Supporting Information for the Paper Entitled: Alex E. Carpenter, Chinglin Chan, Arnold L. Rheingold and Joshua S. Figueroa*

Supporting Information for the Paper Entitled: A Well-Defined Isocyano Analogue of HCo(CO) 4 2. Relative Brønsted Acidity as a Function of Isocyanide Ligation. Alex E. Carpenter, Chinglin Chan, Arnold L. Rheingold and Joshua S. Figueroa* Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla, California 92093-0258 Email: jsfig@ucsd.edu Contents: S1. Synthetic Procedures and Characterization Data... S2 S2. Supplemental 13 C{ 1 H} NMR Chemical Shift Data... S7 S3. Crystallographic Structure Determinations.. S8 S4. References... S14 S 1

S1. Synthetic Procedures and Characterization Data S1.1. General Considerations. Unless otherwise stated all manipulations were carried out under an atmosphere of dry dinitrogen using standard Schlenk and glovebox techniques. Solvents were dried and deoxygenated according to standard procedures. 1 Reagent grade starting materials were purchased from commercial sources and were used as received or purified by standard procedures. 2 The compounds HCo(CNAr Mes2 ) 4, CNAr Mes2, 13 CNAr Mes2, Na[Co(CNAr Mes2 ) 4 ], Co 2 (CO) 6 (CNAr Mes2 ) 2 and Co 2 (CO) 4 (CNAr Mes2 ) 4 were prepared according to literature procedures. 3-7 Benzene-d 6 and THF-d 8 (Cambridge Isotope Laboratories) were dried over NaK/benzophenone, vacuum distilled, degassed and stored over 4 Å molecular sieves for 2 d prior to use. Solution 1 H and 13 C{ 1 H} NMR spectra were recorded on a Bruker AVA 300 MHz spectrometer, Varian Mercury 400 MHz spectrometer, a Varian X-Sens 500 MHz spectrometer, or a JEOL ECA 500 MHz spectrometer. 1 H and 13 C{ 1 H} NMR chemical shifts are reported in ppm relative to SiMe4 ( 1 H and 13 C{ 1 H} δ = 0.0 ppm) with reference to residual-proteo resonances of 7.16 ppm ( 1 H) and 128.06 ppm ( 13 C) for benzene-d6, and 3.58 ppm ( 1 H) and 67.6 ppm ( 13 C) for THF-d 8. 31 P{ 1 H} NMR chemical shifts are reported in ppm relative to a sealed capillary of 85% H 3 PO 4 in aqueous solution. Roomtemperature FTIR spectra were recorded on a Thermo-Nicolet is10 FTIR spectrometer. Samples were prepared either as KBr pellets or as C 6 D 6 solutions injected into a ThermoFisher solution cell equipped with KBr windows. For solution FTIR spectra, solvent peaks were digitally subtracted from all spectra by comparison with an authentic spectrum obtained immediately prior to that of the sample. The following abbreviations were used for the intensities and characteristics of important IR absorption bands: vs = very strong, s = strong, m=medium, w = weak, vw = very weak; b = broad, vb = very broad, bw = broad-weak, sh = shoulder. Combustion analyses were performed by Midwest Microlabs LLC, Indianapolis, IN (USA). S1.2. Acidity Bracketing. The relative acidity of HCo(CO) 4-n (CNAr Mes2 ) n (n = 1-4) was bracketed by addition 1.0 equiv of acid (see: Table 3, text) to a THF d 8 solution of Na[Co(CO) 4-n (CNAr Mes2 ) n ] (n = 1-4). The reaction was then monitored by 1 H NMR spectroscopy for the formation of the corresponding HCo(CO) 4-n (CNAr Mes2 ) n hydride. In a typical experiment, a THF d 8 solution of HCo(CO) 4-n (CNAr Mes2 ) n (0.028 mmol, 1 ml) was treated with 1.0 equiv acid