Supplementary information

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1 Supplementary information METHODS General: All reactions were carried out in flame-dried glassware under Ar. All solvents were purified by distillation over the drying agents indicated and were transferred under Ar: THF, Et 2 O (Mg-anthracene), CH 2 Cl 2 (P 4 O 10 ), MeCN, Et 3 N (CaH 2 ), MeOH (Mg), hexane, toluene (Na/K). Flash chromatography: Merck silica gel 60 ( mesh). IR: Nicolet FT-7199 spectrometer, wavenumbers in cm 1. MS (EI): Finnigan MAT 8200 (70 ev), ESIMS: Finnigan MAT 95, accurate mass determinations: Bruker APEX III FT-MS (7 T magnet). NMR: Spectra were recorded on a Bruker DPX 300 or AV 400 spectrometer in the solvents indicated; 1 H and 13 C chemical shifts ( ) are given in ppm relative to TMS, 19 F chemical shifts are reported in ppm relative to CF 3 COOH; coupling constants (J) are given in Hz. The solvent signals were used as references and the chemical shifts converted to the TMS scale. Melting points: Büchi melting point apparatus B-540 (corrected). Elemental analyses: H. Kolbe, Mülheim/Ruhr. All commercially available compounds (Acros, Fluka, Lancaster, Aldrich) were used as received unless stated otherwise. Tetrakis(dimetylamino)allene (6) 1, N-phenyl(triphenylphosphoranylidene)ethenimine (9), 2 (2,2-diethoxyvinylidene)-triphenylphosphorane (14) 3 and (dimethylaminocarbonylmethylene) triphenylphosphorane (18) 4 were prepared according to the cited literature procedures. (2-Dimethylamino-2-ethoxyvinylidene)-triphenylphosphonium tetrafluoroborate: (Dimethylaminocarbonylmethylene) triphenylphosphorane (2.89 g, 8.3 mmol) was added in portions OEt Ph 3 P to a solution of triethyloxonium tetrafluoroborate (1.62 g, 8.5 mmol) in N dichloromethane (20 ml) at 0 ºC. After stirring for 1 h, the solvent was evaporated BF 4 and the residue purified by flash chromatography (5% MeOH in CH 2 Cl 2 ) to give the desired product as a white solid (2.47 g, 79%). 1 H NMR (300 MHz, CDCl 3 ): = (m, 15H), 3.77 (d, J = 14.7 Hz,1H), 3,36 (c, J = 7.2 Hz, 2H), 3.05 (s, 6H), 0.61 (t, J = 7.2 Hz, 3H) ppm; 13 C NMR (75 MHz, CDCl 3 ): = (d, J = 5.1 Hz), (d, J = 2.9 Hz), (d, J = 10.4 Hz), (d, J = 12.8 Hz), (d, J = 92.6 Hz) ppm; 31 P NMR (121 MHz, CDCl 3 ): = 17.3 ppm; IR (neat): = 1738, 1580, 1436, 1379, 1108, 1033, 996, 875, 719, 691 cm -1 ; HRMS: calcd. for C 24 H 27 NOP: ; found Janousek, Z. & Viehe, H.G. Condensation of dichloromethylenedimethyl-ammonium chloride ("phosgene immonium chloride") with N,N-dialkylcarboxamides. Angew. Chem. Int. Ed. Engl. 10, (1971). Bestmann, H.J. & Schmid, G. Eine neue Synthesemöglichkeit für N-substituierte (Triphenylphosphoranyliden)ketenimine und das (Triphenylphosphoranyliden)thioketen. Chem. Ber. 113, (1980). Bestmann, H.-J., Saalfrank, R.W. & Snyder, J.P. Darstellung des (2,2- Diäthoxyvinyliden)triphenylphosphorans und seine Umsetzung mit Fluorenon. Chem. Ber. 106, (1973). Vicente, J., Chicote, M.-T., Lagunas, M.-C. & Jones, P.G. Synthesis and structural characterization of gold-(i), -(III) and silver(i) complexes of the ylide ligand Ph 3 P=CHC(O)NMe 2. Crystal structure of [(AuPPh 3 ) 2 {µ-c(pph3)c(o)nme 2 }]ClO 4. J. Chem. Soc. Dalton Trans (1991). nature chemistry 1

