Evidence for covalency in transition metal complexes

One final comment in this section we have been concerned with vacuum photoelectron spectroscopy. A similar X-ray photoelectron spectroscopy exists in which the ejected electrons come from inner electron shells. The ejected electron energy is sensitive to the chemical environment from which it originates and so gives information on this. 

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. 

Ligand hyperfine splittings in EPR spectra, discussed in Section 12.6. 

The percentage of ionic character in metal-chlorine bonds as measured by NQR spectroscopy, discussed in Section 12.5.2. 

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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. 

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