Hexa-coordinated

Molecular and electronic structures of penta- and hexa-coordinate silicon compounds. S. N. Tan-dura, M. G. Voronkov and N. V. Alekseev, Top. Curr. Chem., 1986,131, 99 (1008). 

Even for pentacoordinate molecules, and a fortiori for hexa-coordinate ones, graphs may become inadequate because they involve too many points and lines. Matrices have been used to overcome this difficulty . 

This means that there is a possibility of forming a hexa-coordinate intermediate. The finding that the exchange rate constant for Re(VII) is higher than that for Tc(VII) suggests that Re04 can expand its coordination number from 4 to 6 more readily [7,17]. 

Solvation of the divalent copper ion requires some special remarks. In solid state hexa-coordinate Cu2+ shows Jahn-Teller distortion the metal-ligand bonds of four ligands in the equatorial plane are shorter... 

The third class of haemoproteins, with hexa-coordinate low-spin iron, are the cytochromes. First discovered by McMunn in 1884, they were rediscovered in 1925 by David Keilin. Using a hand spectroscope he observed the characteristic absorption (Soret) bands of the three cytochromes a, b and c in respiring yeast cells, which disappeared upon oxygenation. 

In the dithionite-reduced state (Figure 17.11a), PN, the P-cluster can be considered as two [4Fe-3S] clusters bridged by a hexa-coordinate sulfur. In the Pox state, which is oxidized by two electrons relative to PN, two of the iron atoms Fe5 and Fe6 have moved away from the central sulfur atom, and are now coordinated by the amide nitrogen of Cysot87 and the hydroxyl of Seral86, maintaining the irons in a four-coordinate state. 

In simple binary structures, in which no other relevant coordinations (or bonds) can be considered, the ratio m/n will be equal to the stoichiometric compositional ratio. For instance, we will write NaCl6/6 to represent the hexa-coordination (in this case octahedral coordination) of Cl around Na (and vice versa) in sodium chloride. Similarly we will have ZnS4/4 PH3/1 CsCl8/8 CaF8/4 UCI9/3 etc. Moreover, it is possible to add modifiers to the coordination numbers in order to specify not only topological but also geometrical characteristics of the primary coordination sphere. 

Our collective mechanistic studies are consistent with the indicated catalytic cycle. Notably, the catalyst engages primary alcohols in rapid and reversible dehydrogenation, yet the coupling products, which are homoallylic alcohols, are not subject to oxidation as coordination of the homoallylic olefin to the catalyst provides a hexa-coordinate 18-electron complex lacking an open coordination site for p-hydride elimination (Scheme 14). 

Water NMRD profiles acquired for other complexes and proteins always exhibit the same features of hexaaqua nickel(II). As an example, we report here the profile of the hexa-coordinate nickel(II)-substituted bovine carbonic anhydrase II 54,55) (Fig. 15). As in the aqua complex, (i) the low-field profile is flat, (ii) no dispersion appears, the cOg dispersion being quenched in S = 1 complexes with large static ZFS 56) (see Section I.A.5) and the... 

The larger Mg " " ion is hexa-coordinated. The water exchange rate for [Mg(H20)6] " (40) lies between those of the d-transition metals [Co(H20)6] + and [Ni(H20)e] + (Tables I and Table II), and reflects the order of ionic radii of these three ions. The measured activation volume is... 

The eight-sided geometric solid corresponding to a hexa-coordinate metal-ligand complex, as observed with fer-ricyanide [Fe(III)CN6] or calcium ion complex with EGTA. 

For phosphorus involving tetra-, penta- or hexa- coordination the importance of d electrons (even if quantitatively small) must not be underestimated <1968,5) it is especially easy to detect as it strongly decreases the paramagnetic shift from which a net high-field shift arises. To consider these -contributions on a theoretical basis is rather complex<1964,1) and is at present limited to qualitative conclusions/1968,5) The case of the assumed PBr4Q ion, for which a shift of — 150 p.p.m. is observed (compared with —225 for PBr3) is rather interesting in this respect/1969 111... 

Penta-coordinated fluorophosphoranes have been extensively considered by Schmutzlera965,12andl4) and discussed in detail in ref. 1966, 1, p. 281 seq. The special interest was in the existence of two well-defined types of fluorine substituents, as well as for the hexa-coordinated (PVI)e compounds. Fluorine exchange generally occurs by pseudorotationf but steric hindrance, substituent conjugation and low temperatures may stabilize fluorine atoms in their respective position. As a rule (see Section... 

The interpretation of observations on penta- or hexa- coordinated compounds is not so clear as CF3 groups may occupy axial positions, even when competing with fluorine substituents as in (44).<1968,41)... 

The same appears to be true for PVI0 hexa-coordinated compounds such as PFsSe for which 2J(Fax-Feq) is 48 Hz(1968>46> and in (CF3)2PF3He in which all the fluorine atoms are equatorial but can be differentiated as shown in 142 <1968>and finally 143.(1966,33>... 

In the context of bidentate ligands, and in a similar vein to the polyfunctional aryloxides discussed above [141, 142, 146, 147], the chelation of a metal centre by deprotonated j5-diketones is a recurrent feature of monomeric organooxide complexes of aluminium. The employment of such ligands results in the observation of simple hexa-coordinate complexes which incorporate three [RC(0)C(H)C(0)R ] moieties (R = R = Me [177,178], CF3 [179], Ph [180, 181] R = f-Bu, R = CF3 [182]). More recently, the cocrystallisation of M(acac)3 (M = Al, Cr) has allowed the crystallographic study of metal disorder in a series of solid solutions of stoichiometry Ali, cCi x(acac)3 (x = 0.02-0.91) [183, 184]. Chelation of the metal centre similar to that reported in monomeric ] -diketonates has also been noted in the presence of... 

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