Anions conformational differences associated

Conformational Differences Associated with Anion Binding... 

The idea that the instantaneous equilibrium between the different conformations of dissymmetric complex ions could be influenced by ion-pair formation finds some support in recent experiments of Mason and coworkers with diastereoisomeric cobalt(III) complexes. Thus, Mason and Norman (31) have shown that the circular dichroism spectra of D-Co(d-pn)3" and L-Co(d-pn)3" (pn = propylenediamine) are differently changed by oxo-anions such as P04, S04 and 8203 . According to these authors the oxo-anion and D-Co(d-pn)3+ (or D-Coen3" ), but not L-Co(d-pn)3 should have a preferred mutual orientation in the ion pairs. At low concentrations the effect of sulfate and thiosulfate ions on the circular dichroism of D-Co(d-pn)3" is similar to that of phosphate but with these anions at concentrations > 0.2M the previous changes are reversed— probably due to breaking down the specific orientation of the ion pair in the denser ionic atmosphere. It is also interesting that Mason and Norman (32) have found that Co(NH3)6 associated with d-tartaric acid produces a pronounced Cotton effect. These results show that some... 

Stereoelectronic features of vinyl anions are associated with their bent geometries and usual configurational stabilities in solution. Theory predicts a sizable inversion barrier for the parent vinyl anion (28.7kcal/ mol at the MP2/6-31+G(d)//HF/6-31 -nG(d) level). The two conformers of 1-propenyl anion can be formed stereospecifically in the gas phase (via desilylation of E- and Z-l-trimethylsilylpropene) and show different reactivity patterns. ... 

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