Structural stabilities of Mn IV oxides

The molecular orbital energy level diagrams for two clusters, one corresponding to tetrahedral [MnOJ-4 with an interatomic distance Mn4+-02- =174 pm and the other to octahedral [MnOJ-8 with Mn4+-02- = 188 pm, are shown in fig. 11.9. In the ground state of the [MnOJ-8 cluster, the three 3d electrons of Mn4 occupy each of the three molecular orbitals labelled 5 t2g, whereas in the [MnOJ-4 cluster, two of the 3d electrons occupy the two 2e molecular orbitals and the third enters one of the three 512 molecular orbitals. This tetrahedral 512 level lies 2.8 eV above the octahedral 2t2g level, with the result that the [MnOJ-8 cluster is more stable than the [MnOJ-4 cluster. These calculations show that octahedrally coordinated Mn4 cations are more stable in oxides than 

Nevertheless, in the Mn(IV) oxides, some sort of safety valve may be necessary to alleviate the cation-cation repulsive interactions between so many next-nearest-neighbour cations. This may be provided by cation vacancies and 

Racah parameters. The Racah B parameter, which is a measure of interelec-tronic repulsion and exchange interactions, provides a qualitative indication of bond covalency. Values of B derived from optical spectra are lower for transition metals bonded to ligands in a coordination site than for isolated gaseous cations. The nephelauxetic series represents the order of decreasing Racah B parameters and correlates with increasing covalent bonding characters of ligands coordinated to a transition metal. 

Molecular orbitals. Overlap of certain atomic orbitals of the metal and ligand atoms in a coordination site leads to the formation of molecular orbitals. In octahedral coordination, a-bonds are formed by the overlap of hybrid orbitals 

Element distributions in sulphide mineral assemblages. The increased A0, and hence stabilization energies, resulting from Jt-bond formation in sulphides is also responsible for the enrichments of Ni, Cu, Co and Fe in sulphide minerals coexisting with silicates. Such strong chalcophilic properties of these transition elements has profound economic significance in the formation of massive sulphide ore deposits. 

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