ON THE RELATIVE STABILITY OF TETRAGONAL AND TRIGONAL CU(II) COMPLEXESWITH RELEVANCE TO THE BLUE COPPER PROTEINS

The role of the cysteine thiolate ligand for the unusual copper coordi nation geometry in the blue copper proteins has been studied by compar ing the electronic structure, geometry, and energetics of a number of small Cu(II) complexes. The geometries have been optimised with the de nsity functional B3LYP method, and energies have been calculated by mu lticonfigurational second-order perturbation theory (the CASPT2 method ). Most small inorganic Cu(II) complexes assume a tetragonal geometry, where four ligands make sigma bonds to a Cu 3d orbital. If a ligand l one-pair orbital instead forms a pi bond to the copper ion, it formall y occupies two ligand-positions in a square coordination, and the stru cture becomes trigonal. Large, soft, and polarisable ligands, such as SH- and SeH-, give rise to covalent copper-ligand bonds and structures close to a tetrahedron, which might be trigonal or tetragonal with ap proximately the same stability. On the other hand, small and hard liga nds, such as NH3, OH2, and OH-, give ionic bonds and flattened tetrago nal structures. It is shown that axial type 1 (blue) copper proteins h ave a trigonal structure with a pi-bond to the cysteine sulphur atom, whereas rhombic type 1 and type 2 proteins have a tetragonal structure with sigma bonds to all strong ligands. The soft cysteine ligand is e ssential for the stabilisation of a structure that is close to a tetra hedron (either trigonal or tetragonal), which ensures a low reorganisa tion energy during electron transfer.