Effect of substituents on the strength of hypervalent phosphorus-halogen bonds

The strengths of the F2ClP-Cl-, POCl3-Cl-, and PSCl3-Cl- bonds have been determined by measuring thresholds for collision-induced dissociation in a flowing afterglow-tandem mass spectrometer. The results are combined with previously determined values for the PF4-, PF3Cl-, POF4-, and PCl4- systems to determine the effect of adjacent ligands on hypervalent bond strengths. Although the addition of electronegative equatorial ligands strengthens bonding to axial halides, the effect is, in some cases, outweighed by the rearrangement energy of the dissociation products. Computational results indicate that the B3LYP/aug-cc-pVTZ method gives particularly good agreement with experiment among the models used here; however, several less resource-intensive methods give acceptable agreement.