and the resulting mixture was monitored by 1 H NMR for 24 h to detect formation of HCo(CO) 4-n (CNAr Mes2 ) n. By this methodology, a rough estimate of acidity was obtained. S1.3. Synthesis of Na[Co(CO)(CNAr Mes2 ) 3 ] (2-Na). Na[N(SiMe 3 ) 2 ] (0.047 g, 0.255 mmol, 1.0 equiv) was added as a solid to an orange THF solution of HCo(CO)(CNAr Mes2 ) 3 (0.276 g, 0.255 mmol, 5 ml) at 35 C. The solution was stirred for 2 h at 35 C, then for 30 min at room temperature. This produced a red solution that was concentrated to a solid under reduced pressure. The solid was suspended in n- pentane (10 ml), stirred for 20 min and reconcentrated to a solid. This was repeated two additional times. The red solid was then suspended in n-pentane (20 ml) and filtered. The precipitate was dissolved in minimal amount of THF and filtered. n-pentane was layered on top of the THF filtrate and the mixture stored at 35 C. Red, non-diffraction- S 2

quality crystals were then collected and dried in vacuo. 1 H NMR analysis indicated that this crystalline material possessed two molecules of THF. Yield: 0.170 g, 0.133 mmol, 52.1 %. X-ray diffraction quality crystals were obtained from storage of an Et 2 O solution of 2-Na at 35 C for 3 d. 1 H NMR (499.8 MHz, C 6 D 6, 20 C): δ = 6.92 (t, 3H, J = 7 Hz, p-ph), 6.87 (d, 6H, J = 7 Hz, m-ph), 6.84 (s, 12H, m-mes), 2.23 (s, 18H, p-mes), 2.12 (s, 36H, o-mes) ppm. 13 C{ 1 H} NMR (125.7 MHz, C 6 D 6, 20 C): δ = 195.5 (very broad, CNR, ) 137.4, 137.1, 136.1, 135.9, 133.7, 129.5, 128.5, 123.6, 21.3, 20.9 ppm (The CNR resonance is significantly broadened while the CO resonance could not be detected, presumably due to coupling to 59 Co (I = 7/2, 100 %). FTIR (KBr window, C 6 D 6 ): ν CN =1904 (vs), 1865 (vs) cm 1, ν CO = 1830 cm 1 also, 2970 (vw), 2945 (vw), 2915 (w), 2848 (vw), 2009 (m, br), 1579 (m), 1415 (m), 1205 (vw), 1069 (w), 1033 (w), 1011 (vw), 852 (m), 755 (m) 634 (vw) cm 1. Anal. Calcd. For C 84 H 91 N 3 O 3 NaCo (2-Na 2(THF)): C, 79.28; H, 7.21; N, 3.30. Found: C, 78.59; H, 6.63; N, 3.59. S1.4. Synthesis of PPN[Co(CO)(CNAr Mes2 ) 3 ] (2-PPN): At THF solution of Na[Co(CO)(CNAr Mes2 ) 3 ] (2-Na; 0.040 g, 0.035 mmol, 15 ml) was cooled to 35 C and combined with solid PPNCl (0.022 g, 0.037 mmol, 1.05 equiv). The resulting mixture was allowed to warm to room temperature and stir for 1 h. The reaction mixture was then concentrated to a solid under reduced pressure. The resulting residue was extracted with C 6 H 6 (5 ml) and filtered through Celite. The filtrate was then reconcentrated to a solid under reduced pressure. The resulting residue was suspended in n-pentane (10 ml) and filtered. The dark red/purple precipitate was then washed with n-pentane (3 x 10 ml), collected and dried. Yield: 0.031 g, 0.010 mmol, 53.9 %. 1 H NMR (500.2 MHz, C 6 D 6, 20 C): δ = 7.23-7.19 (m, 12H, PPN), 7.15-7.13 (m, 6H, PPN), 7.05 (d, 6H, J = 7 Hz, m-ph), 6.99-6.97 (m, 15H, PPN and p-ph), 6.92 (s, 12H, m-mes), 2.35 (s, 18H, p-mes), 2.27 (s, 36H, p-mes) ppm. 13 C{ 1 H} NMR (100.6 MHz, C 6 D 6, 20 C): δ = 139.2, 138.04, 137.0, 136.2, 135.4, 135.2, 134.6, 131.0 (PPN, ipso-c), 129.7 (PPN), 127.7 (PPN), 126.4 (PPN), 121.4, 21.8 (o-mes), 21.6 (p-mes) ppm (the CNR and CO resonances could not be detected, presumably due to coupling to 59 Co (I = 7/2, 100 %). 