Bonding and stability of the Cr + ion in octahedral fluoride lattices: Results of approximate Hartree-Fock-Roothaan calculations

Abstract

Frozen-core Hartree-Fock-Roothaan calculations at several values of the metal-fluoride distance have been performed for the (CrF 6) 5− cluster in an attempt to study the stability of the Cr +F − bond in fluoride lattices. The separate effects on the 6 A 1 g ground state of the 3 d basis set and the type of corevalence partition have been analyzed. Whereas inclusion of the 3 s and 3 p metallic AO's in the valence shell seems to be unnecessary for obtaining a stable cluster ground state, the diffuse 4 s and 4 p AO's play a significant role in describing the equilibrium geometry of this complex ion. The metal-ligand covalency is examined and related to the information deduced from the curvature of the 6 A 1 g nuclear potential and the orbital energies of the valence MO's. Consideration of the point-charge lattice potential of the NaF leaves the cluster electron density almost unchanged but noticeably decreases the cluster valence energy. It also reduces the size of the cluster by nearly 0.2 Å. As an alternative stabilizing mechanism, a ligand-to-lattice charge transfer has been explored. At the cluster- in-vacuo level it produces electron-deficient clusters more stable than the (CrF 6) 5− unit. As an example, the 7 T 1 u state of the neutral CrF 6 cluster (a system in which the oxidation state of the metal is nominally +1) has been computed and compared with the 6 A 1 g state. This CrF 6 unit turns out to be smaller and more strongly bonded than the (CrF 6) 5− ion but the latter becomes more stable when the point-charge potential of the NaF is taken into account.