Tetrahedral/distorted four-coordinate

It should be noted that four-coordinate Cu(II) complexes do not typically favor tetrahedral coordination, so that distortions for this metal center may be more pronounced upon substitution at the 5-position. See Cotton, F. A. Wilkinson, G. Advanced Inorganic Chemistry (5th Ed.) Wiley-Interscience New York, 1988. 

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

The bond angles for three-coordinate P-, As-, Sb-, and Bi-atoms have been shown by X-ray diffraction to tend toward 90°. The four-coordinate atoms were found to be tetrahedral, the five-coordinate generally trigonal bipyr-amidal for P and As and square pyramidal for Sb and Bi. Steric factors were found to distort these geometries <1996CHEC-II(8)863>. 

Variable coordination modes in compounds of similar general formula but different compositions (heterogeometrism) has been observed in the case of gallium and indium diisobutyldithiophosphinate pair, M(S2PBu12)3, M = Ga or In the gallium compound is four-coordinate, distorted tetrahedral, whereas the indium compound is six-coordinate, distorted octahedral (Scheme 7).134 Other... 

The most extensively studied alcohol dehydrogenases are those of mammalian liver. They are dimeric proteins, with each subunit binding two Zn2+ ions, only one of which is catalytically active. This catalytic Zn2+ ion has distorted tetrahedral geometry, coordinated to one histidine and two cysteine residues. The non-catalytic zinc plays a structural role and is coordinated tetrahedrally to four cysteine residues. 

Tourmaline has a rhombohedral unit cell. The six silicon atoms are each surrounded tetrahedrally by four oxygen atoms, each tetrahedron sharing two of its oxygens with the neighboring tetrahedra to form a six-member ring of composition SieOis the A1 ions are coordinated to six oxygen atoms in a very distorted octahedron. 

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