Nickel complexes with porphyrin, formation

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

A nickel-imido complex and a ruthenium-imido complex boimd by an ancillary porphyrin ligand also react with olefins, in this case to generate aziridine products (Equations 13.74 and 13.75). A pathway for the formation of an aziridine that occurs by a [2+2] addition of the olefin across the metal imido unit, followed by reductive elimination of the aziridine, was proposed for the reaction of the nickel complex, although the azametaUacycle was not observed directly. Because of the lack of coordination site cis to the imido group in the ruthenium-porphyrin system, the transfer of the imido group likely occurs by direct reaction of the imido group with the olefin. ... 

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