Coordination sphere saturation

Outer-sphere. Here, electron transfer from one reactant to the other is effected without changing the coordination sphere of either. This is likely to be the ea.se if both reactants are coordinatively. saturated and can safely be assumed to be so if the rate of the redox process is faster than the rates observed for substitution (ligand tran.sfer) reactions of the species in question. A good example is the reaction. 

On the basis of these results it seems to the present author that inner and outer complexes can reasonably be assumed for the electron transfer to the diazonium ion, but that an outer-sphere mechanism is more likely for metal complexes with a completely saturated coordination sphere of relatively high stability, such as Fe(CN) (Bagal et al., 1974) or ferrocene (Doyle et al., 1987 a). Romming and Waerstad (1965) isolated the complex obtained from a Sandmeyer reaction of benzenediazonium ions and [Cu B ]- ions. The X-ray structural data for this complex also indicate an outer-sphere complex. 

Many of these catalysts are derived from metal complexes which, initially, do not contain metal hydride bonds, but can give rise to intermediate MH2 (al-kene) species. These species, after migratory insertion of the hydride to the coordinated alkene and subsequent hydrogenolysis of the metal alkyl species, yield the saturated alkane. At first glance there are two possibilities to reach MH2 (alkene) intermediates which are related to the order of entry of the two reaction partners in the coordination sphere of the metal (Scheme 1.2). 

Coordinative unsaturation arises from the fact that because of steric and electronic reasons, only a limited number of ligands or nearest neighbors can be within bonding distance of a metal atom or ion. In most transition metal oxides, the oxygen anions in the bulk form closed-packed layers and the metal cations occupy holes among the anions as schematically depicted in Fig. 2.1. In this picture, the oxide ion ligands appear to have saturated the coordination sphere of the bulk cation. 

Following the precedent set by Green,77 the reaction is assumed to proceed through the i73-vinylcarbene complex 70, which is formed by the protonation of the CHPh carbon of 68. This then undergoes carbonyl insertion to afford the 16-electron complex 71, whose coordination sphere is subsequently saturated by iodide, affording the i74-vinylketene product (67). 

The surface of a solid exerts an attractive force on chemical species coming into contact with it owing to incomplete saturation of the coordination sphere of atoms, ions, or molecules at the surface. Adsorption is thus an accumulation of the adsorptive (probe molecules) on the surface of the adsorbent (the solid), giving rise to the adsorbate (or adsorbed phase). 

Shul pin and coworkers have demonstrated, in several papers, that other peroxo vanadium complexes closely related to 36, containing in the coordination sphere amino acids, nitrogen-containing bases or weak carboxylic acids, are effective oxidants of saturated and aromatic hydrocarbons. An accurate account containing this work, together with results related to the use of other transition metals, has appeared415 and all the relevant literature can be found there. 

It is, however, for the transition metals themselves that DFT has proven to be a tremendous improvement over HF and post-HF methods, particularly for cases where tlie metal atom is coordinatively unsaturated. The narrow separation between filled and empty d-block orbitals typically leads to enormous non-dynamical correlation problems with an HF treatment, and DFT is much less prone to analogous problems. Even in cases of a saturated coordination sphere, DFT methods typically significantly ouqierform HF or MP2. Jonas and Thiel (1995) used the BP86 functional to compute geometries for the neutral hexacarbonyl complexes of Cr, Mo, and W, the pentacarbonyl complexes of Fe, Ru, and Os, and the tetracarbonyl... 

A parallel situation appears to obtain for the mixed allyl nitrosyl complex Ru(NO)(C3H5)L2 prepared by Schoonover and Eisenberg (231). This complex which is coordinatively saturated (NO+ and rf -allyl), forms a CO adduct which is assigned a bent nitrosyl structure (231). Further reaction under CO leads to the formation of Ru(CO)3L2 with the possible elimination of acrolein oxime. The coupling of the allyl and nitrosyl ligands can be viewed in this case as nucleophilic attack of NO- on an f/3-allyl species. Unlike in reaction (110), both of the moieties to be coupled lie within the same coordination sphere. The significance of these results is that it lends viability to the notion embodied in (109) in which a migratory insertion of nitrosyl occurs as NO-. 

In some cases the rate of substitution of alkoxide ligands drops oif dramatically before total substitution has occurred. This clearly reflects both the electronic and steric saturation of the metal coordination sphere by the bidentate acac ligands. Hence for titanium it is difficult to substitute the last alkoxide, 9 while for Nb and Ta the reaction stops at the eight-coordinate tris-substitution products (equation 64).240,241... 

The initial coordination of reactants has indeed been proposed to explain the selective oxidation of alkenes in the presence of saturated hydrocarbons. It was argued that, owing to the hydrophobic nature of titanium silicates, the concentration of both hydrocarbons inside the catalyst pores is relatively high and hence the alkenes must coordinate to TiIv. Consequently, the titanium peroxo complex will be formed almost exclusively on Tilv centers that already have an alkene in their coordination sphere, and will therefore oxidize this alkene rather than an alkane which may be present in the catalyst (Huybrechts et al., 1992). Objections to this proposal are based on the fact that the intrinsically higher reactivity of alkenes with respect to saturated hydrocarbons is sufficient to account for the selectivity observed (Clerici et al., 1992). But coordination around the titanium center of an alcohol molecule, particularly methanol, is nevertheless proposed to explain the formation of acidic species, as was previously discussed. In summary, coordination around Tiiv could play a more important role than it does in solution chemistry as a consequence of the hydrophobicity of the environment where the reactions take place. 

According to this hypothesis, the results are modified from what would be expected from classical radical reactions. The interest in this hypothesis is that, with the sole exception of saturated hydrocarbons, it could apply to all the compounds that can be coordinated at the Tiiv center, such as alkenes, aromatics, alcohols, and sulfides. According to this hypothesis, the weak Lewis acidity of Tilv would help to bring the reactant into its coordination sphere. The initial coordination of the reactant would explain the oxidation of methyl-substituted aromatics in the aromatic ring and not in the side chain, even with a radical-type mechanism. 

The hydrido complex [RhH(PPr 3)3] and [Rh2(N2) P(C6H1,)3 4] have been shown to hydrogenate nitriles to primary amines at normal temperature and pressure, with turnover numbers of up to 100 being attained.124 This is a most interesting observation in view of the coordinating ability of primary amines, which might have been expected to saturate the coordination sphere of the rhodium, putting an end to catalysis. 

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