Substitution reactions at octahedral centers. IV. Acid and base hydrolysis of chloropentakis(alkylamine)chromium(III) ions

Abstract

The compounds [Cr(RNH2)5C1](C104)2, where R = H, CH3, C2H5, n-C3H7, and n-C4H9, have been prepared and the replacement of chloride by either water or hydroxide ion subjected to kinetic examination. Below about p H 7 the reaction was acid independent, and AHA* was 22.4 kcal mole-' when ammonia was the inert ligand compared with about 26.4 kcal mole-' when the inert ligand was a primary amine. The corresponding ASA* values increased continuously with R from H to 11-C4HI) At higher pH the rate became inversely dependent upon the acidity of the solution, and AHB* decreased from 26.7 kcal mole-' for ammonia as the inert ligand to 25.4 kcal mole-' for an amine as the inert ligand. For the pH dependent reaction the values obtained for AS,* were much larger than for the p H independent reaction but the pattern of variation as the inert ligands were changed was quite similar in the two cases. The great similarity in the trends of AS* values for the acid and base hydrolysis reactions has been interpreted in terms of a rate-determining step that is primarily dissociative in both cases.

Ligand Substitution Dynamics in Metal Complexes

“…For the replacement of chloride by water in M(RNH 2 ) 5 Cl 2þ , k R¼Me /k R¼H ¼ 22, 0.030 (25 C), and 0.50 (85 C) for M ¼ Co, Cr, and Rh, respectively; for the base hydrolysis, the corresponding ratios are 1.5 Â 10 4 , 225, and 29 and are much larger than can be accounted for by changes in the acidities of the RNH 2 ligands. [136][137][138] These results are consistent with expectations for steric effects in I d , I a , and mid-range I mechanisms for Co, Cr, and Rh, respectively, in the aquation reactions, and for D CB , I Dcb , and mild I dCB in the corresponding base hydrolyses. For aquation and base hydrolysis of Cr(RNH 2 ) 5 Cl 2þ , progression from R ¼ methyl through ethyl and n-propyl to n-butyl results in a less dramatic increase in rate, presumably because increases in either congestion or inductive electron release from the amine ligands favors the dissociative component, and/or because of increasing inhibition of solvation of the complex.…”

Pressure Effects upon the Rates of Aquation of Halopentaamminechromium(III) Ions: Evidence for an Associative Mechanism

“…It is partic~ilarly significant that the aquation rate of this last ion is 22 times ,faster than that of Co(NH,),CI2+, in accordance with steric decompression on going to the transition state via an I, mechanism (23). Solvational differences will also contribute to the rate differences, as pointed out by Parris and Wallace (21), and it is therefore not surprising that the aquation rates increase in Ooflz the Cr(NH2W),-C12+ and Co(NH2R),C12+ series as tlie alkyl chain R is extended beyond methyl; alkyl chain lengthening will make the complexes increasingly "hydrophobic," without significantly increasing steric congestion, as the latter will originate almost entirely in the a carbon atoms.…”

The comparative chemistry of ammine and methylamine complexes of rhodium(III) and cobalt(III)

“…Indeed the methylamine complex aquates 22 times more rapidly than the ammine for M = Co, but 33 times more slowly when M = Cr, at 298 K (3)(4)(5). In the simplest view, this can be attributed to the different roles of steric effects in the mechanistic dichotomy (1,(6)(7)(8), since it is known (9) that marked steric compression exists in both of these methylamine complexes, although there are fewer nonbonded contacts in the chromium one (as expected, since the ionic radius of Cr3' is 61.5 pm as opposed to 52.5 pm for Co3' (10)).…”