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Equilibrium, Kinetic and Leaving Group Effect Studies on Ligand Substitution Reactions of a Simple Model for Coenzyme B 12
Author(s) -
Alzoubi Basam M.,
Hamza Mohamed S. A.,
DückerBenfer Carlos,
van Eldik Rudi
Publication year - 2002
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/1099-0682(200203)2002:4<968::aid-ejic968>3.0.co;2-v
Subject(s) - chemistry , aquation , ligand (biochemistry) , nucleophile , substitution reaction , reaction rate constant , medicinal chemistry , stereochemistry , amine gas treating , equilibrium constant , crystallography , kinetics , catalysis , inorganic chemistry , organic chemistry , biochemistry , physics , receptor , quantum mechanics
The ligand substitution reactions of trans ‐[Co III (en) 2 (Me)H 2 O] 2+ and trans ‐[Co III (en) 2 (Me)NH 3 ] 2+ were studied for CN − , SCN − , N 3 − , and NH 3 as entering nucleophiles. These nucleophiles displace the coordinated H 2 O and NH 3 molecules trans to the methyl group and form the six‐coordinate complex trans ‐[Co(en) 2 (Me)L]. The complex‐formation constant for the displacement of H 2 O by NH 3 was found to be 17.4 ± 1.1 M −1 at 20 °C, and those for the displacement of NH 3 by SCN − and N 3 − were found to be 4.4 ± 1.5 and 3.1 ± 0.4 at 10 °C, respectively. From temperature and pressure dependence studies, activation parameters (Δ H ≠ , Δ S ≠ and Δ V ≠ ) for the reaction of trans ‐[Co III (en) 2 (Me)H 2 O] 2+ with NH 3 were found to be 62 ± 1 kJ mol −1 , +3 ± 5 J K −1 mol −1 and +5.7 ± 0.3 cm 3 mol −1 , respectively, compared to 79 ± 2 kJ mol −1 , +40 ± 6 J K −1 mol −1 and +9.0 ± 0.4 cm 3 mol −1 for the reverse aquation reaction of trans ‐[Co III (en) 2 (Me)NH 3 ] 2+ . Based on the reported kinetic and activation parameters, the substitution of coordinated H 2 O by NH 3 follows an I d mechanism in which the entering nucleophile participates in the transition state. The substitution of NH 3 by SCN − , N 3 − and CN − proceeds via the intermediate aqua complex such that aquation of the amine complex becomes the rate‐determining step at high entering ligand concentration. A detailed comparison with available data in the literature is made.