Coordinated ligands Subject

Metal complexing can subject a ligand to severe internal strain or, alternatively, it can relieve strain or it can freeze the conformation of the coordinated ligand. These modifications often lead to enhanced reactivity by reducing the energy difference between ground and transition state. The subjection of the cyano group to strain when 2-cyanophenanthroline is coordinated to a metal ion, and its resultant accelerated hydrolysis, has been referred to previously (Sec. 

A different type of rearrangement involves cycloaddition of dimethyl acetylenedicarboxylate on to a copper complex which serves to activate the ligand (Scheme 73).229 The reaction is similar in some respects to that shown in equation (42), and indicates the difficulty in the categorization of many of the reactions of coordinated ligands according to mechanism. Consequently, the above account is of necessity somewhat subjective. 

The synthetic utility of reactions of coordinated ligands is an important and varied subject. It is based on the enhancement in reactivity of organic ligands as a consequence of metal coordination. For example, the metal can act as a super add and cause enhanced nucleophilic attack on coordinated carbonyl and imine ligands. The metal ion can also enable the ligand itself to act as a nucleophile, sometimes by direct activation, sometimes by protecting other parts of the ligand and sometimes by a combination of both. 

The coordinated ligands in Cp complexes (as well as other 71-complexes) are subject to reactions of various types, in particular nucleophilic attack reactions. 

Taken together, the above data suggest a stepwise mechanism of Cu(I) transfer from Atxl to Ccc2a as illustrated in Fig. 3 (Brown et al., 1991 Pufahl et al., 1997 Huffman and O Halloran, 2000). When the Cu(I) center can adopt a two-coordinate ligand geometry as in Atxl, the Cu(I) ion is subject to attack by the sulfur ligands of the MXCXXC motif... 

Although polyenes are typically subject to electrophilic attack in the free state, they are rendered amenable to nucleophilic attack as coordinated ligands, a fascinating reversal in chemical reactivity usually referred to by the German term umpolung . It is important to note that nucleophiles (Nu ) usually add to the external face of an arene ligand (i.e., an exo- attack) and tend to reduce the hapticity of the ligands to which they add (Scheme 11). 

The gas phase-chemistry of metal oxide ions has been the subject of several studies, and this area has been reviewed [16,17, 88]. The focus here is on the synthesis and reactions of metal oxides via heterolysis or homolysis of coordinated ligands with X—O bonds. 

Transition metal compounds containing nitric oxide as a coordinated ligand are normally called nitrosyl complexes. However, the term nitrosyl , is only sometimes restricted specifically to complexes which can be regarded as containing a three-electron metal-nitric oxide bond, and the term seems to be used generally for all nitric oxide compounds. Although there are many transition metal-nitrosyl complexes, relatively few also contain metal-carbon linkages and therefore fall within the subject of this chapter. 

Interligand oxygen transfer is a rearrangement that occurs between coordinated ligands on a metal framework. However, because of the importance of this class of reactions, it is considered here as a separate subject. 

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