title: Ligand substitution behavior of a simple model for coenzyme B12.
abstract: The ligand substitution reactions of trans-[CoIII(en)2(Me)H2O]2+, a simple model for coenzyme B12, were studied for cyanide and imidazole as entering nucleophiles. It was found that these nucleophiles displace the coordinated water molecule trans to the methyl group and form the six-coordinate complex trans-[Co(en)2(Me)L]. The complex-formation constants for cyanide and imidazole were found to be (8.3 +/- 0.7) x 10(4) and 24.5 +/- 2.2 M-1 at 10 and 12 degrees C, respectively. The second-order rate constants for the substitution of water were found to be (3.3 +/- 0.1) x 10(3) and 198 +/- 13 M-1 s-1 at 25 degrees C for cyanide and imidazole, respectively. From temperature and pressure dependence studies, the activation parameters delta H++, delta S++, and delta V++ for the reaction of trans-[CoIII(en)2(Me)H2O]2+ with cyanide were found to be 50 +/- 4 kJ mol-1, 0 +/- 16 J K-1 mol-1, and +7.0 +/- 0.6 cm3 mol-1, respectively, compared to 53 +/- 2 kJ mol-1, -22 +/- 7 J K-1 mol-1, and +4.7 +/- 0.1 cm3 mol-1 for the reaction with imidazole. On the basis of reported activation volumes, these reactions follow a dissociative mechanism in which the entering nucleophile could be weakly bound in the transition state.
authors.affiliation: Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany.
authors.forename: M S
authors.initials: MS
authors.lastname: Hamza
authors.forename: C
authors.initials: C
authors.lastname: Dücker-Benfer
authors.forename: R
authors.initials: R
authors.lastname: van Eldik
mesh_terms: D002021:Buffers; D003035:Cobalt / Q000737:chemistry; D003038:Cobamides / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D008956:Models, Chemical; D013053:Spectrophotometry
chemical_list: D002021:Buffers; D003038:Cobamides; D008024:Ligands; D003035:Cobalt; C000913:cobamamide