Ligand poorly modeled

In the case of covalent compounds, crystal-field theory is a poor model for estimating electric field gradients because of the extensive participation of ligand atomic orbitals in the chemical bonds. MO calculations are a much better choice, since the corresponding interactions are considered, and realistic (noninteger) population numbers are obtained for the central metal as well as the ligand atomic orbitals. 

The photosubstitution studies of the Co, Rh, Ir triad are concerned with the hexacoordinated d -M(IIl) complexes however, photoreactions of the d -[IrClj] and a few d -M(I) square-planar complexes also are known. The d -amine complexes of Co(Ill), e.g., [Co(NH3) ] , are poor models for the photosubstitution reactions of other low-spin d complexes, which, like such Co(III) cyano ions as [Co(CN) ] "", have lowest-energy excited states (ES) that are ligand-field (LF) triplets with the (t p fe electronic configuration (Fig. 1). For the Co(III) amines, LF quintets with the (tjP (ep configuration are the lowest states. Regardless of the theoretical implications of such electronic differences, the LF photochemistry of the Co(III) amines contrasts with that of analogous Rh(III) and Ir(III) complexes and of the Co(III)-cyano complexes. For the latter systems in solution, irradiation into LF absorption bands leads to photosubstitution with moderate (ca. 0.1) quantum yields independent of the irradiation wavelength, A. ... 

Reactivity studies of organic ligands with mixed-metal clusters have been utilized in an attempt to shed light on the fundamental steps that occur in heterogeneous catalysis (Table VIII), although the correspondence between cluster chemistry and surface-adsorbate interactions is often poor. While some of these studies have been mentioned in Section ll.D., it is useful to revisit them in the context of the catalytic process for which they are models. Shapley and co-workers have examined the solution chemistry of tungsten-iridium clusters in an effort to understand hydrogenolysis of butane. The reaction of excess diphenylacetylene with... 

Irreversible inhibition is probably due to the alkylation of a histidine residue.43 Chymotrypsin is selectively inactivated with no or poor inhibition of human leukocyte elastase (HLE) with a major difference the inactivation of HLE is transient.42,43 The calculated intrinsic reactivity of the coumarin derivatives, using a model of a nucleophilic reaction between the ligand and the methanol-water pair, indicates that the inhibitor potency cannot be explained solely by differences in the reactivity of the lactonic carbonyl group toward the nucleophilic attack 43 Studies on pyridyl esters of 6-(chloromethyl)-2-oxo-2//-1 -benzopyran-3-carboxylic acid (5 and 6, Fig. 11.5) and related structures having various substituents at the 6-position (7, Fig. 11.5) revealed that compounds 5 and 6 are powerful inhibitors of human leukocyte elastase and a-chymotrypsin thrombin is inhibited in some cases whereas trypsin is not inhibited.21... 

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