Covalency in transition metal complexe

With establishment of the crystal structure, three major features concerning the electronic structure of the blue copper site can be addressed. These features are 1) the nature of the thiolate and thioether bonds, 2) the nature of the ground state wavefunction and 3) the extent of covalency. We have also become strongly involved in using photoelectron spectroscopy as a powerful approach toward determining covalency in transition metal complexes. These will be discussed in turn. 

The determination of the coefficients in the molecular orbitals of Eq. (29), for D41, symmetry, and of Eq. (40), for D2h symmetry, is one of the more important applications of EPR in coordination chemistry. These results are inde the basis for any detailed discussion of metal-ligand bonding and covalency in transition metal complexes. The coefficients are therefore referred to as bonding parameters and are list, in the Tables 2.2, for all compounds where they have been determined with some reliability. 

This is a convenient point at which to bring together the various pieces of evidence indicating covalency in transition metal complexes although it must be remembered that not all are applicable to every species. The pieces of evidence are listed below. 

Other evidence for covalence in transition metal complexes has been obtained from nuclear magnetic resonance spectroscopy (NMR), electronic spectroscopy, nephelauxetic effects, and nuclear quadrupole resonance spectroscopy (NQR) and from the detailed magnetic properties of such complexes. Thus, without giving a detailed description of each of these effects (which is beyond the scope of this book), it can be stated unequivocally that considerable physical evidence shows the existence of some degree of covalence in transition metal complexes. 

Monodentate (monometallic monoconnective) phosphor-1,1-dithiolato ligands are rare. Bidentate (monometallic biconnective) form chelate rings and three sub-types can be distinguished according to the degree of asymmetry (Scheme 2). The most asymmetric type (anisobidentate) occurs when a covalent bond is associated with a secondary bond this takes place mostly in main-group metal complexes. The second type is rare and is the result of the association between a covalent and a dative coordinate bond. The symmetric bidentate bonding (isobidentate) is found mainly in transition metal complexes. 

The electrostatic theory of the preceding section is the starting point for a more complete treatment of the bonding in transition metal complexes, in which the covalency of the interactions is taken into account. 

Early work on the kinetics of photoinduced ET in transition metal complex systems focused exclusively on bimolecular reactions between transition metal chromophores and electron donors or acceptors. However, concomitant with the advances in rapid photochemical kinetic methods and chemical synthetic methodology, emphasis shifted to photoinduced ET in chromophore-quencher assemblies that comprise a metal complex chromophore covalently linked to an organic electron donor or acceptor [24]. These supramolecular compounds afford several... 

In the case of covalent bonding in transition-metal complexes, the qualitative correlation between the observed fine structures and the calculated... 

Soon after the development of the quantum mechanical model of the atom, physicists such as John H. van Vleck (1928) began to investigate a wave-mechanical concept of the chemical bond. The electronic theories of valency, polarity, quantum numbers, and electron distributions in atoms were described, and the valence bond approximation, which depicts covalent bonding in molecules, was built upon these principles. In 1939, Linus Pauling s Nature of the Chemical Bond offered valence bond theory (VBT) as a plausible explanation for bonding in transition metal complexes. His application of VBT to transition metal complexes was supported by Bjerrum s work on stability that suggested electrostatics alone could not account for all bonding characteristics. 

Owen and Thomley (547) have reviewed covalency in transition metal ions and, in particular, in nickel complexes. The NMR contact shift method has been used by Eaton et al. (195) to determine spin densities on organic ligands of paramagnetic molecules. In particular (194), a series of nickel aminotroponeiminates have been studied whereby conjugative and hyperconjugative effects within the molecule may be monitored. Similar studies of contact shifts have been carried out on Co and Ni pyrromethenes and porphyrins (196) and on many transition metal acetylacetonates (193) by Eaton et al. 

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