2 ; elemental analysis calcd.(%) for C 24 H 27 BF 4 ONP: C 62.22, H 5.87, N 3.02; found C 62.15, H 5.82, N Complex 8: (Me 2 S)AuCl (88.4 mg, 0.3 mmol.) was added to a solution of compound 3 (90.7 mg, 0.3 mmol) in THF (2 ml) at room temperature. After stirring for 1 h, the solvents were evaporated, and the residue washed with Et 2 O (2 x 2 ml) to afford the title compound (104 mg, 65%) as an off white solid. 1 H NMR (400 MHz, CD 2 Cl 2 ): = (m, 9H), (m, 6H) ppm; 13 C NMR (100 MHz, CD 2 Cl 2 ): = (d, J = 19.2 Hz), (d, J = 2.9 Hz), (d, J = 11 Hz), (d, J = 12.8 Hz), (d, J = 94 Hz) ppm; 31 P NMR (162 MHz, CD 2 Cl 2 ): 22.2 ppm; IR (neat): = 2096, 1481, 1435, 1108, 996, 738, 690, 663 cm -1. Complex 10: (Me 2 S)AuCl (58.9 mg, 0.2 mmol.) was added in one portion to a solution of N-phenyl- (triphenylphosphoranylidene)ethenimine 9 (75.4 mg, 0.2 mmol) 2 in THF (2 ml) at room temperature. After stirring for 1 h, the solvents were evaporated, and the residue was washed with pentane (2 x 2 ml) to afford the desired compound (106 mg, 87%) as a pale brown solid. 1 H NMR (300 MHz, CD 2 Cl 2 ): = (m, 9H), (m, 6H), (m, 2H), 7.22 (t, J = 13.8 Hz, 1H), 6.96 (d, J = 7.2 Hz, 2H) ppm; 13 C NMR (75 MHz, CD 2 Cl 2 ): = (d, J = 10.0 Hz), (d, J = 6.7 Hz), (d, J = 2.9 Hz), (d, J = 10.0 Hz), (d, J = 12.8 Hz), 127.4, (d, J = 91.3 Hz), (d, J = 2.4 Hz), 33.4 (d, J = 89 Hz) ppm; 31 P NMR (121 MHz, CD 2 Cl 2 ): 22.7 ppm; IR (neat): = 2001, 1738, 1434, 1366, 1229, 1217, 1107, 770, 719 cm -1 ; HRMS: calcd. for C 52 H 40 N 2 AuP 2 : ; found Compound 11: Triphenylphosphine (1.42 g, 5.4 mmol) was added in one portion to a stirred solution of 9-bromomethylene-9H-fluorene (1.40 g, 5.4 mmol) 5 in toluene (30 ml) and the resulting mixture was stirred under reflux overnight. After reaching room temperature, the yellow precipitate was filtered off and washed with toluene. For anion exchange, a solution of the crude bromide salt in water/meoh (7:3, v/v) was poured into a chilled saturated aqueous solution of sodium tetrafluoroborate, causing the precipitation of the desired product as a pale yellow powder (1.21 g, 48%). 1 H NMR (400 MHz, CD 2 Cl 2 ): = (m, 4H), (m, 4H), (m, 8H), 7.55 (d, J = 7.4 Hz, 1H), 7.54 (d, J = 7.4 Hz, 1H), 7.44 (dt, J = 7.5, 0.8 Hz, 1H), 7.30 (dt, J = 7.6, 1.0 Hz, 1H), 7.24 (dt, J = 7.4, 0.8 Hz, 1H), 6.84 (d, J = 14.0, 1H), 6.61 (dt, J = 6.8, 1.1 Hz, 1H), 6.54 (d, J = 7.9 Hz, 1H) ppm; 13 C NMR (100 MHz, CD 2 Cl 2 ): = 162.4, 145.0, 141.8, (d, J = 18.6 Hz), (d, J = 2.3 Hz), (d, J = 11.0 Hz), 134.0, (d, J = 5.3 Hz), (d, J = 13.6 Hz), 129.5, 128.4, 128.2, 123.8, 121.8, 121.2, 119.9, 119.0, 98.9 (d, J = 91.5 Hz) ppm; 31 P NMR (162 MHz, CD 2 Cl 2 ): 15.4 ppm; IR (neat): = 1574, 1438, 1106, 1049, 996, 840, 786, 741, 726, 688 cm -1 ; HRMS: calcd. for C 32 H 24 P: ; found ; elemental analysis calcd.(%) for C 32 H 24 BF 4 P: C 73.03, H 4.60; found C 72.67, H Compound 12: KHMDS (20 mg, 0.1 mmol) was added to a solution of compound 11 (52.6 mg, 0.1 mol) in THF-d 8 (1 ml) at 78 C. After stirring for 5 min, the brown solution was transfered to an NMR tube and characterized at this temperature. 1 H NMR (300 MHz, THF-d 8 ): = 7.82 (d, J = 8.2 Hz, 3H), 7.79 (d, 5 Paul, G.C. & Gajewski, J.J. Unexpected coupling reaction of 9-lithiobromomethylene-9Hfluorene with 6,6-dicyclopropylfulvene. Synthesis (1997). nature chemistry 2

