Bis(mercaptoimidazolyl)(pyrazolyl)hydroborato complexes of zinc, cadmium, and cobalt:: Structural evidence for the enhanced tendency of zinc in biological systems to adopt tetrahedral M[S4] coordination

The bis(2-mercapto-1-methylimidazolyl)(pyrazolyl)hydroborato derivatives [pzBm(Me)](2)Zn, [pzBm(Me)](2)Co, and [pzBm(Me)](2)Cd have been isolated and structurally characterized by X-ray diffraction. Despite their common [pzBm(Me)](2)M composition, each of these complexes adopts a different structure. Thus, (i) the zinc complex exhibits a tetrahedral Zn[S-4] structure in which only the sulfur donors coordinate to zinc, (ii) the cobalt complex exhibits a trigonal-bipyramidal Co[S3NH] structure in which one of the pyrazolyl groups and one of the B-H groups coordinate to cobalt, and (iii) the cadmium complex exhibits a six-coordinate Cd[S4H2] structure in which both B-H groups interact with the cadmium center. These comparisons emphasize that zinc has a greater preference For tetrahedral M[S-4] coordination than does either cobalt or cadmium, an observation that is in accord with the prevalent role of zinc in the structural sites of enzymes.