Kinetics and mechanism of substitution reactions of (N,N-dimethylformamide)pentaammine–cobalt(III) and –rhodium(III) ions in DMF as solvent

Abstract

The exchange of N,N-dimethylformamide (DMF) between M(NH3)5DMF3+ and solvent in acidic DMF solutions proceeds with pseudo-first-order kinetics governed by the parameters kex(55 °C) = 8 × 10−5 and 1.1 × 10−4 s−1, ΔHex* = 113 and 99 kJ mol−1, ΔSex* = +22 and −20 J K−1 mol−1, and ΔVex* = +3.2 and −1.4 cm3 mol−1 for M = Co and Rh respectively. For M = Co (but not Rh), the exchange rate was much faster in the absence of acid, and reduction of Co(III) to Co(II) was significant at low acidities. Solubility problems precluded the measurement of meaningful rate data for the reactions of M(NH3)5DMF3+ with nucleophiles X− in acidic DMF solution except for the case M = Co with X = Br−, for which a limiting first-order rate coefficient 2.5 times kex was obtained for the formation of Co(NH3)5Br2+ at high [Br−] at 55 °C. Despite this last result, the mechanism of simple substitution reactions is evidently dissociative interchange (Id) for pentaamminecobalt(III) complexes in general, but associative interchange (Ia) for pentaamminerhodium(III) analogues.