Complexes semi-coordinate

While the range of copper(II) complexes involving Cu—Br bonds (Table 81c) is more limited than those for Cu—Cl bonds (Table 81b), the structures tend to be analogous, with short Cu—Br distances of 2.4 A and long distances of ca. 3.0 A. Simple iodine complexes of the copper(II) ion are unknown, in view of the ready reduction of copper(II) to copper(I) with the liberation of iodine, nevertheless, copper-iodine bonds are formed if the copper(II) ion is stabilized by complex formation, as in [Cu(bipy)2I]I (420),1299 the structure of which involved the first example of five-coordinate copper(II) stereochemistry.1300 Semi-coordinate iodide... 

There are indications that planar Co(II) complexes in coordinating solvents show high-spin/low-spin equilibria (16, 48). The high-spin states are stabilized by strong axial interaction (38). This is in agreement with the results of semi-empirical calculations showing that, in the base adducts, some quartet levels are even closer to the ground state than in the four-coordinated planar complexes. Their influence in EPR spectroscopy is mainly on the hyperfme parameters. This will be discussed in Chapter VII. 

Addition of NaBr to a solution containing Pd(OAc)2 and 9S3 in methanol gives the orange-brown [PdBr2(9S3)] complex (Eq. 11c), in which Pd(II) interacts with two halide ions and a tridentate 9S3 to yield square pyramidal coordination (Fig. 6 Table 1) [106]. As in the bis(9S3) complex, the coordination sphere contains a semi-coordinated S atom in the apical position [106]. The chloride analogue [137] has a similar structure. 

The mechanism of the cycloaddition reaction of benzaldehyde 2a with Danishefsky s diene 3a catalyzed by aluminum complexes has been investigated theoretically using semi-empirical calculations [14]. It was found that the reaction proceeds as a step-wise cycloaddition reaction with the first step being a nucleophilic-like attack of Danishefsky s diene 2a on the coordinated carbonyl compound leading to an aldol-like intermediate which is stabilized by interaction of the cation with the oxygen atom of the Lewis acid. The next step is the ring-closure step, giving the cycloaddition product. 

The principal mechanistic events include N-N bond formation stage, where the coordinated NO reactant is transformed into the NzO semi-product via dinitrosyl ( M-(NO)2]Z) or dinitrogen dioxide ( M-N202 Z) intermediates, depending on the nature of TMI (vide infra). Simultaneously, the primary (M Z active sites are converted into the secondary [M-0]Z active sites involved in the dioxygen formation cycle [5], The mononitrosyl complexes are usually postulated to be the key intermediate species of this step [2,5,41], whereas the mechanistic role of dinitrosyls and dinitrogen dioxide is more indistinct as yet. 

Figure 4.45 A metal-ligand m,—orbital splitting diagram depicting interaction of the metal-atom d NAO and ligand nL NBO to form semi-localized NLMOs of the coordination complex, with splitting energy Aed. = < d/NLMO — fd> (NAO).

A similar H2 activation mechanism was proposed for the [Pd(NN S)Cl] complexes (5 in Scheme 4.5) in the semi-hydrogenation of phenylacetylene [45] after formation of the hydride 14 (Scheme 4.9), coordination of the alkyne occurs by displacement of the chloride ligand from Pd (15). The observed chemos-electivity (up to 96% to styrene) was indeed ascribed to the chloride anion, which can be removed from the coordination sphere by phenylacetylene, but not by the poorer coordinating styrene. This was substantiated by the lower che-moselectivities observed in the presence of halogen scavengers, or in the hydrogenations catalyzed by acetate complexes of formula [Pd(NN S)(OAc)]. Here, the acetate anion can be easily removed by either phenylacetylene or styrene. 

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