Stability of the gold(I)-phosphine bond. A comparison with other group 11 elements

The stability of gold phosphine complexes of the form [Au(PH₃)_n]⁺ (n = 1-4) and [AuCl(PH₃)_n] (n = 1-3) is analyzed in detail by applying quantum theoretical methods and compared to the coordination behavior of the lighter group 11 elements copper and silver. It is shown that, once [M(PH₃)₂]⁺ or [MClPH₃] (M = Cu, Ag, and Au) is formed, further coordination by PH₃ ligands is relatively weak; i.e., the energy gain to form [M(PH₃)₃]⁺ from [M(PH₃)₂]⁺ is less than 60 kJ mol^-1, and less than 100 kJ mol^-1 to form [MCl(PH₃)₂] from [MClPH₃]. Relativistic effects in gold significantly influence these factors and reduce the tendency for phosphine coordination beyond two-coordination. This implies that the most favored coordination number for gold is two with either a linear P-Au-P or P-Au-X arrangement (X = a strongly coordinating ligand like Cl^-). Instead, X-Au-PH₃ units prefer to interact via close Au-Au contacts (aurophilic interactions) keeping the linear structure approximately intact, while the corresponding copper and silver compounds prefer PH₃ coordination to strongly bound M₂Cl₂ units (M = Cu or Ag) where two chlorine atoms bridge the two metal atoms thus having the formal coordination number of three for copper or silver.