On the relative stability of tetragonal and trigonal Cu(II) complexes with relevance to the blue copper proteins

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The role of the cysteine thiolate ligand for the unusual copper coordination geometry in the blue copper proteins has been studied by comparing the electronic structure, geometry, and energetics of a number of small Cu(II) complexes. The geometries have been optimised with the density functional B3LYP method, and energies have been calculated by multi- configurational second-order perturbation theory (the CASPT2 method). Most small inorganic Cu(II) complexes assume a tetragonal geometry, where four ligands make σ bonds to a Cu 3d orbital. If a ligand lone-pair orbital instead forms a π bond to the copper ion, it formally occupies two ligand positions in a square coordination, and the structure 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 approximately the same stability. On the other hand, small and hard ligands, such as NH3, OH2, and OH-, give ionic bonds and flattened tetragonal structures. It is shown that axial type 1 (blue) copper proteins have a trigonal structure with a π bond to the cysteine sulphur atom, whereas rhombic type 1 and type 2 proteins have a tetragonal structure with σ bonds to all strong ligands. The soft cysteine ligand is essential for the stabilisation of a structure that is close to a tetrahedron (either trigonal or tetragonal), which ensures a low reorganisation energy during electron transfer.


  • Mats H M Olsson
  • Ulf Ryde
  • Björn O. Roos
  • Kristine Pierloot
External organisations
  • Catholic University of Leuven
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Theoretical Chemistry
  • Structural Biology


  • Blue copper proteins, Copper thiolate, Quantum chemical calculations, Rhombic type 1 copper proteins, Trigonal copper complexes
Original languageEnglish
Pages (from-to)109-125
Number of pages17
JournalJournal of Biological Inorganic Chemistry
Issue number2
Publication statusPublished - 1998 Apr
Publication categoryResearch