A bond model for polyoxometalate ions composed of MO6 octahedra (MOk polyhedra with k>4)

Abstract

The literature relating to the bonding in polyoxometalate ions is reviewed. The author's opinions are presented and expanded to give a bond model for polyoxometalate ions of the early transition elements (groups V and VI) composed of MO 6 octahedra (MOk polyhedra with k > 4). This bond model concerns in particular the bond valence (bond lengths) and charge distribution in the polyoxometalate ions and the factors modifying them. It is based on the following commonly used concepts and principles as applied to polyoxometalate ions: -Lewis's octet rule, extended to the decet and dodecet rule for M v and MVI; -prc-drc M----O double bond and the coordinate bond (dative bond); -the resonance concept; -the resonance bond number (or the bond valence concept and the valence sum rule); -polydentate ligands and the chelate effect; -the larger space requirements of unshared electron pairs (cf. the VSEPR model); -the model of multicenter prc-drc multiple bonds for certain #-oxo bridges between metal atoms; -Bronsted's acid/base concept and acid/base equilibria; -Pauling's rules for the acid/base strength of (monomeric) oxoacids/oxoanions; -the law of mass action (Le Chatelier's principle), and others. The result is a set of resonance structures for polyoxometalate species in which three types of resonance can be distinguished. These, in turn, explain: -the cohesion of the strongly distorted tetrahedral M04 building units in the structures by formation of MO 6 octahedra and hence the enhanced stability of the polyoxometalate ions; -the distribution of the formal ionic charge over (nearly) all types of oxygen atoms, but preferably the terminal ones; -the occurrence of positively charged oxygen atoms, caused by charge separation processes; -the enhanced basicity of polyoxometalate ions; and further features. A "meshing effect" defines the increase of the bond valence in inner (bridging) M-O bonds and hence the stabilization of the structures due to extension of the coordination spheres of MO4 tetrahedra. Thus, the bond lengths of the different structure types are governed by a maximization of the bond valence of the inner, bridging (or minimization of the bond valence of the outer, terminal, in contrast to frequent statements in the literature) M-O bonds. The limits of the maximization of the inner bond valences of the polyoxometalate ions are determined -by minimum stoichiometric requirements for corresponding resonance formulae (inevitability of charge on bridging oxygen atoms); -by the necessity to fulfill simultaneously the geometrical relationships of the M-O bond lengths (as defined by their interdependence in the M-O frameworks) and the valence

Structure of Disodium Dimolybdate Synthesized Using Thermodynamically Stable Molybdenum (VI) Oxide Clusters as Precursors

“…However, the used system is well characterized and mainly consists of two dominant complex species, Mo 7 O 24 6− and Mo 8 O 26 4− , 7 which contain octahedral MoO 6 monomers (showing meshing and extension of the MoO 4 tetrahedral building units) that are linked at their vertexes. The vertexes linked in the clusters share an oxygen atom 8,9 . On the other hand, the structure of Na 2 Mo 2 O 7 is described as infinite chains formed from MoO 6 octahedra sharing vertexes within the chains, while MoO 4 tetrahedra bridge adjacent MoO 6 groups 10 …”

Bond length-bond valence relationships with particular reference to polyoxometalate chemistry

“…In doing so the possibility for negative charge on the terminal O atoms and an equivalent positive charge on bridging O atoms according to resonance structure (17b) in Ref. 48 was neglected. Tables 33-36 in Ref.…”

“…In this case the possibility for a negative or positive charge on the bridging O atom (compare, e.g., resonance structures (16d), (17b), (17c), and (17e) in Ref. 48 [46,50]. However, in this way there are great differences between the average bond valences s(ko) and the bond valences s calculated from the bond lengths via a bond length-bond valence function (e.g., up to 0.66 v.u.…”

“…The identification procedure for the outliners was facilitated by a selection according to the bond lengths of the terminal M-O bonds which must lie in the range 1.5 to 2.0 v.u. [44,48] (see also Sect. 3.1).…”

“…In other words: the size of B strongly effects the proportion of the bond valence in inner (bridging) and outer (terminal) M-O bonds and also strongly the results for the charge separation processes (Sect. 2.3.1.1) (the stability of polyoxometalate ions is strongly affected by these factors [44,48]). In this connection it is surprising that in the literature the low accuracy of B is not seen to bring discredit upon the applicability of such B show that B becomes indeed greater and explain how this is accomplished.…”