3 J = 7.7 Hz, 3H), 7.68 (d, J = 7.6 Hz, 2H), (m, 9H), 7.20 (t, J = 7.2 Hz, 2H), 7.04 (d, J = 7.5 Hz, 2H), 6.88 (t, J = 7.5 Hz, 2H ppm; 13 C NMR (75 MHz, THF-d 8 ): = (d, J = 7.4 Hz), (d, J = 2.7 Hz), 139.4, (d, J = 29.9 Hz), (d, J = 17.9 Hz), (d, J = 9.5 Hz), 130.2, (bs), 129.1, (d, J = 11.5 Hz), 126.8, 126.7, 126.2, 125.4, 124.6, 120.5, ppm; 31 P NMR (121 MHz, THF-d 6 ): 3.8 ppm. Complex 13: (Me 2 S)AuCl (88.4 mg, 0.3 mmol) was added to a suspension of compound 11 (157.8 mg, 0.3 mmol) and KHMDS (60 mg, 0.3 mmol) in THF (3 ml) at 78 C. After stirring for 1 h, the mixture was slowly warmed to room temperature and stirring was continued for 2 h. The solvents were evaporated, the residue was extracted with dichloromethane (3 ml), and the product crystallized by slowly diffusing pentane into the dichloromethane phase (83 mg, 41%). 1 H NMR (400 MHz, CD 2 Cl 2 ): = 9.77 (d, J = 8.1 Hz, 1H), (m, 6H), (m, 3H), 7.55 (d, J = 7.7 Hz, 1H), (m, 7H), 7.38 (dt, J = 7.5, 0.7 Hz, 1H), 7.25 (dt, J = 7.8, 1.3 Hz, 1H), 6.98 (t, J = 7.6 Hz, 1H), 6.53 (d, J = 7.8, 1H), 6.32 (dt, J = 7.7, 1.1 Hz, 1H) ppm; 13 C NMR (150 MHz, CD 2 Cl 2 ): = 159.9, 142.3, (d, J = 29.2 Hz), 141.0, (d, J = 11.3 Hz), (d, J = 26.6 Hz), (d, J = 9.2 Hz), (d, J = 3.1 Hz), 130.3, (d, J = 12.0 Hz), 129.4, 128.2, 128.1, (d, J = 85.4 Hz), 125.5, 124.5, 119.8, 119.1(d, J = 1.4 Hz) ppm; 31 P NMR (162 MHz, CD 2 Cl 2 ): 22.9 ppm; IR (neat): = 1481, 1433, 1270, 1104, 996, 828, 780, 729, 710, 687 cm -1 ; HRMS: calcd. for C 32 H 23 PAuClNa: ; found ; elemental analysis calcd.(%) for C 32 H 23 PAuCl. CH 2 Cl 2 : C 52.44, H 3.33; found C 52.88, H (2,2-Diethoxyvinylidene)-triphenylphosphorane (14). 3 1 H NMR (400 MHz, C 6 D 6 ): = (m, 6H), (m, 9H), 4.10 (c, J = 7.1 Hz, 4H), 0,81 (t, J = 7.1 Hz, 6H) ppm; 13 C NMR (100 MHz, C 6 D 6 ): = (d, J = 7.7 Hz), (d, J = 85.5 Hz), (d, J = 9.3 Hz), (d, J = 2.5 Hz), (d, J = 11.2 Hz), 69.3 (d, J = 9.3 Hz), 61.8 (d, J = 4.2 Hz), 13.5 ppm; 31 P NMR (162 MHz, C 6 D 6 ): = 4.1 ppm; IR (neat): = 2975, 1600, 1569, 1475, 1436, 1168, 1095, 1082, 1048, 1013, 997, 912, 890, 758, 743, 690 cm -1. Complex 15: [RhCl(CO) 2 ] 2 (77.7 mg, 0.2 mmol.) was added to a solution of compound 14 (150.4 mg, 0.4 mmol) in THF (2 ml) and the resulting mixture was stirred for 10 min. The precipitate was filtered off and dried in vacuo to afford complex 15 (143.7 mg, 63%) as a very air-sensitive orange solid. 1 H NMR (300 MHz, CD 2 Cl 2 ): = (m, 6H), (m, 9H), 5.13 (bs, 2H), 3.11 (bs, 2H), 0.93 (t, J = 6.9 Hz, 3H), 0.02 (t, J = 6.9 Hz, 3H) ppm; 31 P NMR (121 MHz, CD 2 Cl 2 ): 25.1 ppm; IR (neat): = 2048, 1965, 1531, 1435, 1219, 1102, 1028, 745, 713, 688 cm -1. Complex 16: (Me 2 S)AuCl (88.4 mg, 0.3 mmol.) was added to a solution of (2,2-diethoxyvinylidene)- triphenylphosphorane 14 (112.8 mg, 0.3 mmol) in THF (2 ml) and the resulting mixture was stirred for 1 h. For work up, the solvents were evaporated and the residue was washed with pentane (2 x 2 ml) to afford the complex 16 (159 mg, 87%) as an off white solid. 1 H NMR (400 MHz, CD 2 Cl 2 ): = (m, 6H), (m, 3H), (m, 6H), 4.98 (c, J = 7.0 Hz, 2H), 3.61 (c, J = 7.1 Hz, 2H), 1.40 (t, J = 7.1 Hz, 3H), 0.49 (t, J = 7.0 Hz, 3H) ppm; 13 C NMR (100 MHz, CD 2 Cl 2 ): = (d, J = 9.3 Hz), nature chemistry 3

