Abstract
In 1961 the author proposed (D.W.J. Cruickshank, J. Chem. Soc., (1961) 5486) that bond length variations in compounds of second-row tetrahedral oxyanions were due to variations in dπ— pπ bonding strengths. This hypothesis is reviewed historically and is rejected in its original form on both empirical and theoretical grounds. Ab initio calculations on first and second-row oxides since 1970 are reviewed as regards geometries energies, deformation densities and Roby unique polarization populations. The inclusion of d functions in basis sets for second-row oxides is essential; however, bonding does not occur in the simple manner envisaged in 1961. As compared with the first row, the larger unique populations of second-row d functions are related to larger d orbital penetration effects in second-row atoms, enhanced by larger positive charges.
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