Valence stabilization, mixed crystal chemistry, and electronic transitions in tetrahedral oxo and hydroxo Cr(IV), Mn(V), and Fe(VI) clusters: A theoretic investigation

Ab initio Hartree-Fock SCF (HF-SCF) and multiconfiguration complete active space SCF (CASSCF) calculations have been carried out on tetrahedral M(OH)(4)(z) and MO(4)(z') model clusters (M, z, z': Cr-IV, 0, -4, Mn-V, 1, -3; Fe-VI, 2, -2) in their 3A(2) ground state and in selected triplet and singlet ligand field and charge transfer excited states, Ground state orbital energies and charge distributions supplemented with calculations using a semiempirical approach (Jorgensen) help characterize the stabilization of Cr-IV, Mn-V, and Fe-VI in terms of competing ionic and covalent forces, The crucial role of the Madelung energy in stabilizing these unusual oxidation states is emphasized, Frozen orbitals as obtained by state averaging over d(2) triplet and singlet states are used to compare results from Hartree-Fock and ligand field treatments, Calculations using these orbitals show that interelectronic repulsion parameters in tetra-ore coordinated Cr-IV, Mn-V, and Fe-VI are considerably reduced compared to their free ionic values, Charge transfer states are found to further modify energy levels of d(2) type, leading to an effective lowering of Coulomb repulsion parameters in the order of the e(2), e(1)t(2)(1), and t(2)(2) strong field configurations, Theories of isomorphic substitution for ionic solids are not applicable for the systems under consideration, Comparison between available structural and spectral data shows that Cr-IV, Mn-V, and Fe-VI ions in dilute mixed crystals with isovalent tetrahedral host ions, such as Si-IV, P-V, and S-VI, assume geometries close to those known for stoichiometric phases with CrO44-, MnO43-, and FeO42- tetra hedra. Covalent contributions to the lattice energy from guest (d(2)) and host tetrahedra are additive, and mixing enthalpies are small or negligible, allowing for continuous solid solutions even when guest and host ions, such as Mn-V and P-V, Fe-VI and S-VI, differ considerably in their size. (C) 1997 Academic Press.