Coordination tetrahedral, distorted

We have argued that, once achieved, planar coordination in d systems is stable with respect to higher coordination number or tetrahedral distortion. The question arises then about what circumstances favour planarity in the first place. In particular, we enquire about the occurrence of tetrahedral verses square planar stereochemistry for d complexes. Why, for example, is the [Ni(CN)4] ion planar but [NiClJ tetrahedral ... 

The pyrazole phosphazine ligand, hexakis(3,5-dimethylpyrazolyl)cyclotriphosphazene (16) forms a dizinc complex, that has been structurally characterized, and the analogous copper zinc complex.158 Two ZnCl2 units are coordinated, one distorted tetrahedral and one distorted trigonal-bipyramidal, separated by 7.51 A. Cu1 and ZnCl2 can also be coordinated giving a heteronuclear compound with a Zn Cu separation of 6.798 A. 

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. 

In the X-ray structures of both 3 and 4, the tetrahedral distortion is greater than that found in model A. Since there is an electronic preference for a square planar coordination, the pendant phenyl substituents in 3 and 4 likely result in further steric crowding and therefore in more distorted structures compared to model A. In the QM/MM model B an optimized 0 angle of 30° is found, close to the 34° angle of the X-ray structure. Since the phenyl and trimethyl phenyl groups are accounted for on a steric basis only in model B, the result supports the notion that the severely distorted coordination of the Pd center in 3 is due to a steric effect. 

In both cases the N2O2 coordination of the Ni(II) ion is essentially planar, though there is a slight tetrahedral distortion. However, the diamagnetism displayed by these complexes confirms that they are planar. 

Figure 120 shows the molecular structure of complex [CunL2]2 +, which has a planar CuN2S2 coordination with a slight tetrahedral distortion. 

In Cu,Zn-SOD the copper atom is bound to three histidine groups and the zinc is bound to two histidines and an aspartate oxygen atom (Tainer et al., 1982, 1983) (Fig. 34). The Cu"-Zn distance is 6.3 A. The zinc-ligand geometry is tetrahedral, with a strong distortion toward a trigonal pyramid with aspartic acid at the apex. The coordination of the Cu(II) is tetrahedrally distorted square planar. The axial position of copper is more open on the solvent side than on the protein side probably, water is bound there. The Zn(II) is buried, while the Cu(II) site is solvent accessi-... 

Alcohol dehydrogenases (ADH EC 1.1.1.1), for which several X-ray structures are available ", catalyze the biological oxidation of primary and secondary alcohols via the formal transfer of a hydride anion to the oxidized form of nicotinamide adenine dinucleotide (NAD ), coupled with the release of a proton. Liver alcohol dehydrogenase (LADH) consists of two similar subunits, each of which contains two zinc sites, but only one site within each subunit is catalytically active. The catalytic zinc is coordinated in a distorted tetrahedral manner to a histidine residue, two cysteine residues and a water molecule. The remaining zinc is coordinated tetrahedrally to four cysteine residues and plays only a structural role . 

A = distorted trigonal bipyramidal, B = irregular five-coordinate, C = distorted tetrahedral, D = intermediate between trigonal bipyramidal and square pyramidal, E = distorted tetragonal pyramidal, F = pseudooctahedral, G = linear, H = highly distorted tetrahedral. Axial bonds. 

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