Trigonal and tetragonal Cu II structures

Why does a Cu(II) ion with the ligands in the blue copper proteins assume a trigonal structure, whereas most inorganic cupric conplexes are tetragonal (square planar, square pyramidal, or distorted octahedral) [63,64] We have faced this question by optimising the geometry of a number of models of the type 

Tetragonal structures may also be obtained for models of Cu-alcohol dehydrogenase [39]. When the fourth ligand is uncharged, they are less stable than the corresponding trigonal complexes (in accordance with the crystal structure). However, with OH (which is involved in the reaction mechanism of the enzyme) 

Method Model Basis set Distance to Cu (pm) Scys N SmcI N-N Angle around Cu (°) Scvs N S—S Smci—N n 

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