4 134.3 (d, J = 9.0 Hz), (d, J = 2.4 Hz), (d, J = 12.0 Hz), (d, J = 89.1 Hz), 68.5, 64.3, 15.4, 14.2 ppm; 31 P NMR (162 MHz, CD 2 Cl 2 ): 26.7 ppm; IR (neat): = 1739, 1415, 1379, 1328, 1302, 1098, 748, 714. cm -1. MS (ESI) calcd. for C 48 H 50 O 4 AuP 2 : ; found ; Elemental analysis calcd.(%) for C 24 H 25 AuClO 2 P: C 47.34, H 4.14; found C 47.11, H Complex 17: (Me 2 S)AuCl (176.8 mg, 0.6 mmol.) was added to a solution of compound 14 (112.8 mg, 0.3 mmol) in THF (2 ml) at room temperature. After 1 h, the solvents were evaporated and the residue was washed with ether (2 x 2 ml) to afford the diaurated complex 17 (225 mg, 89%) as a white solid. 1 H NMR (400 MHz, CD 2 Cl 2 ): = 8.03 (bs, 6H), (m, 9H), 5.52 (bc, J = 6.4 Hz), 4.16 (bc, J = 6.9 Hz, 2H), 1.57 (bt, J = 6.5 Hz, 3H), 0.69 (bt, J = 6.7 Hz, 3H) ppm; 13 C NMR (100 MHz, CD 2 Cl 2 ): = 180.7, (d, J = 9.1 Hz), (d, J = 2.8 Hz), (d, J = 12.3 Hz), 74.4, 68.6, 14.3, 13.0 ppm (the quaternary carbons from the phosphine as well as the diaurated carbon could not be detected); 31 P NMR (162 MHz, CD 2 Cl 2 ): 28.1 ppm; IR (neat): = 2971, 1738, 1531, 1435, 1352, 1227, 1103, 1084, 1032, 752, 710, 690. cm -1 ; elemental analysis calcd.(%) for C 24 H 25 Au 2 Cl 2 O 2 P: C 34.26, H 3.00; found C 34.44, H Compound 18: (2-Dimethylamino-2-ethoxyvinylidene)-triphenylphosphonium tetrafluoroborate (7.1 g, 15.3 mmol) and NaNH 2 (900 mg, 23 mmol) were placed in a three necked flask equipped with a dry ice cooled reflux condenser and a stirring bar. The system was cooled to 78 ºC before dry ammonia was condensed (approx. 30 ml) into the flask. After stirring for 1 h under ammonia reflux, the cooling of the reflux condenser was stopped, allowing the ammonia to gently evaporate while the reaction mixture was allowed to reach room temperature. The remaining syrup was extracted with toluene (2 x 20 ml) and the extracts concentrated to a volume of ca. 10 ml. Addition of pentane (20 ml) caused the precipitation of the title compound as an air- and moisture sensitive yellow solid (4.83 g, 84%). 1 H NMR (300 MHz, C 6 D 6 ): = (m, 6H), (m, 9H), 4.10 (c, J = 7.2 Hz, 2H), 2.87 (s, 6H), 1.08 (t, J = 7.2 Hz, 3H) ppm; 13 C NMR (75 MHz, C 6 D 6 ): = (d, J = 28.7 Hz), (d, J = 90.1 Hz), (d, J = 9.6 Hz), (d, J = 3.0 Hz), (d, J = 9.6 Hz), 72.9 (d, J = 141 Hz), 64.0, 40.5, 15.5 ppm; 31 P NMR (121 MHz, C 6 D 6 ): ppm; IR (neat): = 2970, 2930, 1737, 1644, 1581, 1432, 1358, 1136, 1121, 1093, 1063, 667 cm -1 ; HRMS: calcd. for C 24 H 27 NOP: ; found Complex 19: (Me 2 S)AuCl (58.9 mg, 0.2 mmol.) was added to a solution of (2-ethoxy-2- dimethylaminovinylidene)-triphenylphosphorane (92.6 mg, 0.2 mmol) in THF (2 ml) at room temperature. After stirring for 1 h, the solvents were evaporated and the residue was washed with pentane (2 x 2 ml) to afford the title compound (103 mg, 74%) as a pale grey solid. 1 H NMR (300 MHz, CD 2 Cl 2 ): = (m, 6H), (m, 3H), (m, 6H), 3.09 (s, 6H), 3.04 (c, J = 7.2 Hz, 2H), 0.46 (t, J = 7.2 Hz, 3H) ppm; 13 C NMR (100 MHz, CD 2 Cl 2 ): = 171.2, (d, J = 9.0 Hz), (d, J = 2.7 Hz), (d, J = 87.5 Hz), (d, J = 11.9 Hz), 66.0, 42.7, 14.3 ppm; 31 P NMR (121 MHz, CD 2 Cl 2 ): 22.2 ppm; elemental analysis calcd.(%) for C 24 H 26 AuClNOP: C 47.42, H 4.31, N 2.30; found C 47.70, H 4.29, N nature chemistry 4

