17 and a small amount of (HBNMe) 3 ) complex 5 is reformed and hence complete consumption of 5 is not observed. S26

27 Figure S21. Reaction composition over time for the reaction of [RhH 2 (H 3 B NMe 3 )(κ 3 P,O,P Xantphos Et)][BAr F 4] (5) with 2 equivalents of H 3 B NMeH 2 to form [Rh 2 (µ-h)(µ-h 2 BNMeH)(κ 2 P,P Xantphos Et) 2 ][BAr F 4] (17) ( = 5, = 17 and = [RhH 2 (H 3 B NMeH 2 )(κ 3 P,O,P Xantphos Et)][BAr F 4]). S27

28 GC-MS of 1,2 F 2 C 6 H 4 Figure S22. GC-MS graph of 1,2 F 2 C 6 H 4 and identification of contaminants. (a) Full chromatogram of 1,2 F 2 C 6 H 4. The peak at minutes is CH 2 Cl 2 which is used to wash the autosampler syringe. (b) Peak at minutes which is assigned as 1-chloro-2-fluorobenzene. (c) Peak at minutes which is assigned as 2-fluorophenol. (d) Peak at minutes which is assigned as 2-chloro-4-fluorotoluene. Fitting of GPC Data for Polymer Arising From 6 GPC data was initially analyzed using OMNISEC software to obtain suitable peak limits for molecular weight analyses. Weight fraction per log molecular weight increment (dw/d log M) S28

29 versus log molecular weight (log M) plots (i.e. the true molecular weight distribution curve) 21 were obtained using OMNISEC. The sum of two skew Gaussian functions was used to model the molecular weight distribution curve; with parameter estimation achieved using the Levenberg-Marquadt nonlinear least-squares method from MINPACK 22 via the SciPy optimization library for Python. The peak limits found initially using OMNISEC were subsequently applied to the fitted molecular weight distribution curve and the fitted molecular weight parameters were calculated. Crystallography Crystallographic data were collected on a Nonius Kappa-CCD (8) or Agilent SuperNova diffractometer fitted with an Oxford CryoSystems CryoStream unit (3, 4, 6, 12, 14-[BAr Cl 4]). 23 Raw frame data were reduced using CrysAlisPro, including unit cell determination and refinement, integration of intensities and associated correction, or the DENZO-SMN package. 24 The structures were solved using direct methods with SIR92, 25 SuperFlip 26 or SHELXT 27 and refined using full-matrix least squares refinement on all F 2 data using the CRYSTALS program suite 28,29 (3, 4, 6 and 8) or with SHELXL 30 (12 and 14-[BAr Cl 4]) using the interface OLEX2. 31 In certain cases some of the CF 3 groups in the [BAr F 4] anion refined to give highly prolate 32 displacement ellipsoids. For each structure a split site model was used and same distance restraints were applied to maintain a sensible geometry. Thermal similarity and vibrational restraints were also used to ensure the displacement ellipsoids behaved as expected. In general, the hydrogen atoms were located in the difference Fourier maps and refined independently using soft restraints. 29 Hydrogen atoms were refined in this way at intervals following inclusion in the refinement using a riding model. Special Refinement Details Hydrides on the rhodium and the boron atoms in 6 were placed in calculated positions and refined using a riding model. The aryl substituent in 12 is disordered (50:50) over two positions. Restraints were applied to geometries and displacement parameters. All hydrogen atoms are placed in calculated positions using a riding model. Single crystals of 14-[BAr Cl 4] repeatedly grow with two very small dimensions for an X-ray structure determination. A significant fall-off in diffraction intensity at relatively low Bragg angles was observed, nevertheless the refinement converged well and atom connectivity is unambiguous. The final Fourier difference map revealed notable peaks of electron density proximal to each rhodium atom and diametrically opposed across the atom at ca Å, these are of each of magnitude ca e Å 3. In addition, positive residual densities (ca e Å 3 ) on each of the heavy atom sites are observed. Twinning was investigated, but twin scale factors refine to zero for all sensible twin laws. The locations of the electron densities are such that a chemically sensible disorder model could not be generated. These are therefore attributed to Fourier truncation errors. The atoms of one aryl group of the anion displayed prolate ellipsoids. This was treated by modelling the atoms over two sites and restraining their geometries and displacement parameters. The lattice of 14-[BAr Cl 4] contains a number of disordered solvent molecules; these were modelled with the aid of the DSR tool. 33 All hydrogen atoms are placed in calculated positions using a riding model. S29

