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 ac tive space SCF (CASSCF) calculations have been carried out on tetrahed ral 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 wi th calculations using a semiempirical approach (Jorgensen) help charac terize the stabilization of Cr-IV, Mn-V, and Fe-VI in terms of competi ng ionic and covalent forces, The crucial role of the Madelung energy in stabilizing these unusual oxidation states is emphasized, Frozen or bitals as obtained by state averaging over d(2) triplet and singlet st ates are used to compare results from Hartree-Fock and ligand field tr eatments, Calculations using these orbitals show that interelectronic repulsion parameters in tetra-ore coordinated Cr-IV, Mn-V, and Fe-VI a re considerably reduced compared to their free ionic values, Charge tr ansfer states are found to further modify energy levels of d(2) type, leading to an effective lowering of Coulomb repulsion parameters in th e order of the e(2), e(1)t(2)(1), and t(2)(2) strong field configurati ons, Theories of isomorphic substitution for ionic solids are not appl icable for the systems under consideration, Comparison between availab le structural and spectral data shows that Cr-IV, Mn-V, and Fe-VI ions in dilute mixed crystals with isovalent tetrahedral host ions, such a s Si-IV, P-V, and S-VI, assume geometries close to those known for sto ichiometric phases with CrO44-, MnO43-, and FeO42- tetra hedra. Covale nt contributions to the lattice energy from guest (d(2)) and host tetr ahedra are additive, and mixing enthalpies are small or negligible, al lowing for continuous solid solutions even when guest and host ions, s uch as Mn-V and P-V, Fe-VI and S-VI, differ considerably in their size . (C) 1997 Academic Press.