5 Complex 20: GaCl 3 (83.3 mg, 0.47 mmol.) was added to a solution of compound 14 (178 mg, 0.47 mmol) in THF (2 ml) at room temperature and the resulting mixture was stirred for 1 h. The resulting precipitate was filtered off and washed with ether (2 x 2 ml) to afford complex 20 (238 mg, 91%) as a pale brown solid. 1 H NMR (400 MHz, CD 2 Cl 2 ): = (m, 6H), (m, 3H), (m, 6H), 4.68 (bs, 2H), 3.54 (bs, 2H), 1.40 (bs, 3H), 0.51 (bs, 3H) ppm; 13 C NMR (75 MHz, CD 2 Cl 2 ): = 174.1, (d, J = 9.7 Hz), (d, J = 3.2 Hz), (d, J = 12.7 Hz), (d, J = 91 Hz), 72.0, 64.8, 61.5 (d, J = 58 Hz), 14.0, 12.7 ppm; 31 P NMR (162 MHz, CD 2 Cl 2 ): 22.8 ppm; IR (neat): = 1548, 1508, 1435, 1254, 1103, 946, 758, 716. cm -1. MS (EI): 517 (8), 348 (46), 331 (30), 303 (100), 271 (72), 243 (68), 183 (70), 165 (44), 152 (23), 108 (18), 77 (21); elemental analysis calcd.(%) for C 24 H 25 Cl 3 O 2 PGa: C 52.17, H 4.56; found C 51.12, H Complex 21: GaCl 3 (124.8 mg, 0.71 mmol.) was added to a solution of compound 18 (266 mg, 0.71 mmol) in THF (2 ml) at room temperature. After stirring for 1 h, the precipitate was filtered off and washed with ether (2 x 2 ml). Recrystallization from CH 2 Cl 2 /ether afforded the title compound (242 mg, 62%) in form of transparent prisms. 1 H NMR (400 MHz, CD 2 Cl 2 ): = (m, 6H), (m, 9H), 3.21 (s, 6H), 3.44 (c, J = 7.0 Hz, 2H), 0.63 (t, J = 7.1 Hz, 3H) ppm; 13 C NMR (75 MHz, CD 2 Cl 2 ): = 173.3, (d, J = 9.2 Hz), (d, J = 4.4 Hz), (d, J = 12.2 Hz), (d, J = 88 Hz), 67.7, 42.9, 14.4 ppm; 31 P NMR (162 MHz, CD 2 Cl 2 ): 21.3 ppm; IR (neat): = 1548, 1424, 1338, 1182, 1101, 1047, 873, 752, 713 cm -1. MS (EI): 551 (3), 516 (4), 405 (36), 375 (22), 330 (41), 303 (41), 279 (100), 262 (24), 183 (37), 168 (57), 108 (15); Elemental analysis calcd.(%) for C 24 H 26 GaCl 3 ONP: C 52.27, H 4.75, N 2.54; found C 52.32, H 4.78, N nature chemistry 5

6 Computational Studies Density functional theory (DFT) calculations were carried out using Turbomole The BP86 functional 7,8 was employed in combination with the def2-tzvp basis set. 9 The resolution-of-identity (RI) approximation was applied in conjunction with the appropriate auxiliary basis sets to speed up the calculations. 10,11,12 All geometries were fully optimized without symmetry constraints. The resulting structures were confirmed to be minima by force constant analysis. The rigid-rotor harmonic-oscillator approximation was used to compute the zero-point vibrational energies as well as the thermal and entropic corrections needed to determine the reaction enthalpies H and free enthalpies G at 300 K (Table 1). Natural bond order (NBO) analysis 13 was carried out using Gaussian BP86/def2-TZVP results for selected orbital energies, partial charges, and optimised geometries are presented in Tables 2-4 and Figures Ahlrichs, R., Bär, M., Häser, M., Horn, H. & Kölmel, C. Electronic structure calculations on workstation computers: The program system TURBOMOLE. Chem.l Phys. Lett. 162, (1989). Becke, A.D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A 38, (1988). Perdew, J.P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B 33, (1986). Weigend, F. & Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 7, (2005). Eichkorn, K., Treutler, O., Öhm, H., Häser, M. & Ahlrichs, R. Auxiliary basis sets to approximate Coulomb potentials (Erratum of Chem. Phys. Letters 240 (1995) 283). Chem. Phys. Lett. 