31 P{ 1 H} (121.4 MHz, C 6 D 6, 20 C): δ = 22.1 (s, PPN) ppm. FTIR (KBr window, C 6 D 6 ): ν CN = 1894 (vs), 1858 (vs) cm 1, ν CO = 1830 cm -1 also, 2965 (vw), 2915 (w), 2851 (w), 1624 (m), 1574 (m), 1510 (w), 1410 (w), 1274 (s), 1258 (s), 1186 (m), 1111 (s), 1033 (s), 852 (w), 755 (w), 725 (w), 692 (w), 639 (w) cm 1. Suitable combustion analysis was not obtained. S1.5. Characterization Data for HCo(CO)(CNAr Mes2 ) 3 (2-H) in THF-d 8 : 5 1 H NMR (499.8 MHz, THF-d 8, 20 C): δ = 7.29 (t, 3H, J = 7 Hz, p-ph), 7.02 (d, 8H, J = 7 Hz, m- Ph), 6.76 (s, 12H, m-mes), 2.22 (s, 12H, p-mes), 1.86 (s, 24H, o-mes), 12.46 (s, 1H, Co-H) ppm. S1.6. Synthesis of Na[Co(CO) 2 CNAr Mes2 ) 2 ] (3-Na): Sodium amalgam (Na(Hg); 0.030 g, 1.3 mmol, 10 equiv, 0.1 wt%) was added to a stirring THF solution of Co 2 (CO) 4 (CNAr Mes2 ) 4 (0.200 g, 0.126 mmol, 10 ml). The resulting mixture was stirred for 1 h then filtered through Celite. The orange filtrate was concentrated to a solid under vacuum. The solid residue was stirred in n-pentane (10 ml) for 20 min then reconcentrated to a solid. This step was repeated two additional times. Thereafter, the yellow product was suspended in n-pentane (2 ml), filtered, then washed with n-pentane S 3

(3 x 1 ml). The bright yellow precipitate was collected and dried in vacuo. Yield: 0.124 g, 0.152 mmol, 60.2 %. Single crystals of Na[Co(CO) 2 (CNAr Mes2 ) 2 ] (3-Na) were obtained by layering n-pentane on top of a concentrated THF solution followed by storage at 40 C. 1 H NMR (499.8 MHz, THF-d 8, 20 C): δ = 7.11 (t, 2H, J = 7.6 Hz, p- Ph), 6.92 (d, 4H, J = 7.6 Hz, m-ph), 6.80 (s, 8H, m-mes), 2.28 (s, 12H, o-mes), 1.97 (s, 24H, p-mes) ppm. : 1 H NMR (400.1 MHz, C 6 D 6, 20 C): δ = 6.95 (s, 8H, m-mes), 6.94 (t, 2H, J = 7 Hz, pph), 6.87 (d, 4H, J = 7 Hz, m-ph), 2.31 (s, 12H, o-mes), 2.12 (s, 24H, p-mes) ppm. 13 C{ 1 H} NMR (100.6 MHz, THF-d 8, 20 C): δ = 216.1 (very broad, CO), 196.5 (very broad, CNR), 137.3, 137.2, 136.8, 136.4, 133.8, 129.8, 128.9, 124.1, 21.5, 21.0 ppm. FTIR (KBr window, THF): ν CN = 1891 (s, br) cm 1, ν CO = 1852 cm 1 also, 1613 (w), 1576 (m), 1415 (m), 847 (w), 749 (w) cm -1. FTIR (KBr Pellet): ν CN = 1918 (vs), 1882 (s) cm -1, ν CO = 1793 (s) cm -1 also, 2976 (vw), 2948 (vw), 2917 (w), 2859 (vw), 2049 (m), 2026 (sh), 1613 (w), 1577 (m), 1460 (m), 1416 (m), 1374 (w), 1049 (m), 1027 (sh), 852 (m), 797 (m), 755 (m), 630 (w), 544 (w) cm -1. Anal. Calcd. For (C 52 H 50 CoN 2 O 2 ) (C 4 H 8 O) 3 : C, 74.40; H, 7.22; N, 2.71, Found: C, 74.55; H, 6.78; N, 3.53. S1.7. Synthesis of PPN[Co(CO) 2 (CNAr Mes2 ) 2 ] (3-PPN): A THF solution of Na[Co(CO) 2 (CNAr Mes2 ) 2 ] (3-Na; 0.060 g, 0.073 mmol, 100 ml) was frozen in cold well. Upon thaw, solid PPNCl (0.042 g, 0.077 mmol, 1.05 equiv) was added. The resulting mixture was then allowed to slowly warm to room temperature and react for 3 h. The reaction mixture was then concentrated to a solid under reduced pressure. The solid was dissolved in C 6 H 6 (20 ml) and filtered through Celite. The filtrate was then concentrated to a yellow solid under reduced pressure. The solid was suspended in a 9:1 n- pentane/c 6 H 6 (10 ml) and filtered. The precipitate was washed with n-pentane (3 x 5 ml). The precipitate