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 P31{H1} 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.
