Simple molecular-orbital model for the correlation of Mössbauer quadrupole splitting with stereochemistry in organotin(IV) compounds

Abstract

Mössbauer quadrupole splitting in organotin(IV) compounds is interpreted in terms of the additive approximation, in which the total electric field gradient at the ¹¹⁹Sn nucleus is written as a sum of partial field gradient tensors. Localized orbitals are shown to provide the natural framework for discussion of additive electric field gradients, and it is conjectured that the existence of a suitable localization transformation is a necessary condition for additivity. It is shown that the partial field gradient associated with a given ligand is different for tetrahedral, trigonal-bipyramidal-apical, trigonal-bipyramidal-equatorial, and octahedral co-ordination positions. In particular, the partial field gradient for octahedral co-ordination is about 70%(experiment indicates 75%) of that for tetrahedral. Absolute numerical values for partial field gradient parameters cannot be obtained from experiment for any of the above co-ordination positions, but the evaluation of relative parameters is discussed, and working values are given for a variety of ligands in tetrahedral or octahedral structures. Quadrupole splittings calculated by use of these values agree with observed splittings to within 0·4 mm s^–1 or better. Alternatively, disagreement may be used as evidence of large distortions or incorrect assignment of structure. It is shown that a literal'point-charge' treatment of the effect of distortions from idealized geometry cannot be justified in molecular-orbital terms. Illustrative calculations are performed for tetrahedral systems, but experimental data indicates that it is better to ignore small distortions in quantitative discussion of the magnitude of quadrupole splitting. Extension of the model to octahedral complexes of low-spin Fe(II) is briefly discussed.