30 Table S2. Crystallographic data for complexes 3, 4, 6, 8, 12, 14-[BAr Cl 4]. Compound Chemical formula C 62 H 52 BF 24 OP 2 Rh C 66 H 60 BF 24 OP 2 Rh C 62 H 66 B 2 F 24 NOP 2 Rh Formula weight Temperature (K) Crystal system Monoclinic Monoclinic Orthorhombic Space group P2 1 /c P2 1 /c Pca2 1 a (Å) (2) (2) (10) b (Å) (2) (2) (10) c (Å) (3) (2) (2) α (deg) β (deg) (13) (9) 90 γ (deg) V (Å 3 ) (14) (2) (8) Z ρ (calcd) (g cm 1 ) µ (mm 1 ) Reflections collected Unique reflections Restraints/parameters 1596/ / /894 R int R 1 [I > 2σ(I)] wr 2 [I > 2σ(I)] GooF Residual electron density (e Å 3 ) 2.13, , , 0.70 CCDC no Compound [BAr Cl 4] Chemical formula C 46 H 64 Cl 2 O 2 P 4 Rh 2 C 45 H 55 F 2 OP 2 Rh C 89 H 92 B 2 Cl 10 O 2 P 4 Rh 2 Formula weight Temperature (K) Crystal system Triclinic Monoclinic Triclinic Space group P 1 P2 1 /m P 1 a (Å) (3) (5) (4) b (Å) (3) (4) (9) c (Å) (4) (5) (10) α (deg) (15) (4) β (deg) (14) (4) (3) γ (deg) (13) (3) V (Å 3 ) (6) (14) (4) Z ρ (calcd) (g cm 1 ) µ (mm 1 ) Reflections collected S30

31 Unique reflections Restraints/parameters 0/ / /1192 R int R 1 [I > 2σ(I)] wr 2 [I > 2σ(I)] GooF Residual electron density (e Å 3 ) 0.86, , , 1.21 CCDC no Figure S23. Molecular structure of the cationic portion of 3, displacement ellipsoids are shown at 30% probability level, H atoms and [BAr F 4] anion are not shown. Selected bond distances (Å) and angles (º): Rh P1, (11); Rh1 P2, (12); Rh1 O1, 3.294(3); Rh1 (C24/C25), 2.200(5)/2.182(4); Rh1 (C29/C30), 2.183(4)/2.169(4); P1 Rh1 P2, (4). S31

32 Figure S24. Molecular structure of 8, displacement ellipsoids are shown at 30% probability level, H atoms are not shown. Selected bond distances (Å) and angles (º): Rh1 Cl (8); Rh1 P (7); Rh1 P (8); Rh1 O (2); P1 Rh1 P (3). S32

33 Computational Details The geometry optimization was run with Gaussian 03 Revision D and performed using the BP86 35,36 functional. Rh and P centres were described with Stuttgart RECPs and associated basis sets, 37 with added d-orbital polarization on P ( = 0.387) 38 and 6-31G** basis sets for C, H, O and B. 39,40 Stationary points were fully characterized via analytical frequency calculations as minima with all positive eigenvalues. Frequency calculations were run using Gaussian 09 Revision D Atoms in Molecules 42 analyses were performed with the AIMALL program 43 using the same functional and basis sets described above. NBO calculations were run with NBO using the same functional and basis sets described above. Geometry Optimizations P P O Rh B Rh O P P Figure S25. Geometry optimized structure of the cationic portion of 14-[BAr Cl 4]. Bond Computed (Å) Crystal (Å) Rh-B Rh-P Rh-O Table S3. Comparison of key distances between experimental and computed structures of 14- [BAr Cl 4]. S33

34 P O Rh B P Rh P O P Figure S26. Geometry optimized structure of 14-[BAr Cl 4]-(Xantphos-H). Bond Computed (Å) Rh-B 1.87 Rh-P 2.27 Rh-O 2.33 Table S4. Key distances of the computed structure of 14-[BAr Cl 4]-(Xantphos-H). S34

35 QTAIM Results Rh B Rh Figure S27. Molecular graph of 14-[BAr Cl 4] with 2D contour plot of the electron density in the {RhBP} plane with projected stationary points and bond paths. Bond critical points (BCP) are shown in green and ring critical points (RCP) in red. BCP ρ(r) 2 ρ(r) ε H(r) B Rh B Rh Table S5. Calculated QTAIM parameters (a.u.) for selected BCPs in 14-[BAr Cl 4]. ρ(r) = electron 2 density, ρ(r) = Laplacian of the electron density, ε = bond ellipticity, H(r)= total energy density. S35

36 NBO Results LV B E = 75.5 kcal mol -1 LP Rh LV B E = 25.7 kcal mol -1 LV B E = 15.1 kcal mol -1 LP Rh LV B E = 12.9 kcal mol -1 Figure S28. Computed NBOS of 14-[BAr Cl 4] corresponding to 2 d-orbitals donating to 4 lowvacancy boron orbitals (s, px, py, and pz). S36

37 Localized Molecular Orbitals π-interaction HOMO -5 π-interaction HOMO -8 σ-interaction HOMO -12 Figure S29. Computed Kohn-Sham molecular orbitals equating to the multiple bonding in the Rh-B-Rh interaction in 14-[BAr Cl 4]. S37