Effect of Substituents on the Strength of Hypervalent Phosphorus−Halogen Bonds

Abstract

The strengths of the F 2 ClP-Cl -, POCl 3 -Cl -, and PSCl 3 -Clbonds 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 4 -, PF 3 Cl -, POF 4 -, and PCl 4systems 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 resourceintensive methods give acceptable agreement.

Bond strengths in POCl3−, POCl2−, and PSCl2−

“…Therefore, vibrational and rotational frequencies were calculated using the B3LYP/augcc-pVTZ model to give a consistent set of frequencies, given in Table 1. The calculated frequencies reported here for POCl and PSCl average 2% lower than experiment, which is typical for this type of system [14,21]. Recent work on closely related molecules with this model suggests it gives generally good agreement with experiment [22].…”

“…The calculated values are 13-39 kJ mol −1 below experiment; the G3 calculations are in better agreement than the B3LYP calculations. B3LYP/aug-cc-pVTZ calculations of P Cl − bond strengths in 10-electron complexes are in good agreement with experiment [14], but corresponding bond strengths in group 14 complexes such as ACl 5 − (A = Si, Ge, Sn) are 10-44 kJ mol −1 lower than experimental results [31]. The reason for these inconsistent results is not clear.…”

“…Other computational results for several of these species have been reported previously [5,8,14], but not at a consistent computational level. The bond lengths calculated with the B3LYP/aug-cc-pVTZ method are slightly longer than the experimental values, which is typical [14,27]. The bond angles are in generally good agreement.…”

“…Thus, secondary reactions have a significant impact on the ion distribution. Substantial amounts of POCl 4 − are also created, as discussed previously [14]. Approximately 10 5 collisions with the buffer gas cool the metastable ions to room temperature.…”

“…A related study of POCl 4 − and PSCl 4 − has also been completed recently [14]. An advantage of these gas-phase experiments is that the effects of solvation on the bonding [15] are eliminated, making the experimental results directly comparable to computational results.…”