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(RNH(2))(5)H2O](3+) (M=RH, R=H, ME, ET, PR M=CR

摘要

The kinetics of the anation reactions of [M(RNH(2))(5)H2O](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 Delta V-not equal 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 shifts the I-a-I-d classification to the I-d side, although no dramatic changes in the reaction mechanism are found. It is concluded that a More O'Ferrall 'continuous' type of approach to the mechanism classification of the substitution reactions is much more useful in this case. [References: 47]