The scale of nucleophilicity

One of the most common ligands, particularly in this area of chemistry, is the monotertiary phosphine. This ligand is amenable to variation by simple perturbation of the alkyl- and aryl-substituents. Several studies have been reported in which the relative nucleophilicities of the metal complexes have been correlated, on a semi-quantitative basis, with the electron-releasing capability of the phosphine ligand. 

Comparison of the relative nucleophilicities of substituted pyridines and [(r -C5H5)Co(CO)L] (L = phosphine) towards Mel° 

In a further study, the reactions of tra .y-[IrCl(CO)L2] (L = PEtPhj or PEtjPh) and t/am-[IrCl(CO) P(CgH4Z-p)j 2l (Z = Cl, F, H, Me or MeO) with alkyl halides have been investigated (Ugo et al 1972). The reactions with both methyl and benzyl halides were affected by the phosphines in the order PEtPhj PEtjPh PPhj. The anomalous position of PEtjPh has been ascribed to steric effects, and this is consistent with the lower entropies of activation for reaction (7) when L = ethyl-substituted phosphines. 

Relatively little systematic work on the nucleophilicities of metal complexes has been performed. Of particular interest would be the definition of a scale of nucleophilicity based on isoelectronic metals with the same coligands. The variety of different types of nucleophiles and the dichotomy of electron transfer vs nucleophilic mechanisms precludes any extensive comparison. However, it is possible to compare the relative nucleophilicities within a given group. 

The relative rates of reaction between Mel and [(ti -CsHj) M(CO)(PPhj)] (M = Co. Rh or Ir) are in the order Co(l.O), Rh(1.4), Ir(8.0) (Hart-Davis and Graham, 1970), whereas for the reaction between pentafluoropyridine and [M(CO)2(PPhj)2] the order is Co(l.O), Rh(857), Ir(357) (Booth et al., 1975). Although both studies demonstrate the greater nucleophilicity of the heavier members of the triad, it is clear that the relative reactivities of the members can vary. Such behaviour is also manifest in the acid strengths of transition metal hydrides (Pearson, 1985 Shriver, 1970 Jordan and Norton, 1982 and references therein Ziegler, 1985). 

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The scale of nucleophilicity of anionic complexes from rates of reactions of anions (as sodium salts in DME) with alkyl halides (vs. [Co(CO)4] = 1) is assembled in Table 2". The nucleophilicities vary by over seven orders of magnitude. Anion nucleo-philicities are solvent dependent. Addition of either a crown ether or TMED to Na[Mn(CO)j] in THE causes the rates of reaction of the anion with PhCH2Br to decrease. In the absence of a crown ether or TMED, the Na ion is associated with a carbonyl oxygen in the anion. This ion pairing increases the nucleophilic character of the anion by lowering the entropy of activation of the reaction. 

The scope of the review 2 Introduction to metal complexes as nucleophiles 2 Types of metal-based nucleophiles 5 The scale of nucleophilicity 6 The influence of ligands on the nucleophilicity 7 The nucleophilicity of metal complexes 12 The influence of the solvent on the nucleophilicity 14 Stereochemical changes at a saturated carbon centre 18 Reaction of coordinatively-saturated complexes 18 Reactions of coordinatively-unsaturated complexes 22 Reactions of MCjSn 27... 

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