Seven-coordinate system

For a substitution reaction involving a six-coordinated species we can envision two possible transition states. One involves an Sn2 mechanism (substitution, nucleophilic, and bimolecular) where a six-coordinate complex accepts the ligand to be added. This produces a seven-coordinate system as the intermediate that then releases one of the substituted ligands, thus returning to a new six-coordinate system. The second possible transition state involves an Sn2 mechanism... 

It is not surprising that the substitution processes on seven-coordinate 3d metal ions follow an interchange, rather than a limiting dissociative mechanism. In the case of seven-coordinate Fe(III) complex we even found an associative interchange mechanism for the substitution of solvent molecules as a result of the high n-acceptor ability of the fully conjugated pentadentate ligand system present in its equatorial plane 44). 

In addition to the halide systems there are a number of thallium(I) salts of thallium(III) complex anions in which the overall stoichiometry implies, incorrectly, the presence of thallium(II) species. These include the sulfate, selenate,369 acetate370 and oxalate371 derivatives. An X-ray determination has confirmed that Tl(OAc)2 is indeed Tl[Tl(OAc)4], in which the anions are linked through seven-coordinate Tl+, with some evidence for a stereochemically active inert pair.370 In T1[T1(0H)(S04)2], the structure consists of sheets of linked anions with Tl+ ions.372... 

A-Benzoylphenylhydroxylamine (BPHA) reacts with Nb20 in bioling acidic aqueous solutions to yield a monomeric seven-coordinated niobium compound [NbO(BPHA)3]. The hydroxylamine ligand could be displaced by tropolone or 8-quinolinol, and the ratedetermining step was shown to be associative (equation 38).427 Oxoniobium and tantalum(V) 8-quinolinolates have also been reported. The optimal conditions for the formation and extraction of the [Nb0(C6H402)3]3 ion formed in the niobium(V)-pyrocatechol system, have been investigated.428... 

Figure 2.20 illustrates the unit vectors er and in a cylindrical coordinate system. Based on a geometric interpretation in the limit A0 - 0, develop expressions for der/d8 and deg/dd. Explain why the remaining seven unit-vector derivatives vanish (e.g., der/dr= 0). 

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