Effects on Bonding of Metal Complexation

This section briefly discusses the effect of metal complexation on the electronic structure of C60 and draws together some information that has been presented in previous sections. 

In terms of the Dewar-Chatt model of bonding, for v metal complexation one double bond is effectively removed from the fullerene conjugation system due to extensive interaction between metal d orbitals and the fullerene HOMO and LUMO (7). The remaining 29 double bonds then behave almost identically to uncomplexed C60 with their IR, Raman, UV-vis, and 13C NMR spectra showing only slight perturbations, mainly as a result of diminution of symmetry effects. Nevertheless, it is important to state that the fullerene metal interaction is not confined purely to the former s HOMO and LUMO, and that other molecular orbitals are energetically suitable for interaction 89,90). The spectroscopic evidence cited for the preceding statement is as follows  

Electrochemical studies Adducts undergo similar electrochemical processes, with reduction couples shifted to slightly more negative potentials owing to the lower electron affinity of the cage. This is due to 

Raman studies on [M(PPh3)2(i72-C60)] and [C60 M(PEt3)2 n] (M = Ni, Pd, Pt n = 1,6) showed slight shifts to lower wavenumbers, suggesting some M — tt back-bonding (17). 

Diffraction studies showed only slight changes in fullerene C-C bond lengths and angles. 

UV-vis spectroscopy showed only small changes on complex-ation. 

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B. tr-Bonded Complexes Effects on Bonding of Metal Complexation Physical Properties and Chemical Reactivity... 

For example vibrational spectra and the study of lattice modes, from which information on effective charges may be obtained via a dielectric model, the mechanical and elastic properties of crystals, proton and other NMR which have provided much interesting information on the bonding of metal complexes, and optical effects such as harmonic generation. 

The presence of residual unbound transition-metal ions on a dyed substrate is a potential health hazard. Various eco standards quote maximum permissible residual metal levels. These values are a measure of the amount of free metal ions extracted by a perspiration solution [53]. Histidine (5.67) is an essential amino acid that is naturally present as a component of perspiration. It is recognised to play a part in the desorption of metal-complex dyes in perspiration fastness problems and in the fading of such chromogens by the combined effects of perspiration and sunlight. The absorption of histidine by cellophane film from aqueous solution was measured as a function of time of immersion at various pH values. On addition of histidine to an aqueous solution of a copper-complex azo reactive dye, copper-histidine coordination bonds were formed and the stability constants of the species present were determined [54]. Variations of absorption spectra with pH that accompanied coordination of histidine with copper-complex azo dyes in solution were attributable to replacement of the dihydroxyazo dye molecule by the histidine ligand [55]. 

The most important reaction of this type is the formation of imine bonds and Schiff bases. For example, salicylaldehyde and a variety of primary amines undergo reaction to yield the related imines, which can be used as ligands in the formation of metal complexes. However, it is often more desirable to prepare such metal complexes directly by reaction of the amine and the aldehyde in the presence of the metal ion, rather than preform the imine.113 As shown in Scheme 31, imine formation is a reversible process and isolation of the metal complex results from its stability, which in turn controls the equilibrium. It is possible, and quite likely, that prior coordination of the salicylaldehyde to the metal ion results in activation of the carbonyl carbon to amine nucleophilic attack. But it would be impossible for a precoordinated amine to act as a nucleophile and consequently no kinetic template effect could be involved. Numerous macrocyclic chelate systems have been prepared by means of imine bond formation (see Section 61.1.2.1). In mechanistic terms, the whole multistep process could occur without any geometrical influence on the part of the metal ion, which could merely act to stabilize the macrocycle in complex formation. On the other hand,... 

A consistent model permitting rationalization and prediction of the solvato-chromic behaviour of coordination compounds with MLCT absorption has been described [428]. According to this qualitative model, the changing relationship between the metal-ligand bond dipolarities in the ground and MLCT excited state determines whether the complex is negatively, positively, or not solvatochromic [428]. For comprehensive reviews on solvent effects on electronic spectra of metal complexes, see references [15, 17]. 

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