Structure Changes Prior to Ligand Exchange

Detection and measurement of the two isomeric forms in solution again is most convenient by proton NMR spectroscopy, in which the isotropic proton hyperfine contact shifts in the paramagnetic tetrahedral isomers make recognition easy, and the amount of the shift reflects the proportion of paramagnetic species in solution (107,110-116, 119). Other methods, including UV/vis spectroscopy (106, 112, 114, 116, 118, 119), magnetic moment determination (105, 106, 109, 110, 115,116,118,119), dipole moment measurement (106,109,114), and IR spectroscopy (118), have also been employed. 

The next point to consider is how the two isomers interconvert. From the possible mechanisms, the most obvious conceptually is the intramolecular motion shown in Fig. 9, often referred to as a digonal twist, which leads to a direct change of ligand positions. The problem here lies in the fact that such a motion is forbidden. 

An inherent weakness in applying this reasoning to the chemistry of transition metal complexes, however, is that it does not take energy considerations fully into account. The Woodward-Hoffmann treatment of organic systems in reality compares two pathways, allowed 

The most direct information on the isomerization processes has been gained from NMR studies of the phosphine complexes [NiX2L2]. The rates of some of the planar-to-tetrahedral interconversions are similar to the NMR time scale. In these cases, separate signals can be observed for the two isomers at low temperatures, but at the high-temperature limit, averaged resonances are seen (102-104). These observations allow reaction rates and activation parameters to be calculated. 

All of the relevant bis-chelate complexes, [Ni(biL)2], undergo the interconversion faster than the [NiX2L2] examples discussed earlier. As a consequence, the low-temperature limit has not been reached in NMR studies of these compounds, and rate data (other than minimum limits) are lacking. Nevertheless, the concensus is that these, too, isomerize by a first-order intramolecular twist. 

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Diol complexes, 9 211 d ion complexes, heavier, structure changes prior to ligand exchange, 34 258-259 Diopside, 4 61 crystal structure of, 4 51 p-Dioxane complexes, osmium, 37 312... 

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