Bonding and charge distribution in isopolyoxometalate ions and relevant oxides—A bond valence approach

Abstract

Refined bond length-bond valence relationships for M−O bonds have been applied to isopolyoxometalate and some oxide structures (M=MoVI, WVI, VV, NbV, TaV). The M−O bond valences and the distribution of the (negative) ionic charge over the different types of oxygen atoms have thus been obtained. Additional negative charge and an equivalent positive charge were found to exist on certain types of oxygen atoms. The reasons for the observed distribution of the bond lengths and bond valences in the M−O frameworks and of the charge over the oxygen atoms have been analyzed. The main factors determining the distribution are the stabilization of the polymeric structures by strengthening of the inner M−O bonds of the M−O frameworks through acceptance of the negative charge by the terminal oxygen atoms and of the positive charge by (approximately) linearly bridging oxygen atoms and/or (angularly) bridging OH groups and coordinated H2O molecules. Both charge situations are restricted by the necessity to fulfill simultaneously the geometrical conditions of the M−O frameworks (interdependence of the M−O bond lengths), the bond length-bond valence relationships of the M−O bonds, and the valence sum rule for the M and O atoms and by avoidance, as far as possible, of very high negative charge on terminal oxygen atoms. Similar but much less detailed calculations and analyses were undertaken by some authors to locate the positions of OH groups and coordinated H2O molecules as well as to elucidate the hydrogen bridge system of polyoxometalates. Other authors have attempted to define, among other features, the charge distributions and basicities of the oxygen atoms in polyoxometalate ions. Our results differ considerably from those of the second group of authors, and reasons for the differences are discussed. Additionally, a number of general questions are treated in detail: the conditions for the extension of the coordination sphere of the protonated tetrahedral monometalate ions and the reasons for the occurrence of distorted MO6 octahedra (MOk polyhedra); the question of the coordination number of the addenda elements M in special cases; the amphoteric character of the species; the protonation sites; the relationship between charge distribution and basicity (as defined by the protonation constants) of polyoxometalate ions; the strength of the integration of the different types of MO6 octahedra (MOk polyhedra) within the M−O frameworks (kinetic inertness of the structures); the stabilization of the polymeric character of the different M−O frameworks by the “meshing effect” and by “normal” bonding; the question of a “trans influence”; the question of ions (guests) encapsulated by (uncharged or charged) M−O frameworks (hosts); and others. In summary, the bond valence model, which is strictly fulfilled within the accuracy of the bond lengths for polyoxo species, proves to be a powerful tool for studying bonding questions and yields a fascinating picture of the polyoxometalate structures.