242, (1995). Eichkorn, K., Weigend, F., Treutler, O. & Ahlrichs, R. Auxiliary basis sets for main row atoms and transition metals and their use to approximate Coulomb potentials. Theor. Chem. Acc. 97, (1997). Weigend, F. A fully direct RI-HF algorithm: Implementation, optimised auxiliary basis sets, demonstration of accuracy and efficiency. Phys. Chem. Chem. Phys. 4, (2002). Glendening, E.D., Reed, A.E., Carpenter, J.E. & Weinhold, F. NBO Version 3.1. Frisch, M.J. et. al. Gaussian 03, Revision D.01, Gaussian Inc., Wallingford CT, nature chemistry 6

7 Table 2 Orbital energies (ev) in the free internal ligands L 2, the complexes A = L 1 C L 2, and the mono-aurated complexes A(AuCl) (BP86(RI)/def2-TZVP): lone pair and * acceptor orbital in L 2, (HOMO-1) and (HOMO) lone pair in A, (HOMO) in A(AuCl). Ligand L 2 A A(AuCl) * G CO F CNPh D C(OEt) E C(OEt)(NMe 2 ) Table 3 Partial charges from NBO analysis. A A(AuCl) A(AuCl) 2 q(c) [a] q(c) [b] q(c) [a] q(c) [b] q(aucl) q(c) [a] q(c) [b] q(aucl) 2 B PPh G CO F CNPh D C(OEt) E C(OEt)(NMe 2 ) [a] Central carbon atom. [b] Carbon atom which is directly bonded to the central carbon atom. Figure 8 Optimized geometries (BP86(RI)/def2-TZVP) of 14 (left) and 18 (right). The ethyl groups in 14 lie approximately in the plane (C1-C2-O1-O2), whereas in 18 the ethyl group and one of the methyl groups of NMe 2 adopt an out-of-plane conformation (with regard to C1-C2-O1-N1). Hydrogen atoms are omitted for clarity. nature chemistry 7

8 Figure 9 Optimized geometries (BP86(RI)/def2-TZVP) of the gold complexes 16 (left) and 19 (right). Hydrogen atoms are omitted for clarity. Table 4 Selected geometric parameters for the structures 14, 16, (distances in, angles in degree). Calculated values (BP86(RI)/def2-TZVP) are given in black, crystallographic data are shown in red for comparison (two data for 21, which has two independent molecules in the unit cell) C1-C2-O1-C3 1.9 (5.8) (-3.3) (-26.2) (-129.4/126.9) C1-C2-O2-C (-169.5) (-170.4) (166.6) C1-C2-N1-C (28.0/-25.0) C1-C2-N1-C (-139.8/146.4) P-C1-C (125.6) (114.3) (115.6) (118.2/118.2) C1-C (1.314) (1.363) (1.383) (1.365/1.381) C2-O (1.379) (1.333) (1.335) (1.349/1.367) C2-O (1.345) (1.341) (1.335) C2-N (1.349/1.351) nature chemistry 8

9 Validation of the BP86/def2-TZVP approach: The optimized geometries agree well with the available X-ray crystal structures in the case of the complexes 14, 16, 20, and 21 (see Table 4 above). For further validation, we have carried single-point calculations at the optimised BP86/def2-TZVP structures using larger basis sets (BP86/def2-TZVPP, BP86/def2-QZVPP) as well as a standard hybrid functional (B3LYP/def2-TZVP, B3LYP/def2-TZVPP), and determined the reaction energies for the first and second complexation step (see equations 1 and 2 in Table 1 of the paper). The corresponding results are collected in Tables 5 and 6. It is obvious that the computed reaction energies are quite insensitive to such variations in the DFT approach. Going from BP86/def2-TZVP to BP86/def2-TZVPP causes changes in the reaction energies of typically kcal mol 1, and further extension of the basis in BP86/def2-QZVPP leads to consistent shifts between +0.6 and +0.9 kcal mol 1. Replacing the BP86 by the B3LYP functional for a given basis set yields changes in the reaction energies of