A relativistic muffin-tin free Xα molecular orbital study of metal tetrakis tetrahydroborates: M(BH₄)₄, M = Zr, U

Muffin-tin free DV-Xα molecular orbital calculations employing self-consistent numerical basis sets have been performed for the tetrakis tetrahydroborates of zirconium and uranium. For the latter compound, both a nonrelativistic and a fully relativistic solution have been obtained. These calculations essentially confirm the results of a recent extensive Xα-SW study on the electronic structure of tetrahydroborates. Very good agreement with experimental ionization potentials is found in the present calculation. The assignment of the most important ligand metal bonding orbitals to the first PE band of Zr(BH₄)₄ supports that of the Xα-SW study, but is at variance with the results of a previous STO-DV-Xα investigation. A relativistic treatment is mandatory for a proper description of the metal ligand bonding in the uranium complex. In the nonrelativistic calculation the contribution of U 5f orbitals to metal ligand covalency is overestimated, the participation of the U 6d orbitals in such bonding is underestimated. These findings parallel the results of a previous Xα-SW study on uranocene, U(C₈H₈)₂, where this effect has been rationalized as a pecularity of the Xα method.