Crystallographic studies, transition metal copper complexes

The measurement and interpretation of the d—d spectra of copper(II) complexes have provided a highly fruitful field for coordination chemists in recent years. There are two reasons for this first, copper(II) has a one hole electronic configuration which should render the d—d spectra amenable to theoretical anal3mis. Secondly, the structural chemistry of copper(II) encompasses a remarkable variety of stereochemistries and coordination numbers. This can to some extent be rationalised in terms of the Jahn-Teller effect, and a vast amount of crystallographic data has been accumulated. Indeed, it is probably true to say that more structural parameters have been measured for the coordination compounds of copper(II) than for those of any other transition metal ion. Such data are, of course, a prerequisite for extensive theoretical studies. Moreover, the d—d spectra are sensitive to the detailed geometry about the metal atom, and interest in the ligand field splitting naturally accompanies stmctural studies of copper(II) compounds. 

Another source of interest came from biochemistry. Research on the blue copper proteins revealed unusual electronic properties (redox potential and kinetics, EPR and optical behavior) that were suspected of arising from interaction of the copper ion with a thioether group from methionine [7]. While crystallographic studies established a weak interaction (Cu -- - S 2.9 A) [8,9,10], its influence on the electronic properties of the Cu site is now considered questionable. Nevertheless, the controversy regarding the blue eopper proteins, like the analogy to phosphines, served to focus attention on the broad issue of how thioether coordination affects the electronic structure of transition metal ions. Homoleptic thioether complexes provide the best way of assessing these consequences, since no other groups obscure the effect of thioether coordination. 

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