Polyhedron, inner coordination

Sharma and Reed, 1976)]. In proteins the coordination number 4 is most common, where the zinc ion is typically coordinated in tetrahedral or distorted tetrahedral fashion. The coordination polyhedron of structural zinc is dominated by cysteine thiolates, and the metal ion is typically sequestered from solvent by its molecular environment the coordination polyhedron of catalytic zinc is dominated by histidine ligands, and the metal ion is exposed to bulk solvent and typically binds a solvent molecule (Vallee and Auld, 1990). The inner-sphere coordination number of catalytic zinc may increase to 5 during the course of enzymatic turnover, and several five-coordinate zinc enzyme—substrate, enzyme product, and enzyme-inhibitor complexes have been studied by high-resolution X-ray crystallographic methods (reviewed by Matthews, 1988 Christianson and Lipscomb, 1989). The coordination polyhedron of zinc in five coordinate examples may tend toward either trigonal bipyramid or octahedral-minus-one geometry. 

The parameters of the inner coordination polyhedron are totally consistent with those reported for a number of organo-cobalt(III) complexes (21) with one exception. The average cobalt-nitrogen (planar) distance, 1.866 A, is shorter, although only slightly, than in other organo-cobalt(III) complexes. The crystal structures of three cobaloxime complexes, which are the most closely related to our organo-cobalt complexes, have mean Co-N (planar) distances which vary between 1.88 and 1.90 A (22,23, 24). 

For any referential sphere, of course, the coordination number is CN=8 and the corresponding polyhedron is a cube. Such situation is obvious if we take inland particle as reference yet, the same is inferred if the reference particle is from comer since elementary cell comer is common to eight cubes, the reference particle is surrounded by the eight inner spheres of the neighboring cubes. 

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