The Br 3 P-Brand I 3 P-Ibond strengths 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 PF 4and PCl 4systems to determine the effect of the terminal atom on hypervalent bond strengths. The bond strengths correlate very well with the difference in the electronegativities of phosphorus and the halogen atoms. Computational results at the B3LYP/6-311+G(d) level indicate that PCl 4and PBr 4have seesaw geometries and are good examples of three-center, four-electron bonding. PF 4has a similar geometry, but the bonding is more ionic, whereas PI 4is tetrahedral, consistent with greater steric crowding.
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