Hexacoordinate complexes

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. 

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). 

Roewer et al. have used salene-type ligands for the preparation of neutral penta- and hexacoordinated silane complexes.834-836 Interestingly, the reaction of the acid form of the salene ligand reacts with organotrichlorosilanes in the presence of diethylamine to form pentacoordinated enamine silane complexes 849-851, whereas the disodium salt of the salene ligand reacts with phenyltrichlorosilane providing the hexacoordinated chlorosilane complex 852 (Scheme 119).834 The pentacoordinated complex 849 was also obtained when the hexacoordinated complex 852 was reacted with triethylamine (Scheme 119). 

With iV,iV,iV, iV -tetramethylethylenediamine (TMEDA), HSiCl3 forms the expected hexacoordinated complex (TMEDA)SiHCl3 858, whereas with A,iV,iV, iV -tetraethylethylenediamine (TEEDA), a facile redistribution reaction takes place, which gives rise to the formation of the hexacoordinated complex (TEEDA)SiH2Cl2 859 and SiCU... 

PHYSICAL ORGANIC CHEMISTRY NOMENCLATURE Hexacoordinate complex,... 

Fig. 6. The hexacoordinate complex (lb) optimized from a model of the X-ray crystal structure (PDBid 1J8U). The conformer la has the imidazole ring in His290 rotated 180° compared to that of the X-ray crystal structure (and lb) and is slightly more stable than lb. Distances are given in angstroms.

When either the 2(6)2 + solution resulting from this process or a solution prepared from a sample of isolated solid 2(6)2 + (BF4 )2 were electrochemically reduced at — IV, the tetracoordinate catenate was quantitatively obtained. The cycle depicted in Fig. 14.3 was thus completed. The changeover process for the monovalent species is faster than the rearrangement of the Cu(II) complexes, as previously observed for the previously reported simpler catenate.16 In fact, the rate is comparable to the CV timescale, and three Cu species are detected when a CV of a CH3CN solution of 2(6)2 + (BF4 )2 is performed. The waves at + 0.63 V and —0.41V correspond, respectively, to the tetra- and hexacoordinate complexes mentioned above. By analogy with the value found for the previously reported copper-complexed catenane,16 the wave at —0.05 V is assigned to the pentacoordinate couple (Fig. 14.4b). 

All these observations led Bernard260 to a study of the behaviour of the spirophosphorane 206 in the presence of trimethylamine or pyridine (Scheme 56) an insoluble complex (212) rapidly precipitates with the former, whereas the addition of the latter progressively shifts the PH NMR doublet of 206 to high field, with <5 = — 94 ppm (for one equivalent added) or S = — 69 ppm (for two equivalents). This was interpreted as resulting from the formation of the hexacoordinated complex 213 in rapid equilibrium with 206. Similar results were reported by Munoz and coworkers261 and by Schmidtpeter et al262,... 

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

The first ionic hexacoordinate complexes, silicon-tris-acetylacetonate cations (176), were reported as early as 1903 by Dilthey196a and Rosenheim and coworkers196b. Subsequently many other /i-diketonate complexes were studied197-199, and the subject was extensively reviewed7,200 and will not be discussed further here. 

TABLE 21. 29Si chemical shifts for ionic hexacoordinate complexes (in DMSO)203... 

Geometrically unrestricted hexacoordinate complexes are generally octahedral, as, for 219... 

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