was then recrystallized at 40 C by layering pentane on top of a concentrated THF solution. Yield: 0.050 g, 0.038 mmol, 52.0 %. Single crystals of Na[Co(CO) 2 (CNAr Mes2 ) 2 ] (3-PPN) were obtained from a concentrated solution of C 6 H 5 F/Et 2 O/n-pentane (7:2:1 by volume) followed by storage at 40 C. 1 H NMR (500.2 MHz, C 6 D 6, 20 C): δ = 7.28-7.23 (m, 12H, PPN), 7.18-7.16 (m, 6H, PPN), 7.07 (d, 4H, J = 7 Hz, m-ph), 7.05-7.03 (m, 12H, PPN), 7.02 (s, 8H, m-mes), 6.99 (t, 2H, J = 7 Hz, p- Ph), 2.36 (s, 24H, o-mes), 2.35 (s, 12H, p-mes) ppm. 13 C{ 1 H} NMR (100.6 MHz, C 6 D 6, 20 C): δ = 137.7, 136.84, 136.78, 135.7, 134.7, 134.0, 131.1 (PPN, ipso-c), 129.4 (PPN), 127.5 (PPN), 125.5 (PPN), 122.5, 21.7 (o-mes), 21.3 (p-mes) ppm (CNR and CO resonances could not be detected, presumably due to coupling to 59 Co (I = 7/2, 100 %). 31 P{ 1 H} (121.4 MHz, C 6 D 6, 20 C): δ = 22.0 (s, PPN) ppm. FTIR (C 6 D 6 ; KBr Window): ν CN = 1882 (s, br, sh) cm 1, ν CO = 1840 cm -1 also, 3051 (vw), 2945 (w, br), 2920 (w), 2848 (w), 1580 (m), 1480 (m), 1438 (s), 1411 (m), 1286 (vw), 1266 (vw), 1180 (vw), 1113 (m), 1069 (w), 997 (w), 880 (vw), 847 (w), 753 (w), 722 (m), 694 (w) cm -1. FTIR (KBr Pellet): ν CN = 1884 (sh) cm 1, ν CO = 1846 (vs, br) cm -1 also, 3061 (vw), 2956 (w), 2917 (w), 2856 (w), 1996 (w), 1576 (m), 1438 (m), 1416 (m), 1266 (m), 1113 (m), 1030 (vw), 1002 (vw), 853 (w), 750 (w), 725 (m), 695 (w) cm 1. Anal. Calcd. For C 88 H 80 CoN 3 O 2 P 2 : C, 79.31; H, 6.06; N, 3.16. Found: C, 79.40; H, 6.04; N, 3.11. S1.8. Synthesis of HCo(CO) 2 (CNAr Mes2 ) 2 (3-H): A thawing Et 2 O suspension of Na[Co(CO) 2 (CNAr Mes2 ) 2 ] (3-Na; 0.150 g, 0.184 mmol, 10 ml) was combined with a 35 C Et 2 O suspension of 3,5-dimethylbenzoic acid (0.027 g, 0.189 mmol, 1 ml, 1.03 equiv) S 4

and allowed to stir for 2 h while slowly warming to room temperature. Thereafter, the reaction mixture was concentrated to a yellow solid under reduced pressure. The material was dissolved in C 6 H 6 (10 ml) and filtered through Celite. The filtrate was then concentrated to a solid, under reduced pressure, then suspended in n-pentane (10 ml). The suspension was filtered and the resulting pale yellow precipitate washed with n- pentane (3 x 1 ml) and MeCN (3 x 1 ml). Yield: 0.062 g, 0.0781 mmol, 42.4 %. Single crystals of HCo(CO) 2 (CNAr Mes2 ) 2 were obtained by layering n-pentane on top of a concentrated THF solution pre-cooled to 40 C. 1 H NMR (499.8 MHz, C 6 D 6, 20 C): δ = 6.94 (t, 2H, J = 7 Hz, p-ph), 6.90 (s, 8H, m-mes), 6.86 (d, 4H, J = 7 Hz, m-ph), 2.26 (s, 12H, p-mes), 2.02 (s, 24H, o-mes), 12.29 (s, 1H, Co-H) ppm. 1 H NMR (499.8 MHz, THF-d 8, 20 C): δ = 7.11 (t, 2H, J = 7 Hz, p-ph), 7.42 (d, 4H, J = 7 Hz, m-ph), 6.80 (s, 8H, m-mes), 2.78 (s, 12H, p-mes), 1.97 (s, 24H, o-mes), 13.01 (s, 1H, Co-H) ppm. 13 C{ 1 H} NMR (100.6 MHz, C 6 D 6, 20 C): δ = 201.5 (CO), 172.6 (CNR), 138.9, 137.7, 136.1, 135.8, 134.9, 129.4, 128.9, 128.4, 21.3 (p-mes), 20.3 (o-mes) ppm. FTIR (C 6 D 6 ; KBr Window): ν CN = 2140 (m), 2076 (vs) cm 1, ν CoH = 2014 (m, sh) cm 1, ν CO = 1993 (s), 1963 (vs) cm 1 also, 3053 (vw), 2973 (vw), 2920 (m), 2848 (w), 1582 (m, sh), 1455 (s), 1427 (s), 1072 (m) 1036 (m), 880 (vw), 850 (w), 753 (w), 686 (vw) cm 1. Anal. Calcd. For