Introduction to metal complexes as nucleophiles

The idea of a simple metal complex acting as a nucleophile is conceptually relatively simple. All that is required is a metal centre (the metal and its coligands) which is relatively electron-rich, and which has a vacant site, or a potentially vacant site (via rapid dissociation of a ligand). This is typified by the reaction shown in (2) between the five-coordinate, square-based pyramidal [Co(dmgH)2py] (py = pyridine, dmgH = dimethylglyoxime) and RX. 

The analogy between (1) and (2) is obvious. However, if the metal nucleophile contains more than one vacant site the liberated group X can bind to the metal, as shown in the archetypal reaction of this type shown in (3), involving the square-planar //an5-[lrCl(CO)(PPh3)2]. 

It is worthwhile discussing the salient features of reaction (3) at this stage, since they will encompass many of the problems which will recur throughout 

Obviously a discussion of the oxidative addition reactions which proceed by an electron-transfer mechanism is outside the scope of this review for a thorough appraisal of this area the reviews by Halpern (1970), Lappert and Lednor (1976), Stille and Lau (1977), Deeming (1972, 1974), Kochi (1978), together with the recent article by Hill and Puddephatt (1985) and references therein, should be consulted. For the remainder of this chapter we will be concerned entirely with systems in which the nucleophilicity of the metal complex has been either unambiguously demonstrated, or at least strongly implicated. It is important to be conscious of the spectrum of transition-state structures which could exist in the interaction between a metal centre and a molecule of RX as shown in Fig. 1. 

At the extremes of this series the metal interacts with the carbon centre and the group X exclusively, whereas an intermediate form involves a three-centred transition state where equal interaction between the metal and both the carbon centre and X occurs. Strictly, only the transition states involving the interaction between the metal centre and the carbon centre are of relevance to the discussion herein. For this reason we have included briefly the mechanistic studies which are beginning to appear in the literature on the homogeneous activation of hydrocarbons. 

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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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