Coordination chemistry in two dimensions: the angular overlap model for actinyl complexes

The angular overlap model has been applied to the study of actinyl coordination complexes AnO(2)L(n) (An = U, Np, Pu, Am; n = 4, 5, 6) using a 13-orbital sd(5)f(7) manifold. After allowing for a triply bonded (sigma + 2 perpendicular to pi) linear OdropAndropO substructure, a seven-orbital submanifold d(3)(dsigma,ddelta}f(4){fdelta,fphi} remains for sigma and pi bonding to the ligands in the equatorial plane. The two fdelta orbitals in this manifold cannot be used for sigma bonding to the equatorial ligands leaving only a smaller five-orbital d(3) {dsigma,ddelta)f(2)(fphi) manifold suitable for equatorial or bonding. Pentagonal bipyramidal complexes of the type AnO(2)L(5), which are known experimentally to be favored for monodentate L ligands, use this five-orbital submanifold most effectively to form or bonds to all five equatorial ligands as indicated by the angular overlap parameters calculated for these orbitals. Hexagonal bipyramidal complexes of the type AnO(2)L(6) are less favorable as formation of a bonds to all six equatorial ligands (L) must necessarily either disturb the orbitals required for the linear OdropAndropO substructure or use high-energy actinide p orbitals. Angular overlap calculations on octahedral complexes of the type AnO(2)L(4) show very uneven overlap between the orbitals in the d(3) {dsigma, ddelta}f(2){fphi} manifold with the four equatorial ligands. Thus the angular overlap model supports the experimental observation that the most favorable actinyl coordination geometry is the pentagonal bipyramidal AnO(2)L(5) in the absence of ligands with special properties.