ARE STRONG GOLD GOLD INTERACTIONS POSSIBLE IN MAIN-GROUP X(N)A(AUPR3)M MOLECULES

The isolobal analogy between a proton and gold phosphine unit (AuPR3) and the existence of molecules such as CH5+, suggests the possibility of stabilization of Au-Au linkages by coordination to a main group fragment. Calculations of the extended Huckel type on AX(n)(AuPR3)m molecules show that since the energies of the orbitals associated with Au-Au bonding in the neutral (AuPR3)m fragments lie above empty levels of the neutral AX(n) fragments to which they are coordinated, there is invariably formal electron transfer between the two on coordination. The result is a large diminution of Au-Au bonding. This electronic situation for the main group case is quite different to that for transition metal ML(n)(AuPR3)m molecules which such formal charge transfer does not occur and stronger AuAu interactions are possible. Really strong Au-Au interactions are found only to occur in systems isoelectronic with H(n)(n-2)+ such as in the known molecule (AuPR3)4I2, and may be expected in the as yet unknown species (AuPR3)3I. The case of As(AuPR3)4+ is a particularly interesting one. Here the C4v geometry is one where close Au-Au contacts are possible. It is suggested however that such enhanced Au-Au interactions are only one of the reasons behind the adoption of this geometry. Comparisons are made with the nontetrahedral geometry of SiLi4.