The xenon-fluoride bond dissociation energy in XeF3- has been measured by using energy-resolved collision-induced dissociation studies of the ion. The measured value, 0.84 +/- 0.06 eV, is higher than that predicted by electrostatic and three-center, four-electron bonding models. The bonding in XeF3- is qualitatively described by using molecular orbital approaches, using either a diradical approach or orbital interaction models. Two low-energy singlet structures are identified for XeF3-, consisting of Y- and T-shaped geometries, and there is a higher energy D3h triplet state. Electronic structure calculations predict the Y geometry to be the lowest energy structure, which can rearrange by pseudorotation through the T geometry. Orbital correlation diagrams indicate that that ion dissociates by first rearranging to the T structure before losing fluoride.

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.

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Bond strengths for a series of Group 15 tetrachloride anions ACl4 (A = P, As, Sb, and Bi) have been determined by measuring thresholds for collision-induced dissociation of the anions in a flowing afterglow-tandem mass spectrometer. The central atoms in these systems have ten electrons, which violates the octet rule: the bond dissociation energies for ACl4- help to clarify the effect of the central atom on hypervalent bond strengths. The 0 K bond energies in kJ mol(-1) are D(Cl3A-CL-) = 90 +/- 7,115 +/- 7,161 +/- 8, and 154 +/- 15, respectively. Computational results using the B3LYP/LANL2DZpd level of theory are higher than the experimental bond energies. Calculations give a geometry for BiCl4 that is essentially tetrahedral rather than the see-saw observed for the other tetrachlorides. NBO calculations predict that the phosphorus and arsenic systems have 3C-4E bonds, while the antimony and bismuth systems are more ionic.

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The strengths of the SeCl2−Cl-, SeOCl2−Cl-, SeBr2−Br-, and TeCl2−Cl- bonds have been measured to be 133 ± 9, 150 ± 10, 110 ± 6, and 170 ± 7 kJ mol-1, respectively, by determining thresholds for collision-induced dissociation in a flowing afterglow-tandem mass spectrometer. These values are stronger than the previously measured D(SCl2−Cl-) and D(SOCl2−Cl-), both 85 ± 8 kJ mol-1. Bond energies and other properties for these molecules and TeOCl3 - have also been computed by several high-level computational techniques, including B3LYP/aug-cc-pVTZ. There is excellent agreement between the experimental and computational energetics for SeCl3 -, SeOCl3 -, and TeCl3 -. The bond strengths increase in the order S < Se < Te.

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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.