Tetrahedral Coordination Spheres

The presence of halides in the coordination sphere with aldehydes is important as zinc halides are typically used in organic synthesis as organic carbonyl activators. Large excesses of aldehydes and anhydrous zinc halides were necessary. Both monomeric and polymeric structures with halide bridges were observed. Tetrahedral geometries were observed for mixed ligand, aldehyde, and halide complexes.353... 

Varying ratios of the ligands 7V-(2-thiophenyl)-2,5-dimethylpyrrole and V-methylimidazole were used to form tetrahedral zinc complexes with S4, S3N, and S2N2 coordination spheres. X-ray structural analyses and IR spectra were recorded for all compounds and the relevance to zinc finger proteins was discussed. The comparison to cobalt and cadmium structures showed only minor differences, supporting the theory that changes on substituting these metals into zinc proteins would be minor.538... 

LADH contains a tetrahedral zinc which is coordinated by one histidine nitrogen and two cysteine sulfurs. One aim is to make synthetic analogs of this coordination sphere that also feature the catalytically important water or hydroxide ligand in the fourth coordination site. Analogs containing bound substrates such as alcohols are also of interest. 

A novel cadmate complex has been formed by the reaction of Cd(NH2>2 with I C=CH in the presence of acetylene in liquid ammonia.250 The potassium salt, K2Gd(CCH)4-2NH3 191, has been structurally characterized. The cadmium center is tetrahedrally coordinated to four acetylide units with which it forms Cd-C bonds of 2.23-2.25 A (Figure 29). The acetylide ligands are 7r-coordinated to two crystallographically distinct potassium ions whose coordination sphere is completed by two ammonia molecules. 

In the diagram shown in Figure 6.9, a sulfur atom has been inserted at the cluster s center in agreement with current thinking.24 The M center is attached to protein side chains at only two locations, cysteine 275 (at Fel) and histidine 442 (at Mo), and is unusual in having three-coordinate irons rather than the normal four-coordinate tetrahedral iron coordination sphere.29b The molybdenum forms... 

With the Ti4+ ions acting as Lewis acid centers, a strong interaction with ammonia and water with these centers is expected. There is in fact abundant spectroscopic evidence for the coordination of NH3 and H20 molecules to tetrahedral Ti4+ centers and for the corresponding expansion of their coordination spheres. 

The first spirogallanate [Li(thf)12-crown-4][(PhC=CPhPhC=CPh)2Ga] 281 was isolated from the reaction of 1,4-dilithiotetraphenylbutadiene and GaCl3.198 The gallium center adopts a severely distorted tetrahedral coordination sphere with bond angles ranging from 87.4(2)° to 131.8(2)° (Figure 14). 

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