The unusual yellow color of Au2(dppm)(SR)2 (R = 4-tolyl; dppm = diphenylphosphinomethane) is attributed to a red-shift in the S→Au charge transfer caused by destabilization of the sulfur highest occupied molecular orbital (HOMO). Variable temperature experiments show two broad bands at -80°C in the 31P{1H} NMR spectrum of Au2(dppm)(SR)2 and the activation energy for interconversion is 10 kcal/mol. Only one sharp band is observed down to -80°C in the spectrum of the white complex, Au2(dppe)(SR)2 (dppe = diphenylphosphinoethane). Molecular mechanics calculations on Au2(dppm)(SR)2 and Au2(dppe)(SR)2 reveal that, for Au2(dppe)(SR)2, a series of maxima and minima, separated by 2.5 kcal/mol, occur every 120° which is consistent with rotation around an unhindered carbon-phosphorus single bond. The Au atoms are not within bonding distance in any conformation. Computational results for Au2(dppm)(SR)2 indicate one minimum energy structure in which the Au-P bonds are anti. There is a high energy conformation (9 kcal/mol above the global minimum) where overlap between golds is maximized. The implications of gold-gold bonding in this complex are discussed. The steric influence of the thiolate ligand has been examined by synthesizing a series of dinuclear gold(I) complexes in which the steric properties of the thiolate are varied: Au2(dppm)(SR)2 (R = 2,6-dichlorophenyl; 2,6-dimethylphenyl; 3,5-dimethylphenyl). The 2,6-disubstituted complexes are white, while the 3,5-dimethyl complex is yellow. These results, along with VT-NMR experiments, are consistent with the conclusion that the more sterically-bulky thiolates hinder the close approach of the golds in the dinuclear complexes.
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