Second-sphere ligand field effects on oxygen ligator atoms and experimental evidence - the transition metal oxygen bond in oxidic solids

The properties of the M-O bond in oxidic solids, where M is a low-valent 3d transition metal ion such as Ni-II, Cu-II, Co-II or Cr-III, are very varia ble depending on the structure and constitution of the respective compound, and are studied by optical and EPR spectroscopy. Specifically, high-valent cations in the cationic coordination of the oxygen ligator atoms beside M determine the bonding behaviour of oxygen towards the transition metal ions . Applying a newly developed extension of the angular overlap model (AOM) t o fit the d-d spectra and the EPR and hyperfine tensor components (Cu2+). t he bonding parameters e(sigma) and e(pi) for a great variety of structures and solids are evaluated. In these calculations a 2s-2p hybridisation on ox ygen is anticipated, where the number and the geometrical arrangement of th e high-valent cations (Nb-V, Sb-V; W-VI, Te-VI, etc.) determine the kind of hybridisation (sp, sp(2), sp(3)) and the directional properties of the hyb rid orbitals. A wide range of AOM parameters-e(sigma) ranging from similar to 3 to similar to 5 x 10(3) cm(-1) in the case of Ni2+ - is obtained, the most striking changes being observed, if a d(0) configurated cation (Nb-V, W-VI) is substituted by a cation with d(10) configuration (Sb-V, Te-VI). Pa rticularly interesting are anisotropies in the pi-interactions in the case of sp2 hybridised oxygen atoms, which lead to large symmetry splittings of the d-d transitions. The e(pi) and e(pi) energies, which constitute the lig and held parameter Delta, turn out to be rather complex bonding parameters. They can be classified according to the local symmetry of the M-O bond in the respective crystal structure and correspond to the sigma- and pi-overla p capacities resulting from the modeling of the oxygen bonding properties b y strongly contrapolarising cations in the oxygen coordination sphere. Addi tional information, specifically about the degree of covalence within the t ransition metal-oxygen bond, comes from the nephelauxetic ratio beta and th e mixing coefficient alpha of Cu-II in the ground state MO-both parameters deduced from the experiment. The obtained results can be nicely interpreted -by defining effective electronegativities of oxygen towards the transition metal. Thus, high-valent and small cations as S-VI and P-V are strongly co ntrapolarising and generate highly ionic oxygen atoms comparable to the flu oride ions. The collected data are finally used to define oxygen ligand fie lds with the property to stablise the higher oxidation states of the late 3 d metal ions (Cu-III, Ni-III, Ni-IV). (C) 1998 Elsevier Science S.A. All ri ghts reserved.