Abstract
The simple analytical model of Mayer for the qualitative study of bond order in three-centre four-electron bonds, corresponding to the apical hypervalent bonds in sulfuranes has been extended by taking into account a d orbital on the central sulfur atom. This seems to be a useful extension of Mayer's results, which were based on the model of Rundle and Musher, using only an sp basis set. The maximal sulfur-apical ligand bond order attainable by an sp model in symmetric sulfuranes is only 0.5, whereas, according to our previous ab initio calculations, this bond order is sometimes close to the value of 1, characteristic of a usual single bond. It is shown by a simple extension of Mayer's considerations that, in the extreme case when one electron is promoted to a d orbital, the bond order due to d participation may reach the value of 0.5. At relatively small d orbital populations (0.05–0.3) the increase of the bond order has been found to be roughly equal to the d-orbital population.
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