On the electronic structure of metal tetrahydroborates: quasi-relativistic X.alpha.-SW study of M(BH4)4 (M = zirconium, hafnium, thorium, uranium)

Abstract

Quasi-relativistic SCF-X..cap alpha.. scattered-wave calculations are presented for the series of metal tetrakis(tetrahydroborates) M(BH/sub 4/)/sub 4/ (M = Zr, Hf, Th, U). The bonding in the transition-metal compounds is that of tetrahedral d/sup 0/ complexes with strong covalent contributions from ..pi.. bonding ligand group orbitals. In the actinide complexes, 5f orbitals of the central atom are found to be important to covalent ligand-metal bonding but less than in the corresponding di-..pi..-(8)annulene compounds. The ligand field manifold of U(BH/sub 4/)/sub 4/ and the corresponding spin-orbit constant agree well with experimental data. Photoelectron spectra are reproduced satisfactorily with the exception of a uniform shift. An explanation is presented why the first ligand-derived band in the spectra of the actinide complexes is narrower than that in the transition-metal compounds. The difference between the zirconium and hafnium complexes in the splitting of the second band is found to be a relativistic effect due to a more effective covalent mixing of the Hf 6s orbital. 37 references, 5 figures, 6 tables.