Reactions at Labile Transition Metal Centers

Ni2+ was very popular in the early days of the investigation of mechanisms of complex formation, since the time-scale for its reactions with simple ligands was so convenient for the then recently developed stopped-flow technique. However, interest has now moved on to other first-row cations, especially to Cu2+. A review of the kinetics and mechanisms of formation of tetraazamacrocyclic complexes concentrates on Ni2+ and Cu2+, and their reactions with cyclam and similar ligands (267). The tetra(4 -sulfonatophenyl)porphyrin complexes of Ni2+ and of Cu2+ react immeasurably slowly with cyanide, but their IV-methyl derivatives do react, albeit extremely slowly. The relevant time scales are hours for removal of Ni2+, months for the removal of Cu2+, by 10-4 M cyanide at pH 7.4 (268). 

Rate constants for complex formation between Mn2+, Co2+, Ni2+, Cu2+ and Zn2+ and 5,10,15,20-tetraphenylporphyrin in acetonitrile correlate with rate constants for acetonitrile exchange at these cations, though of course the reactions with the porphyrins are much (104 to 106 times) slower than the corresponding rates of solvent exchange (269). 

The kinetics of reaction of Co2+, Ni2+, Cu2+, or Zn2+ with terpy in DMSO-water mixtures are complicated, possibly both by the terdentate 

Reactions of NO with water-soluble Fe(II), and Co(II), porphyrin complexes are very fast (k 109 M-1 s 1) and characterized by small positive 

Kinetic studies on complex formation reactions of the tripodal tetra-mine complex [Co(Me6tren)(H20)]2+ with pyridine, 4-methylpyridine, and imidazole yielded activation parameters AH1, and AS. Activation parameters and dependences of rate constants on incoming ligand concentration indicated that the formation mechanism ranged from dissociative for the weaker and bulkier incoming ligands (py, 4-Mepy) to associative for the more basic and less bulky imidazole 2-methylimida-zole occupies an intermediate position (280). 

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