C 52 H 51 CoN 2 O 2 : C, 78.57; H, 6.47; N, 3.52, Found: C, 78.28; H, 6.49; N, 3.53. S1.9. Synthesis of Na[Co(CO) 3 (CNAr Mes2 )] (4-Na): In three separate flasks, sodium amalgam (Na(Hg); 0.047 g, 2.07 mmol, 10 equiv, 0.05 wt %) was added to a stirring THF solution of Co 2 (CO) 6 (CNAr Mes2 ) 2 (0.200 g, 0.207 mmol, 15 ml). The resulting mixtures were then stirred for 1 h, combined and filtered through Celite. The orange filtrate was then concentrated to a solid under vacuum. The solid residue was then stirred in n-pentane (10 ml) for 20 min then reconcentrated to a solid containing roughly a 2:1:1mixture of Na[Co(CO) 3 (CNAr Mes2 )] (4-Na) to Na[Co(CO) 2 (CNAr Mes2 ) 2 ] (3-Na) and Na[Co(CO) 4 ] as assayed by 1 H and 13 C{ 1 H} NMR spectroscopy (see Section S2 for 13 C{ 1 H} NMR spectrum of Na[Co(CO) 4 ]). Bulk separation of the mixture was then achieved by repeated fractional crystallization from a THF/n-pentane mixture at 40 C in the following manner: the mixture was dissolved in a minimal amount of THF and filtered. n-pentane was then layered on top of the filtrate and the mixture was stored at 40 C for 2 d to afford orange crystals of Na[Co(CO) 2 (CNAr Mes2 ) 2 ] (3-Na). The mother liquor was isolated and the crystallization process repeated to afford predominantly yellow crystals of Na[Co(CO) 3 (CNAr Mes2 )] which were collected, dried in vacuo and then recrystallized, again, from a concentrated solution of THF with pentane layered on top at 40 C. Yield: 0.120 g, 0.237 mmol, 57.2 %. 1 H NMR (499.8 MHz, THF-d 8, 20 C): δ = 7.16 (t, 1H, J = 7 Hz, p-ph), 6.98 (d, 2H, J = 7 Hz, m-ph), 6.86 (s, 4H, m-mes), 2.27 (s, 6H, p-mes), 2.06 (s, 12H, o-mes) ppm. 13 C{ 1 H} NMR (100.6 MHz, THF-d 8, 20 C): δ = 137.6, 136.9, 136.5, 136.4, 132.9, 129.7, 128.6, 124.5, 21.2 (p-mes), 20.6 (o-mes) ppm. (CNR and CO resonances could not be detected, presumably due to coupling to 59 Co (I = 7/2, 100 %). FTIR (KBr Pellet): ν CN = 1932 (s) cm 1, ν CO = 1870 (vs), 1846 (s) cm -1 also, 2952 (w), 2919 (w), 2877 (w), 1606 (w), 1580 (m), 1489 (w), 1444 (m), 1415 (m), 1378 (m), 1046 (s), 893 (w), 863 (m), 852 (m), 815 (vw), 803 (m), 786 (w), 756 (m), 562 (s), 545 (s), 525 (s), cm 1. Repeated attempts to obtain a suitable elemental analysis were unsuccessful, presumably due to non-stoichiometric loss of solvents coordinated to alkali-metal cations. S 5

S1.10. Synthesis of PPN[Co(CO) 3 (CNAr Mes2 )] (4-PPN): A THF suspension of PPNCl (0.041 g, 0.071 mmol, 2 ml, 1.2 equiv) was added to a stirring THF solution of Na[Co(CO) 3 (CNAr Mes2 )] (4-Na; 0.030 g, 0.059 mmol, 13 ml) at room temperature. Upon addition, the reaction mixture turned from yellow to orange. The mixture was stirred for 1 h, then filtered through Celite. The filtrate was then concentrated to a solid. The material was then suspended in n-pentane (10 ml) and stirred for 20 min. Thereafter, the reaction mixture was concentrated to a solid. This process was repeated twice. The resulting orange material was then suspended in n-pentane (10 ml) and filtered. The precipitate was washed with a 3:1 (by volume) n-pentane/benzene solution (3 x 5 ml) then a n- pentane solution (4 x 10 ml). Yield: 0.027 g, 0.026 mmol, 37.3%. X-ray diffraction quality crystals were obtained from