An SCF–MS–Xα study of the bonding and nuclear quadrupole coupling in dihalogenocuprates(I), CuX –2(X = Cl, Br), dihalogenoaurates(I), AuX –2(X = Cl, Br, I), and bis(cyano)aurate(I), Au(CN) –2

Abstract

SCF–MS–Xα calculations of the electronic structure of the linear dihalogenometallates(I) CuCl^–₂, CuBr^–₂, AuCl^–₂, AuBr^–₂, AuI^–₂ and of Au(CN)^–₂ have been carried out to investigate the bonding and nuclear quadrupole coupling in these ions. Good agreement between the calculated and observed ⁶³Cu and ¹⁹⁷Au coupling constants is obtained for all of these systems, and the calculations produce the correct sign (negative) for the field gradient at the gold nucleus in Au(CN)^–₂, and predict negative signs for the field gradients at the Cu and Au nuclei in the other MX^–₂ species. The origin of this field gradient is mainly the presence of electron density in the Cu 4p_z- or Au 6p_z-orbitals, which undergo a contraction relative to the free atom orbitals on bond formation. The valence d-shell makes a positive contribution to the total field gradient, and this is mainly due to promotion of electron density from the d_z²- to the s-orbital, and causes a significant reduction in the magnitude of the field gradient at the metal nucleus. The calculations provide an explanation for the observed general increase of the Mössbauer quadrupole splitting with the chemical isomer shift for several linearly coordinated gold(I) compounds, and also explain small systematic departures from this relationship in the case of the halogeno complexes AuX^–₂.