Hexacoordination

Eig. 1. Types of chelates where (1) represents a tetracoordinate metal having the bidentate chelant ethylenediamine and monodentate water (2), a hexacoordinate metal bound to two diethylenetriamines, tridentate chelants (3), a hexacoordinate metal having triethylenetetramine, a tetradentate chelant, and monodentate water and (4), a porphine chelate. The dashed lines iadicate coordinate bonds. 

Chromium(III) forms stable salts with all the common anions and it complexes with virtually any species capable of donating an electron-pair. These complexes may be anionic, cationic, or neutral and, with hardly any exceptions, are hexacoordinate and octahedral, e.g. ... 

P,0-Heterocycles with P-atom hexacoordinated owing to donor interaction, participation in enzymatic reactions 98ACR535. 

The disappearance of the first band at 600 cm 1 in IR spectra of melts K2TaF7 - KBr takes place at a lower KBr concentration compared to both KF and KC1 containing systems (see Fig. 78, b). If KBr concentration equals or exceeds 0.5 mol fraction, a single band, which is attributed to hexacoordinated complex ions is observed. In the case of the K2TaF7 - KI molten system (Fig. 78, c) this phenomenon is observed at KI concentration as low as 0.4 mol fraction. 

The third factor that strongly affects the equilibrium between hexa- and heptacoordinated complexes (85) is the nature of the second, outer-sphere cations. Increasing the ionic radii of the cations causes the equilibrium in Equation (85) to shift to the left, forming mostly hexacoordinated complexes MeF6 The mechanism of interionic equilibrium in fluoride melts can be presented schematically as follows ... 

Higher temperatures, increased K2TaF7 concentrations and other factors mentioned above shift the equilibrium in Equation (174) to the left and lead to the formation of coarser tantalum particles. Form this point of view, it can be concluded that smaller hexacoordinated complexes, TaF6 lead to the formation of coarser tantalum powder, whereas the predominant presence of larger heptacoordinated complexes ions initiates the formation of finer particles. 

Nakajima reported the use of a chiral bipyridine N,N -dioxide 18 in the desym-metrization of acyclic meso epoxides (Figure 7.3). Although the enantioselectivity was not as high as in the method developed by Fu for meso-stilbene oxide (90% ee vs. 94% ee), it was higher for the same aliphatic epoxide (74% ee vs. 50% ee) [57]. Nakajima showed that mono-N-oxide derivatives 19 and 20 were much less effective than 18 in tenns of both yield and enantioselectivity, and accordingly proposed a unique mechanism for 18 involving a hexacoordinate silicon intermediate coordinated to both N-oxides of the catalyst. 

Photochemistry of hexacoordinate complexes of the heavier transition metals. P. C. Ford, J. D. Petersen andR. E. Hintze, Coord. Chem. Rev., 1974,14, 67-105 (109). 

While ytterbium(II) benzamidinate complexes have been known for many years/ the synthesis of the first divalent samarium bis(amidinate) required the use of a sterically hindered amidinate ligand, [HC(NDipp)2l (Dipp = C6H3Pr2-2,6)/ As illustrated in Scheme 54, the dark green compound Sm(DippForm)2(THF)2 (DippForm = [HC(NDipp)2] ) can be prepared by three different synthetic routes. Structural data indicated that hexacoordinated... 

R2Sn(rV)] derivatives of N-Bz-Gly-Gly and N-Bz-Gly-Gly-Gly were found to involve both dicarboxylate binding to yield hexacoordinated Sn centers, and dimeric tetraorganodistannoxanes in which the carboxylates bind... 

Mossbauer spectroscopy has also been widely used to investigate the structures of dialkylstannylene derivatives of carbohydrates in the solid state. The usual magnitude of A = 2.78-3.07 mms indicated a coordination number larger than four, with Sn centers in a penta- or hexacoordinated environment. 

Kumar Das et al. obtained a unique hexacoordinated tetraorganotin compound, bis C,N-[3-(2-pyridyl)-2-thienyl] diphenyltin(IV). The crystal... 

Let us examine the properties of molecules where a central atom M is surrounded by five ligands A, B, C, D, E. We assume that the ligands are at the vertices of a trigonal bipyramid. This assumption is adequate for most pentacoordinate complexes but we ougth to mention that the description of stereoisomerization we propose could be applied if another polytopal form—the tetragonal pyramid, for example—was the stable one. The same type of description has already been undertaken for hexacoordinate octaedral complexes . ... 

First isolated from D. desulfuricans (28), desulfoferrodoxin (Dfe) was also isolated from D. vulgaris (29). D is a 28-kDa homodimer that contains two monomeric iron centers per protein. These iron centers were extensively characterized by UV/visible, EPR, resonance Raman, and Mossbauer spectroscopies (30). The data obtained were consistent with the presence of one Dx-like center (center I) and another monomeric iron center with higher coordination number (penta or hexacoordinate), with 0/N ligands and one or two cysteine residues (center II). Comparison of known Dfx sequences led to the conclusion that only five cysteines were conserved, and that only one of them could be a ligand of center II (31). 

Chemical assemblies around hexacoordinated phosphorus atoms can be anionic, neutral or cationic depending upon the nature of the hgands. This is described in the following paragraphs. 

When the three ligands are the same, procedures similar to the ones developed by Hellwinkel [16,17], Koenig [25-28] and coworkers can be utilized -that is the one-time addition of three equivalents of bidentate ligands to PCI5 to afford the hexacoordinated phosphate anions. Ligands as varied as malonic acid,3,3,3-trifluoro-2-hydroxy-2-trifluoromethyl-propionic acid or oxalic acid have been used to form 5,6, and 7,respectively [29]. Aromatic 1,2-diols are also particularly good ligands as tetrachloropyrocatechol [30, 31], 4-methylpyro-... 

Fig. 4 Statistical distribution among possible hexacoordinated phosphates from the reaction of three different ligands (aa,bb and cc, 1 1 1 ratio) and PCl5.L,L =(aa),or (bb),or (cc)...

Scheme 1 Synthesis of hexacoordinated phosphates 14 containing 1,3,2-dioxaphosphorinane rings...

Following a similar protocol, novel hexacoordinated phosphate anions 19 to 22 bearing two different dioxo ligands could be simply prepared as their di-methylammonium salts from the reaction of tetrachlorocatechol derived phos-phoramidite 23 with a variety of symmetrical diones other than o-chloranil (a-diketones or orf/zo-quinones) and subsequent addition of tetrachlorocatechol to the corresponding phosphorane 24 (Scheme 3) [42]. 

Akiba and coworkers reported a new type of anionic hexacoordinated phosphorus derivative exemplified by compound 29 bearing a stable three-membered dioxaphosphirane ring [48]. Phosphoranide 34, generated from P-H phosphorane 35 with KH in the presence of 18-crown-6, was exposed to dioxygen to give crystalline 29 (50%). Phosphate 29 was found to be quite stable as a solid to ordinary room light as well as to the air at ambient temperature for several months (Scheme 5). 

Scheme 5 Synthesis of hexacoordinated phosphate 29 containing a dioxaphosphirane ring...

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