Heptacoordination

In addition, a chlorine ion most probably replaces the fluoride ion located on the rotation diad axis of the heptacoordinated complex with C2v symmetry otherwise splitting of the band would be observed. Replacement of the ligand in any other position would reduce the symmetry from C2v to Cj. 

The most common property of molten systems containing tantalum or niobium is the ionic equilibrium between hexa- and heptacoordinated complexes, with the general compositions MeFg and MeF6X(n+I) ... 

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 ... 

The anions MeF6 and X approach each other closely to form the heptacoordinated complex MeF6X(n+1)", or separate from one another, according to the polarization potential of the outer-sphere cation (alkali metal cation -M+). This process is unique in that the mode frequencies of the complexes remain practically unchanged despite varying conditions. This particular stability of the complexes is due to the high charge density of Ta5+ and Nbs+. 

The scheme of the interaction mechanism (Equation 88) testifies to an electro-affinity of MeFe" ions. In addition, MeFe" ions have a lower negative charge, smaller size and higher mobility compared to MeF6X(n+1> ions. The above arguments lead to the assumption that the reduction to metal form of niobium or tantalum from melts, both by electrolysis [368] and by alkali metals, most probably occurs due to interaction with MeF6 ions. The kinetics of the reduction processes are defined by flowing equilibriums between hexa-and heptacoordinated complexes. 

It should be noted that in addition to changes in K2NbF7 or K2TaF7 concentrations that afford control over the complex structure and electrolysis parameters, the cation type also affects the equilibrium between the complex ions. The heptacoordinated complexes become increasingly dominant when progressing along the cation series from Li to Cs. 

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. 

Brand-new results show the existence of heptacoordinated silicon as described in some of the following papers of this chapter, which also contribute to the discussion of mechanistical pathways in the course of nucleophilic substitution reactions at silicon. From these results one may speculate whether compounds with octa- and nonacoordinated silicon may be characterised in the near future. Although it is a problem to assign coordination numbers in -w-bound systems, it is worthwhile to note Jutzi s dccamethylsilicocene with a formal Si-coordination number ten in the oxidation state +2 in this context. With respect to Si(U)-compounds it should be stated that there are further derivatives with the... 

Going back to mechanistic studies it is not clear if the reactions of nucleophiles with hexacoordinated silicon compounds are pure nucleophilic substitutions or if they take a more complex route. However there is another challenge to find whether the silicon atom can accept being in heptacoordination. Such a possible situation has been observed with a tricapped tetrahedron structure of a silane which has been proved to be isosteric with the corresponding germane of which the X-ray structure determination has been carried out. 

From these results we can conclude that heptacoordination at silicon cannot be excluded a priori as a possible intermediate in the course of reactions at silicon. 

A diaryldihydrosilane with a hexacoordinated silicon center, produced through intramolecular coordination, is reported not to react with benzaldehyde, although the silane is capable of reducing silver ion to silver metal.113 There is also a report of a heptacoordinate silicon hydride species with the ability to transfer hydride to trityl cation while remaining inactive toward methanol.108114... 

Compounds having highly coordinate group 14 elements such as hexa- and heptacoordinate silicon26 and pentacoordinate germanium27 are also synthesized by this method (Scheme 3). 

The role of intramolecular n - ct donor-acceptor interactions in compounds TeCl2(OR)2 and TeCl3OR (R=alkyl with B-donor atom) has been elucidated experimentally and with help from ab initio calculations.110 Solid (Me0CH2CH20)2TeCl2 exhibits a network of intramolecular O- -Te and intermolecular O- -Te (267.4 pm) and Cl- -Te (357.7 pm) contacts involving heptacoordinated tellurium. 

R = Me, Bu. Compounds of the series with 244 X = / -Me, j9-NH2, / -Br are monomeric. Compounds of the series with X groups in orffro-positions show dimeric structure and have heptacoordinated Sn atoms. The great variation of bond strength in these compounds can be appreciated from the bond lengths in the dimeric case with R = Bu and X = 0-Cl ... 

Three Sn—O distances ca 0.21 nm, ca 0.25 nm 245 and ca 0.29 nm. The author favours a monomeric structure over a dimeric one, which would imply heptacoordination for the Sn atoms. 

The only example of a ketothiolate adopts a dimeric structure [Bi(PhCH S CH2C 0 Ph)3]2[CH2Cl2] in which one of the three S,0 chelate ligands effects a secondary interaction with a neighboring bismuth [Bi---S 3.494(5) and 3.551(5) A] to impose heptacoordination 62... 

The quite reactive Re(III) heptacoordinated complex, [Re (terpy)2(H20)]3+, denoted by ReCR20)TER, is also included. 

Several triphenylgermanium hydrides have been prepared where the hydride atom is encapsulated by substituents on the phenyl ligands,95,108-111 as illustrated in Equations (84)-(86).110,111 The germanium atom of 42a-c are formally hexacoordinate and that of 43 is heptacoordinate. A tribenzobarellenegermane compound with a trihydro-germane moiety attached to the bridgehead carbon was prepared and characterized (Equation (87)), and this species was found to be exceptionally air stable.1... 

The same happens for the last haemerythrin model complex [Fe2(Htpp-do)(C6H5C02)]3 + (Htppdo = V,V,A A+tetrakis(6-pivalamido-2-pyridyl-methyl)-l,3-diaminopropa-2-ol), in which one Fe(II) atom is heptacoordinate, and the other Fe(II) atom is hexacoordinate. At low temperature (—50°C), in acetone solution this complex reversibly reacts with dioxygen.37 The only electrochemical information is that, in MeCN solution, the original trication exhibits two one-electron oxidations ( Fe(II)Fe(II)/Fe(II)Fe(III) = +0.29 V VS. NHE Fe(II)Fe(III)/Fe(III)Fe(III) =... 

The two heptacoordinated MO units differ from each other in having two different distance for the seventh oxygen. There are only minor difference in other Sm—0 bond distances. The observed bond distances are as follows ... 

Fig. 12. Bond distances and angles in the heptacoordinated dimeric form of Pr DPM) 3 complex...

Fig. 14. The heptacoordinated polyhedron of Yb(acac)3 acimin) complex. The coordinated acimin oxygen (0(7)) is shaded...

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