typically less than 1 (2) kcal mol 1 for the first (second) complexation step. The reported BP86/def2-TZVP results are thus stable at the DFT level. We further note that the BP86/def2-TZVP approach has previously been validated for carbon(0) complexes against ab initio MP2 and CCSD(T) reference calculations (see ref. 5 of the paper), and it has been applied successfully in several theoretical studies of carbon(0) complexes (see refs. 5-7, 9, and 13 of the paper). Table 5 Reaction energies E (kcal mol 1 ) for first complexation step (equation 1) Table 6 Reaction energies E (kcal mol 1 ) for second complexation step (equation 2) A BP86/def2- BP86/def2- BP86/def2- B3LYP/def2- B3LYP/def2- TZVP TZVPP QZVPP TZVP TZVPP A BP86/def2- BP86/def2- BP86/def2- B3LYP/def2- B3LYP/def2- TZVP TZVPP QZVPP TZVP TZVPP nature chemistry 9

10 Crystallographic Summaries CCDC contain the supplementary crystallographic data for this paper. This information can be obtained free of charge from The Cambridge Crystallographic Data Centre via C6 C4 N1 C5 N2 C1 C7 C8 H2A C2 N3 C9 O8 O5 O2 H2B C3 N4 Cl2 O1 C10 O7 O6 Cl1 O3 C11 O4 Figure 10 Structure of compound 23 in the solid state. X-ray Crystal Structure Analysis of compound 23: C 11 H 26 Cl 2 N 4 O 8, M r = g mol -1, colorless, crystal size 0.30 x 0.25 x 0.10 mm, monoclinic, space group P2 1 /c, a = (13) Å, b = (17) Å, c = (13) Å, = (2), V = (4) Å 3, T = 100 K, Z = 4, D calc = g cm 3, = Å, (Mo-K ) = mm -1, empirical absorption correction (T min = 0.62, T max = 0.75), Nonius KappaCCD diffractometer, 2.27 < < 33.94, measured reflections, 7371 independent reflections, 6278 reflections with I > 2 (I), Structure solved by direct methods and refined by full-matrix least-squares against F 2 to R 1 = [I > 2 (I)], wr 2 = 0.207, 242 parameters, H atoms riding, S = 0.970, residual electron density +1.1 / -2.3 e Å -3. X-ray Crystal Structure Analysis of complex 16: C 24 H 25 Au Cl O 2 P, M r = g mol -1, colorless, crystal size 0.10 x 0.04 x 0.02 mm, monoclinic, space group P2 1 /n, a = (10) Å, b = (10) Å, c = (11) Å, = (2), V = (3) Å 3, T = 100 K, Z = 4, D calc = g cm 3, = Å, (Mo-K ) = mm -1, nature chemistry 10

11 Semi-empirical absorption correction (T min = 0.42, T max = 0.75), Nonius KappaCCD diffractometer, 2.61 < < 37.07, measured reflections, independent reflections, reflections with I > 2 (I), Structure solved by direct methods and refined by full-matrix least-squares against F 2 to R 1 = [I > 2 (I)], wr 2 = 0.058, 262 parameters, H atoms riding, S = 1.225, residual electron density +1.8 / -1.5 e Å -3. X-ray Crystal Structure Analysis of complex 17: C 25 H 27 Au 2 Cl 4 O 2 P, M r = g mol -1, colorless, crystal size 0.18 x 0.10 x 0.08 mm, monoclinic, space group P2 1 /n, a = (4) Å, b = (4) Å, c = (4) Å, = (1), V = (14) Å 3, T = 100 K, Z = 4, D calc = g cm 3, = Å, (Mo-K ) = mm -1, Semiempirical absorption correction (T min = 0.15 T max = 0.75), Nonius KappaCCD diffractometer, 3.04 < < 33.12, measured reflections, independent reflections, 8332 reflections with I > 2 (I), Structure solved by direct methods and refined by full-matrix least-squares against F 2 to R 1 = [I > 2 (I)], wr 2 = 0.083, 307 parameters, H atoms riding, S = 1.041, residual electron density +2.2 / -2.7 e Å -3. X-ray Crystal Structure Analysis of complex 10: C 26 