Other Reactions of Co-ordinated Ligands with Nucleophiles

The reaction of a co-ordinated ligand with a nucleophile may be one of two types, which differ both in mechanistic detail and in synthetic consequence. The first type is that which we have already considered in some detail for carbonyl groups in Chapter 3. In these reactions, the nucleophile is added to an unsaturated carbon centre. This results in a change of hybridisation at carbon (Fig. 4-1). Specifically we have only so far discuss- 

Metals and Ligand Reactivity, New Edition. Edwin C. Constable Copyright 1996 VCH Verlagsgesellschaft mbH, Weinheim ISBN 3-527-29278-0 

4 Other Reactions of Co-ordinated Ligands with Nucleophiles 

The second type of reaction involves the attack of the nucleophile upon a saturated centre, with concomitant loss of a suitable leaving group. This second type of reaction corresponds to a classical SN2 process (Fig. 4-2). 

Although the loss of some anionic or other good leaving group is the usual consequence of such reactions at carbon centres, this is not the case when the electrophilic centre is an element such as silicon, sulfur, phosphorus or a transition metal with available d orbitals in the valence shell. In these cases, a simple addition reaction may occur to give a product of increased co-ordination number. This may involve simple reactions such as the addition of an anion to a neutral molecule or more complex processes (Fig. 4-3). 

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In conclusion, the hydrolytic and other reactions of co-ordinated amino acid derivatives with nucleophiles may proceed by two major routes. The first involves a moderate acceleration by general acid catalysis of a monodentate TV-bonded ligand, whilst the second may involve very dramatic rate increases (by a factor of a million or so) associated with didentate chelating TV O-bonded ligands. There is little evidence for the widespread involvement of co-ordinated nucleophiles attacking the carbonyl in amino acid derivatives, although some special, and well characterised, examples with cobalt(m) complexes are considered in the next chapter. 

Many of the studies purporting to show nucleophilic attack upon co-ordinated heterocyclic ligands have been performed with complexes of 5-nitro-l,10-phenanthro-line, 8.20. Unfortunately, this ligand proves to be a less than optimal choice, as nitroalke-nes undergo facile reactions with nucleophiles and, in contrast to simple pyridines, the free ligand reacts with hydroxide ion. Hydroxide ion and other nucleophiles react with 8.20 at the 6-position to give the stabilised anion 8.21 (Fig. 8-20). It is almost certain that the majority of the reactions of 5-nitro-l,10-phenanthroline complexes with hydroxide... 

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