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

Abstract

. J. Chem. 55,3166 (1977).For the aquation of (CH3NH2)SRhC12+, the first order rate coefficients are represented by AH,,* = 101.9 kJ mol-' and AS,,* = -50.2 J K-' mol-' in 0.1 M HCIO,, while for base hydrolysis the rate is first order in [(CH3NH2),RhC12+] and [OH-] at ionic strength 0.10 M and the rate coefficients (in M-' s-') are represented by AHOH* = 108.6 kJ mol-' and ASoH* = 74.1 J K-I mol-'. Acid dissociation constants are reported for (RNH2),MOHZ3+ (R = H or CH,; M = Rh or Co), and these, combined with spectral data, show CH3NH2 to be a poorer electron donor than NH3 in complexes of this type, contrary to expectations. The comparative kinetics of reactions of (RNH2),MC12+ support the assignment of an I, mechanism to aquation when M = Rh or Cr, Id to aquation when M = Co, and D,, for base hydrolysis in all these cases. THOMAS WILSON SWADDLE. Can. J. Chem. 55,3166 (1977). [Traduit par le journal]

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.…”