On the Electronic Structure of Metal Tetrahydroborates: Quasi-Relativistic Xα-SW Study of M(BH₄)₄ (M = Zr, Hf, Th, U)

Quasi-relativistic SCF-Xα scattered-wave calculations are presented for the series of metal tetrakis(tetrahydroborates) M(BH₄)₄ (M = Zr, Hf, Th, U). The bonding in the transition-metal compounds is that of tetrahedral d⁰ complexes with strong covalent contributions from π 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-π-[8]annulene compounds. The ligand field manifold of U(BH₄)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.