How bridging ligands and neighbouring groups tune the gold-gold bond strength

Gold-gold interactions in small polynuclear complexes are analysed using extended Huckel calculations. They are influenced by the nature of the ligand donor atoms, by the bridging ligands, but most by the formal oxidation state of the metal. Au-Au bonds are much stronger in complexes of Au(II) and Au(III), but a weak interaction between two d(10) centres exists for Au(I) complexes, owing to mixing of the s and p orbitals with the d orbitals. Phosphines induce stronger metal-metal bonds when coordinated trans to the Au-Au bond in [Au(II)[(CH2)(2)PPh(2)]L](2) (Ph=phenyl), but have the opposite effect when bonded orthogonally to the metal-metal axis in Au(I) binuclear species. When two gold atoms are bridged by a single carbon atom, belonging either to mesityl (Mes = 2,4,6-Me(3)C(6)H(2)) or CR(2), the former produces stronger Au(I)-Au(I) interactions, reflected in shorter distances. Formal oxidation states are proposed for the gold atoms in two mixed-valence clusters, [Au-4(C6F2)(2){(PPh(2))(2)CH}(2)(PPh(3))(2)](ClO4)(2) and [{(2,4,6-C6F3H2)Au(CH(2)PPh(2)CH(2))(2)Au}Au-2(CH(2)PPh(2)CH(2))(2)Au](ClO4)(2). The results suggest a higher oxidation state for the outer gold atoms, in both the T-shaped tetranuclear cluster and the Au-6 linear chain.