Seven-coordinate intermediates

These observations are, indeed, consistent with an associative activation, generation of a seven-coordinated intermediate (easier in the case of Mo and W than for Cr because their larger sizes produce less steric hindrance) by attack taking place directly upon the metal atom, that is, with an A or a limiting 5 2 mechanism, accompanied by a reaction sequence involving dissociative activation similar to scheme (24) above, viz. 

Discussion of the mechanism of photosubstitution in Cr(III) complexes has largely concerned the possibility of a seven-coordinate intermediate. Such an associative type mechanism is not proven by any means, but there is some circumstantial data to support it and some data that tends to rule out a purely dissociative mechanism involving the photoinduced formation of a discrete five-coordinate intermediate capable of being competitively scavenged by nucleophiles in the medium. 

There are several possible dissociative pathway, in which a five-coordinated intermediate of either tbp or sp geometry are formed. One of them is shown below. There are no evidence for the actual occurrence of associative pathway through seven coordinate intermediate. 

Reversible fac-mer isomerization of the [LaCl3(HMPA)3] complex studied by variable temperature 1H NMR spectroscopy gave evidence for an associative mechanism [82], However it is not possible to distinguish between associative interchange mechanism and seven coordinate intermediate. An equilibrium mixture of [LaCl3(HMPA)3] and [LaCl3(HMPA)4] is also a possibility. 

It is possible to imagine a situation in which a solvent molecule is in such close proximity to the metal ion at the instant a hexacoordinated metal complex is excited that neither rotation nor translation is needed prior to formation of a seven-coordinate intermediate. In this event, k, would not decrease markedly as the solvent viscosity is increased. 

The mechanistic assignments have been mainly based on stereochemical and theoretical, rather than on kinetic grounds. In the absence of evidence for well-defined five- or seven-coordinate intermediates, a clear-cut distinction between the several mechanistic possibilities is likely to remain difficult. Studies of photophysics and photochemistry as a function of solvent mobility may prove fruitful in elucidating the primary mechanisms. 

The reaction of (Co(EDTA)p (EDTA= ethylenediaminetetraacetate) with [OH] proceeds via parallel oxidation of the metal center and abstraction of hydrogen from the ligand the reaction of [Co(NTA)] (NTA = nitrilotriacetate) with [OH]" proceeds via abstration only. Oxidation of these complexes by Brj, [(NCS) ], O and alkylperoxy radicals occurs via an inner-sphere mechanism, possibly involving seven-coordinated intermediates . 

Figure 6. Depiction of the equilibrium between two possible six-coordinate intermediate structures generated via ligand dissocciation from a seven-coordinate intermediate Mn(ll) complex.

Slow rate-determining gain of one ligand to produce a seven-coordinate intermediate is the key to this process. We can represent this reaction as a sequence of a slow and therefore rate-determining formation of a seven-coordinate intermediate, followed by the fast loss of a ligand which is only significant if the ligand lost differs from the one added (5.38). 

Although the nature of seven-coordinate compounds is of intrinsic interest, perhaps of greater significance is the role of expanded coordination spheres as intermediates in the chemistry of octahedral complexes. The preparation of the thermodynamically unfavorable tmn5-Mo(CO)2-(diphos)2 isomer (5) via a seven-coordinate intermediate is an exemplary case of stereochemical control. Plausible seven-coordinate intermediates in the reactions of olefins with pentacarbonyltungstenphenylcarbene (6) can exert stereochemical control of the product distributions. 

The rate of exchange of bonded water with bulk solvent gives evidence that the process is associative, that is, it occurs through a seven-coordinate intermediate. The two reported determinations give (averaged) kinetic parameters of k, = 2.15 x 10 s = 7.8kcal mole ... 

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