storage of a saturated fluorobenzene solution at 35 C for 2d. 1 H NMR (500.2 MHz, C 6 D 6, 20 C): δ = 7.32-7.28 (m, 12H, PPN), 7.20-7.16 (m, 6H, PPN), 7.11-7.07 (m, 12H, PPN), 7.04 (d, 2H, J = 2H, m-ph), 7.02 (s, 4H, m- Mes), 7.00 (t, 1H, J = 7 Hz, p-ph), 2.40 (s, 12H, o-mes), 2.24 (s, 6H, p-mes) ppm. 13 C{ 1 H} NMR (100.6 MHz, C 6 D 6, 20 C): δ = 137.6, 137.1, 136.6, 136.3, 133.9, 133.5, 131.1 (PPN, ipso-c), 129.4 (PPN), 127.5 (PPN), 127.1 (PPN), 123.9, 21.5 (p-mes), 21.1 (o-mes) ppm. (CNR and CO resonances could not be detected, presumably due to coupling to 59 Co (I = 7/2, 100 %). 31 P{ 1 H} (121.4 MHz, C 6 D 6, 20 C): δ = 21.9 (s, PPN) ppm. FTIR (KBr Pellet): ν CN = 1901 (s) cm 1, ν CO = 1852 cm -1 also, 3058 (w), 2953 (w), 2915 (w), 2853 (w), 2037 (m), 2007 (m), 1613 (w), 1576 (m), 1482 (w), 1435 (m), 1413 (w), 1299 (sh), 1282 (sh), 1263 (m), 1182 (w), 1160 (w), 1116 (m), 995 (w), 848 (w), 799 (w), 744 (w), 722 (s), 691 (s), 636 (w), 544 (s), 530 (s) 498 (m) cm -1. FTIR (C 6 D 6 ; KBr Window): ν CN = 1894 (vs) cm 1, ν CO = 1858 (vs) cm 1 also, 3060 (vw), 2961 (vw), 2919 (w), 2853 (vw), 2039 (w), 2009 (s), 1615 (w), 1577 (m), 1480 (w), 1455 (w), 1441 (w), 1416 (w), 1301 (w), 1284 (w), 1265 (w), 1184 (w), 1119 (s), 1069 (w), 1056 (w), 1031 (m), 848 (vw), 752 (w), 742 (w), 723 (m), 693 (m), 548 (s), 533 (m) cm 1. Suitable combustion analysis was not obtained. S1.11. Synthesis of HCo(CO) 3 (CNAr Mes2 ) (4-H): A thawing Et 2 O suspension of Na[Co(CO) 3 (CNAr Mes2 )] (4-Na; 0.031 g, 0.062 mmol, 10 ml) was combined with a 35 C Et 2 O suspension of 3,5-dimethylbenzoic acid (0.010 g, 0.067 mmol, 2 ml, 1.08 equiv) and allowed to stir for 10 min while slowly warming to room temperature. The reaction mixture was then filtered through Celite and concentrated to a solid under reduced pressure. Yield: 0.022 g, 0.046 mmol, 74.2 %. Note: HCo(CO) 3 (CNAr Mes2 ) decomposes at room temperature over the course of ~30 min to afford a complex mixture of HCo(CO) 2 (CNAr Mes2 ) 2, Co 2 (CO) 6 (CNAr Mes2 ) 2 and free CNAr Mes2. 1 H NMR (500.2 MHz, C 6 D 6, 20 C): δ = 6.94 (t, 1H J = 7 Hz, p-ph), 6.89 (s, 4H, m-mes), 6.82 (d, 2H, J = 7 Hz, m-ph), 2.19 (s, 6H, o-mes), 2.02 (s, 12H, p-mes), 12.21 (s, 1H, Co-H) ppm. 13 C{ 1 H} NMR (100.6 MHz, C 6 D 6, 20 C): δ = 198.5 (CO), 165.4 (CNR), 139.3, 138.4, 135.6, 134.0, 130.9, 129.4, 128.9, 128.4, 21.1 (p-mes), 20.2 (o-mes) ppm. FTIR (C 6 D 6 ; KBr Window): ν CN = 2161 (m) cm 1, ν CoH = 2024 (m, sh) cm 1, ν CO = 2059 (m), 1993 (vs) cm 1 also, 2956 (m), 2920 (m), 1613 (m), 1579 (m), 1560 (m), 1460 (m), 1414 (m), 1116 (m), 1066 (m), 1036 (m), 849 (m), 758 (m), 694 (m) cm 1. As a result of its thermal instability, suitable elemental analysis was not obtained. S 6

S2. 13 C{ 1 H} NMR Chemical Shift Data for the Isocyanide Carbons in Na[Co(CNAr Mes2 ) 4 ] (1-Na) and the Carbonyl Carbons in K[Co(CO) 4 ]. S2.1 13 C{ 1 H} NMR Spectrum of Na[Co(CNAr Mes2 ) 4 ] (1-Na). 