Steric effects on the substitution of pentaamine-chromium(III) and -rhodium(III) complexes. Anation reaction rate constants as an indicative of the dissociative shift of the mechanism on crowding the [M(RNH2)5H2O] (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) complexes |
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Authors: | Gabriel Gonzlez Manuel Martinez |
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Institution: | Department de Química Inorgànica, Universitat de Barcelona, Diagonal 647, E-08028, Barcelona, Spain |
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Abstract: | The kinetics of the anation reactions of M(RNH2)5H2O]3+ (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) with several ligands (H3PO4/H2PO4?, H3PO3/H2PO3?, CF3COO?, Br?, Cl?, SCN?) have been studied at different temperatures and acidities at I = 1.0 M (LiClO4. Results obtained for the anation rate constants and thermal activation parameters are compared with the previously published data for R = H, in order to establish the effects of the amine substituents in the reaction mechanism proposed for the substitution reactions of these complexes. The results obtained are interpreted on the basis of a mechanism where the bond formation process is more important in the substitution on M = Cr complexes than in that of the M = Rh complexes, as already pointed out for the published ΔΛ≠ values for the water exchange on these systems. A simple Langford-Gray classification becomes inadequate to describe these situations where the increase of the steric demand of the amine substituents shifta the Ia-Id classification to the Id side, although no dramatic changes in the reaction mechanism are found. It is concluded that a More O'Ferall ‘continuous’ type of approach to the mechanism classification of the substitution reactions is much more useful in this case. |
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Keywords: | Kinetics mechanism Substitution reactions Chromium complexes Rhodium complexes Amine complexes |
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