BOND CHARACTER EFFECTS ON BOND VALENCES, ANGLES, AND ENERGIES

If the bond-valence model is to be developed for predicting full structures, then the links between bond valences (lengths), angles, and energies must be elucidated. The more we can demonstrate quantitative links between them, the more we can constrain model parameters to ensure wider applicability. Here we show that all these properties are strongly related to one another, and are predictable if we also take into account the bond character, meaning the degree of ionicity, covalency, or metallicity. The bond character (or at least the general tendency toward a given type of bond character) of an atom pair can be described in terms of the electronegativity difference and the average electronegativity. It has long been established that the valence of a bond between a given type of atom pair in crystals tends toward a particular expectation value, and this is highly correlated with the electronegativity of the cation involved. We use experimental and calculated structures of oxide molecules with nominal X-O bond valences of 1 v.u. to show that, given the same bond valences, the bond XOX bond angle is strongly dependent on the X-O electronegativity difference, except among the more covalent bonds, where the bond angles are more strongly dependent on ligand-ligand interactions. However, we also show that bond dissociation energies for a large number of diatomic and other small molecules with 1 v.u. bonds are a function of bond character, and that the transition between dependence of the bond angles on bond character to ligand-ligand interactions corresponds to a change in the bond energy due to the well known “lone-pair bond weakening effect”. That is, an increase in the stereoactivity of lone pairs as bonds become more covalent causes a weakening of the bond energy if more acute bond angles cannot be accommodated.