13 C NMR chemical shifts of isocyanide and carbonyl resonances in [CoL 4 ] anions are often challenging to detect due to coupling to 59 Co (I = 7/2, 100 %). This coupling often induces rapid relaxation and severe line broadening. It was, however, possible to detect the 13 C{ 1 H} NMR (125.7 MHz, C 6 D 6 ) chemical shift for Na[Co(CNAr Mes2 ) 4 ]. This was located using isotopically enriched 13 CNAr Mes2 using a 500 MHz Varian Xsens spectrometer equipped with a high sensitivity cryoprobe. The isocyanide resonance is centered at 198.0 ppm (C 6 D 6 ) with a half-peak line width of 502 Hz (Figure S2.1). Figure S2.1 Isocyanide Na[Co( 13 CNAr Mes2 ) 4 ]. 13 C{ 1 H} NMR (125.7 MHz, C 6 D 6 ) chemical shift for S1.2 13 C{ 1 H} NMR Spectrum for K[Co(CO) 4 ] The 13 C{ 1 H} NMR (125.7 MHz, THF) chemical shift for K[Co(CO) 4 ] was obtained from a concentrated THF solution (>5 M) of K[Co(CO) 4 ] that was prepared by reductive cleavage of Co 2 (CO) 8 using the method of Ellis and co-workers. 8 With prolonged scanning, an octet ( 1 J C-Co = 277 Hz; 59 Co I = 7/2, 100 %) centered at 215.5 ppm was detected. The observed coupling constant agrees well with the value previously reported for [Co(CO) 4 ] ( 1 J C-Co = 287 Hz), which was measured from 13 C satellites in solid-state 59 Co spectra. 2 The spectrum in Figure S2.2 appears to be the first example where the expected octet in [Co(CO) 4 ] is well resolved. This data was used to identify the [Co(CO) 4 ] anion during the synthesis of 4-Na. S 7

Figure S2.2. Carbonyl 13 C{ 1 H} NMR (125.7 MHz, THF-d8) for [Co(CO) 4 ]. 1 J Co-C = 277 Hz. S3. Crystallographic Structure Determinations. S3.1. General. Single-crystal X-ray structure determinations were carried out using Bruker P4, Platform or Kappa X-ray diffractometers equipped with Mo or Cu radiation sources (sealed tube or rotating anode), low-temperature cryostats and CCD detectors (Bruker APEX or Bruker APEX II) at the UCSD Small-Molecule Crystallography Facility. All structures were solved by direct methods using SHELXS and refined by fullmatrix least-squares procedures utilizing SHELXL within the Olex2 small-molecule solution, refinement and analysis software. 9,10 For cases of significant solvent disorder SQUEEZE was implemented. 11 S3.2. Structure Solution Details. For Na[Co(CO)(CNAr Mes2 ) 3 ] Et 2 O (2-Na Et 2 O), the Et 2 O solvent molecule of co-crystallization and the sodium cation, exhibited two-site positional disorder. The occupancies of the two components of the positional order were fully modeled and anisotropically refined. For Na(THF) 3 [Co(CO) 2 (CNAr Mes2 ) 2 ] (3- Na(THF) 3 ), two-site positional disorder in the THF molecule coordinated to the Na + cation was fully modeled and anisotropically refined. S 8

Figure S3.1. Molecular structure of [Na(Et 2 O)][Co(CO)(CNAr Mes2 ) 3 ] (Et 2 O) (2-Na(Et 2 O)). Selected distances (Å) and angles ( ): Co1-C1 = 1.777(4), Co1-C2 = 1.796(4), Co1-C3 = 1.786(4), C1-C4 = 1.779(4), C1-Co1-C2 = 101.0(2), C1-Co1-C3 = 106.5(2), C1-Co1-C4 = 107.4(2), C2-Co1-C3 = 112.6(2), C2-Co1-C4 = 112.8(2), C3-Co1-C4 = 115.2(2). Figure S3.2. Molecular structure of HCo(CO) 2 (CNAr Mes2 ) (3-H). Selected distances (Å) and angles ( ): Co1-C1 = 1.846(2), Co1-C2 = 1.782(2), Co1-C3 = 1.774(2), Co1-C4 = 1.833(2), Co1- H1 = 1.448, C1-Co1-H1 = 177(1), C1-Co1-C2 = 103.13(9), C1-Co1-C3 = 98.31(9), C1-Co1-C4 = 96.96(8), C2-Co1-C3 = 118.6(1), C3-Co1-C4 = 118.55(9), C4-Co1-C2 = 114.92(9). S 9

