Coordination of the Metal Atom

Several crystallographic studies of the Mo DMSO reductase (Mo-DMSOR) of Rhodobacter capsulatus and Rhodobacter sphaeroides have been accom- 

The Nature of Molybdenum and Tungsten Centres in Oxo-transfer Enzymes 

The catalytic centres of W oxo-transfer enzymes appear to belong to the DMSOR family, as exemplified by the structure of the aldehyde oxidoreductase 

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If the reaction temperature is raised to 430 K and the carbon monoxide pressure to 3 atm, coordination of the metal atom in the rearranged product occurs via the phosphorus site, as in 159 (M = Cr, Mo, W) [84JOM(263)55]. Along with this product (M = W) at 420 K, formation of the dimer of 5-phenyl-3,4-dimethyl-2//-phosphole, 160 (the a complex), is possible as a consequence of [4 - - 2] cycloaddition reactions. Chromium hexacarbonyl in turn forms phospholido-bridged TiyP)-coordinatedcomplex 161. At 420 K in excess 2,3-dimethylbutadiene, a transformation 162 163 takes place (82JA4484). 

In crystals of the complex of AuCl with a strained ligand system such as dibenzocyclopheptatrienylphosphine, there is no evidence for significant 7r-complexation. By contrast, the corresponding copper(i) and silver(i) complexes clearly show -coordination of the metal atoms to the C=C bond (Scheme 76).313... 

Cp2M(S4) complexes with a tetrahedral coordination of the metal atoms and half-chair conformation of the MS4 rings these interactions are insignificant (25, 40). 

The use of organomagnesium reagents as bases leads to complexation of the nitrile imines (e.g., 141), which has been found to have a strong effect in promoting syn selectivity in reactions with methyl 2-(l-hydroxyalkyl)acrylates via coordination of the metal atom with the alcoholic oxygen (e.g., leading to the formation of 142). Lithium complexation had little effect (78). 

There are two aspects of the structural chemistry of cyano complexes that we shall discuss here the coordination of the metal atom, and relative values of interatomic distances and vibrational stretching frequencies.1 The correlation between the distances and frequencies and the very limited available thermodynamic data is considered briefly at the end of the section. 

The minimal structure demand for the OMe-group is leading to the maximal coordination of the metal atoms resulting in the fact that the majority of methoxides — both crystalline and amorphous — are polymeric. The powder diffraction studies have shown that many methoxides (and sometimes ethox-... 

EXAFS studies have provided important clues on the coordination of the metal atoms. At the V K-edge the features of the absorption edge are consistent with V11 or Viv in a distorted octahedral environment with Fe, S, and O or N atoms as the nearest neighbors to the V atom. The Fe K-edge is dominated by Fe—S and Fe—Fe interactions consistent with the structure proposed for FeMoco. These results emphasize the structural similarity between the cofactor centers of MoFe and VFe proteins. 

The coordination of the metal atoms varies between a bi-capped trigonal prism (C.N. = 8) at the mirror planes normal to b (in the central portion of the strips) -lamellae of YF3 type - and a baddeleyite-like polyhedron (C.N. = 7) at the apb [a. Sect. 4.4.]. 

Substitution Variants. The fluorite-type structure is maintained in principle when alkaline earth elements are replaced partially by rare-earth elements. Charge compensation is achieved by occupation of additional interstitial anionic sites.The coordination of the metal atoms may increase from 8 to 9 or even 10 by this. Another way of charge balance is the partial replacement of fluorine by oxygen to form oxyfluorides. Since the possible interstitial positions provide pathways for anion disorder and movement, this class of materials shows fluoride ionic conductivity. 

Many structural studies have been made on the species present in the vapors above oxysalts such as alkali metal nitrates, chlorates, perrhenates, and so on. In part, the interest in such work lies in determining the mode of coordination of the metal atom to the oxyanion in the monomeric vapor species. Both gas electron diffraction and matrix isolation have been employed to seek an answer to this question. 

So far we have been concerned with the type of coordination of the (metal) atoms inserted between the c.p. layers. The way in which the double ayeisAA etc. are superposed also, of course, determines the coordination around the c.p. atom. This is not of special interest in MX2 compounds since the contacts to one side are only of the van der Waals type. For example, in M0S2, with the structure AbA BaB... each S has a trigonal prismatic arrangement of nearest neighbours (3 Mo + 3 S), but the S-S contacts are not of much chemical interest. The situation... 

The feature common to the structures of Ge3N4, AIN, Ca3Nj and the numerous ternary nitrides with the antifiuorite structure is the tetrahedral coordination of the metal atoms, the c.n. of N being 3, 4, 6, and 8 respectively. It would seem that in these structures the determining factor is the type of coordination around the metal ions. 

The layer structures are of two main types, with octahedral or trigonal prismatic coordination of the metal atoms The simplest structure of the first kind is the Cdl2 (C6) structure of TiS2, ZrS2, HfS2, TaS2, and PtS2. (It is stated that Ta 2 has... 

The structures in which there is trigonal prism coordination of the metal atom contain pairs of adjacent S layers which are directly superposed (and therefore not close-packed), but the multiple S-M-S layers are then packed in the same way as simple layers in normal c.p. sequences. The simplest structures of this kind are illustrated in Fig. 4.11 (p. 130), in which the nomenclature is similar to that used for mica polytypes. No example of the (2T) structure is yet known, but examples of three structures are ... 

BiFs has the a-UFc structure (p. 994) in which there is octahedral coordination of the metal atom. (An early e.d. study established the trigonal bipyramidal configuration of the molecules NbCls, NbBrj, TaClj, and TaBrj. ... 

A comparison of the structures of compounds of Be, Mg, Zn, and Cd reveals an interesting point we exclude Hg from this series because there is practically no resemblance between the structural chemistries of Zn and Hg apart from the fact that one form of HgS has the zinc-blende structure. In the following group of compounds italic type indicates tetrahedral coordination of the metal atom in the crystal in other cases the metal has 6 octahedral neighbours. 

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