H 20 Au Cl N P, M r = g mol -1, colorless, crystal size 0.27 x 0.25 x 0.25 mm, triclinic, space group P 1, a = (2) Å, b = (3) Å, c = (3) Å, = (10), = (10), = (10), V = (5) Å 3, T = 100 K, Z = 2, D calc = g cm 3, = Å, (Mo-K ) = mm -1, empirical absorption correction (T min = 0.71 T max = 0.99), Nonius KappaCCD diffractometer, 2.92 < < 39.39, measured reflections, independent reflections, reflections with I > 2 (I), Structure solved by direct methods and refined by full-matrix least-squares against F 2 to R 1 = [I > 2 (I)], wr 2 = 0.076, 271 parameters, H atoms riding, S = 1.024, residual electron density +1.4 / -4.2 e Å -3. X-ray Crystal Structure Analysis of complex 21: C 24 H 26 Cl 3 Ga N O P, M r = g mol -1, colorless, crystal size 0.28 x 0.10 x 0.09 mm, monoclinic, space group P2 1 /n, a = (3) Å, b = (10) Å, c = (4) Å, = (10), V = (3) Å 3, T = 100 K, Z = 8, D calc = g cm 3, = Å, (Mo-K ) = mm -1, empirical absorption correction (T min = 0.46 T max = 0.74), Nonius KappaCCD diffractometer, 2.93 < < 27.50, measured reflections, independent reflections, 9515 reflections with I > 2 (I), Structure solved by direct methods and refined by full-matrix least-squares against F 2 to R 1 = [I > 2 (I)], wr 2 = 0.114, 565 parameters, H atoms riding, S = 1.012, residual electron density +2.3 / -1.1 e Å -3. nature chemistry 11

12 X-ray Crystal Structure Analysis of complex 20: C 24 H 25 Cl 3 Ga O 2 P, M r = g mol -1, colorless, crystal size 0.23 x 0.19 x 0.16 mm, monoclinic, space group P2 1 /c, a = (2) Å, b = (4) Å, c = (4) Å, = (10), V = (10) Å 3, T = 100 K, Z = 4, D calc = 1.488g cm 3, = Å, (Mo-K ) = mm -1, empirical absorption correction (T min = 0.49 T max = 0.78), Nonius KappaCCD diffractometer, 2.98 < < 36.35, measured reflections, independent reflections, 9367 reflections with I > 2 (I), Structure solved by direct methods and refined by full-matrix least-squares against F 2 to R 1 = [I > 2 (I)], wr 2 = 0.085, 282 parameters, H atoms riding, S = 1.041, residual electron density +0.5 / -0.9 e Å -3. X-ray Crystal Structure Analysis of complex 13: C 33 H 25 Au Cl 3 P, M r = g mol -1, colorless, crystal size 0.10 x 0.08 x 0.04 mm, orthorhombic, space group Pbca, a = (8) Å, b = (17) Å, c = (2) Å, V = (8) Å 3, T = 100 K, Z = 8, D calc = 1.488g cm 3, = Å, (Mo-K ) = mm -1, empirical absorption correction (T min = 0.50 T max = 0.86), Nonius KappaCCD diffractometer, 2.09 < < 36.58, measured reflections, independent reflections, reflections with I > 2 (I), Structure solved by direct methods and refined by full-matrix leastsquares against F 2 to R 1 = [I > 2 (I)], wr 2 = 0.049, 343 parameters, H atoms riding, S = 1.108, residual electron density +2.5 / -0.9 e Å -3. nature chemistry 12

13 Structural Information on Selected Starting Materials Figure 11 View of the published structure of compound 3 shows the core of this ligand to be slightly bent, with a P-C-CO angle of Figure 12 View of the published structure of compound 14 shows the core of this ligand to be highly bent, with a P-C-C(OEt) 2 angle of Daly, J.J. & Wheatley, P.J. Structure of triphenylphosphoranylideneketen. J. Chem. Soc. A (1966). Burzlaff, H., Voll, U. & Bestmann, H.-J. Die Kristall- und Molekülstruktur des (2,2- Diäthoxyvinyliden)-triphenylphosphorans. Chem. Ber. 107, (1974). nature chemistry 13