Figure S3.3. Molecular structure of Na(THF) 3 [Co(CO) 2 (CNAr Mes2 ) 2 ] (3-Na). Selected distances (Å) and angles ( ): C1-Co1 = 1.803(2), C2-Co1 = 1.796(2), C3-Co1 = 1.764(2), C4-Co1 = 1.761(2), C1-Co1-C2 = 108.06(7), C1-Co1-C3 = 103.47(8), C1-Co1-C4 = 102.76(8), C2-Co1-C3 = 109.13(8), C2-Co1-C4 = 111.01(8), C3-Co1-C4 = 121.25(9). Figure S3.4. Molecular structure of PPN[Co(CO) 2 (CNAr Mes2 )] (Et 2 O) (3-PPN (Et 2 O)). Selected distances (Å) and angles ( ): Co1-C1 = 1.819(7), Co1-C2 = 1.820(8), Co1-C3 = 1.745(8), Co1-C4 = 1.737(8), P1-N3 = 1.590(6), P2-N3 = 1.575(7), C1-Co1-C2 = 111.0(3), C1-Co1-C3 = 111.3(3), C1-Co1-C4 = 108.7(3), C2-Co1-C3 = 108.2(3), C2-Co1-C4 = 109.5(3), C3-Co1-C4 = 108.0(4). S 10

Figure S3.5. Molecular structure of Na(Et 2 O)[Co(CO) 3 (CNAr Mes2 )] (4-Na(Et 2 O). Selected distances (Å) and angles ( ): C1-Co1 = 1.816(4), C2-Co1 = 1.777(4), C3-Co1 = 1.772(4), C4-Co1 = 1.747(4), C1-Co1-C2 = 106.3(2), C1-Co1-C3 = 116.6(2), C1-Co1-C4 = 106.9(2), C2-Co1-C3 = 107.9(2), C2-Co1-C4 = 110.6(2), C3-Co1-C4 = 108.5(2). Figure S3.6. Molecular structure of PPN[Co(CO) 3 (CNAr Mes2 )] (C 6 H 5 F) (4-PPN (C 6 H 5 F)) solvent molecule of crystallization omitted. Selected distances (Å) and angles ( ): Co1-C1 = 1.806(2), Co1-C2 = 1.749(2) Co1-C3 = 1.747(2), 1.747(2), P1-N2 = 1.577(1), 1.582(2), C1-Co1-C2 = 116.15(10), C1-Co1-C3=108.39(9), C1-Co1-C4 = 108.2(10), C3-Co1-C4 = 112.32(10). S 11

Table S3.1. Crystallographic data collection and refinement information. [Na(Et 2 O)][Co(CO) (CNAr Mes2 ) 3 ] (2-Na(Et 2 O)) HCo(CO) 2 (CNAr Mes2 ) 2 (3-H) [Na(THF) 3 ] [Co(CO) 2 (CNAr Mes2 ) 2 ] (3-Na(THF) 3 ) Formula C 80 H 85 CoN 3 NaO 2 C 52 H 51 CoN 2 O 2 C 64 H 74 CoN 2 NaO 5 Crystal System Monoclinic Monoclinic Triclinic Space Group P2 1 /c P2 1 /n P1 a, Å 13.7522(7) 15.658(3) 12.5478(19) b, Å 18.5133(9) 13.440(3) 13.359(2) c, Å 26.4525(14) 20.941(4) 18.742(3) α, deg 90 90 75.175(9) β, deg 100.868(2) 104.796(8) 78.801(7) γ, deg 90 90 68.362(7) V, Å 3 6614.0(6) 4260.8 2805.5(8) Z 4 4 2 Radiation (λ, Å) Mo-Kα, 0.71073 Mo-Kα, 0.71073 Mo-Kα, 0.71073 ρ (calcd.), g/cm 3 1.208 1.239 1.223 μ, mm -1 0.316 0.445 0.364 Temp, K 100(2) 100(2) 100(2) θ max, deg 25.456 27.612 28.491 data/restraints/ parameters 12186/0/825 9736/0/529 13999/30/695 R 1 0.0517 0.0379 0.0394 wr 2 0.1023 0.0921 0.0897 GOF 1.000 1.027 1.029 S 12

Table S3.1 (con t). Crystallographic data collection and refinement information. PPN [Co(CO) 2 (CNAr Mes2 ) 2 ] (Et 2 O) (3-PPN (Et 2 O)) Na(Et 2 O) [Co(CO) 3 (CNAr Mes2 )] (4-Na(Et 2 O)) PPN [Co(CO) 3 (CNAr Mes2 )] (C 6 H 5 F) (4-PPN ( C 6 H 5 F)) Formula C 92 H 90 CoN 3 O 3 P 2 C 64 H 70 Co 2 N 2 Na 2 O 8 C 70 H 60 CoFN 2 O 3 P 2 Crystal System Monoclinic Triclinic Triclinic Space Group C2/c P1 P1 a, Å 24.378(4) 11.2666(14) 12.5064(7) b, Å 16.575(3) 12.1163(14) 15.3040(8) c, Å 38.687(6) 12.1953(12) 16.2449(9) α, deg 90 104.217(8) 106.515(2) β, deg 98.267(2) 111.985(8) 96.958(3) γ, deg 90 90.688(8) 96.827(2) V, Å 3 15469(4) 1486.4(3) 2920.4(3) Z 8 1 2 Radiation (λ, Å) Mo-Kα, 0.71073 Mo-Kα, 0.71073 Mo-Kα, 0.71073 ρ (calcd.), g/cm 3 1.208 1.295 1.270 μ, mm -1 0.316 0.628 0.401 Temp, K 100(2) 100(2) 100(2) θ max, deg 25.393 26.113 25.506 data/restraints/ parameters 14088/36/894 5765/0/360 10440/0/718 R 1 0.1075 0.0471 0.0367 wr 2 0.2048 0.0969 0.0781 GOF 1.146 1.016 1.018 S 13

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