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 AnO2L n (An = U, Np, Pu, Am; n = 4, 5, 6) using a 13-orbital sd5f7 manifold. After allowing for a triply bonded (σ + 2 ⊥ π) linear O≡An≡O substructure, a seven-orbital submanifold d3{dσ,dδ}f4{fδ,fϕ} remains for σ and π bonding to the ligands in the equatorial plane. The two fδ orbitals in this manifold cannot be used for σ bonding to the equatorial ligands leaving only a smaller five-orbital d3{dσ,dδ}f2{fϕ} manifold suitable for equatorial σ bonding. Pentagonal bipyramidal complexes of the type AnO2L5, which are known experimentally to be favored for monodentate L ligands, use this five-orbital submanifold most effectively to form σ bonds to all five equatorial ligands as indicated by the angular overlap parameters calculated for these orbitals. Hexagonal bipyramidal complexes of the type AnO2L6 are less favorable as formation of σ bonds to all six equatorial ligands (L) must necessarily either disturb the orbitals required for the linear O≡An≡O substructure or use high-energy actinide p orbitals. Angular overlap calculations on octahedral complexes of the type AnO2L4 show very uneven overlap between the orbitals in the d3{dσ,dδ}f2{fϕ} 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 AnO2L5 in the absence of ligands with special properties.