Bond length structure, influence

This work is more concerned with a fourth use of apparent atomic valences. The interest is in compounds in which there is no ambiguity concerning true valences and whose structures are (presumably) not in question. It is then asked why the apparent valence differs from the true valence on the occasions when it does so significantly. The hypothesis, clearly anticipated by Zachariasen [13] and, for molecules, by Bartell [41], is that bond lengths are influenced not only by valence but also by next-nearest ( non-bonded ) interactions [42]. 

Crystal structures have been determined (Figure 2.56) for RhCl3(PEt2Ph)3 and RhBr3(PMe2Ph)3 (in both cases wer-isomer) - in each case, the Rh-X bond length shows the trans-influence of a tertiary phosphine [110]. 

However, despite the rather dramatic change in coordination geometry that is observed upon comparing [TpBut Me]CuCl and [TpBut]CuCl (41), only rather minor perturbations are observed in comparing the structures of the Cud) dimers [TpBut]Cu 2 (37) and [TpBut,Me]Cu 2 (22). Thus, both the average Cu-N bond lengths and also the Cu - Cu separations in [TpBut Me]Cu 2 and [TpBut]Cu 2 are very similar. Nevertheless, although the coordination environment about each copper center is similar, the 5-methyl substituent does influence the fluxional nature of the molecule in solution. Thus, whereas [TpBut]Cu 2 is fluxional on the NMR time scale at room temperature, with a static structure that is only observed at -56°C, [TpBut Me]Cu 2 exhibits a static H NMR spectrum at room temperature. Furthermore, a static spectrum for... 

---