Bond lengths oxyanions

Table 3.36. Geometries and NBO/NRT descriptors of common oxyanions XOmn (see Fig. 3.91), showing symmetry, bond length Rxo, NRT bond order bxo and central-atom valency Vx, atomic charges Qx and Qo, and d-orbital occupancy dx for representative first- and second-row species...

Calculations of the structure of the mesoionic thiopyrylium-3-olate 74 suggest that the C-S bond lengths are similar to those in the pyrylium cation at ca. 168 pm, perhaps supporting the fully charge-separated betaine structure. However, the charge at oxygen is closer to 0.5 than the 1.0 expected for such a structure. Furthermore, the nucleus-independent chemical shift value is appreciably lower than that for the thiopyrylium cation. These data point toward an ylidic structure with an acceptor moiety rather than an aromatic cation and an exocyclic oxyanion <2002IJQ(90)1055>. 

There exists substantial evidence that in reactions that involve oxyanions or amines as bases or as nucleophiles, their partial desolvation, as they enter the transition state, typically has made greater progress than bond formation. In the context of the PNS, this partial loss of solvation represents the early loss of a reactant stabilizing factor and hence reduces the intrinsic rate constant. As discussed at some length in our 1992 chapter,4 for strongly basic oxyanions this desolvation effect often manifests itself in terms of negative deviations from Br Ansted plots and/or in abnormally low p or pnuc values.58,188 In fact, a number of cases have been reported where the pnuc value was close to zero or... 

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