Base-catalysed linkage isomerization of pentaamminechromium(III) complexes of urea and formamide

Abstract

The complexes Cr(NH₃)₅L³⁺ [L = OCHNH₂, OC(NH₂)₂ and OC(NHMe)₂] have been synthesized as the O-bound isomers. At 25ºC in aqueous base (pH c. 10-14) they isomerize to the deprotonated N-bound isomers without competitive hydrolysis. Reacidification regenerates the O-bound isomer rapidly and completely. The second-order rate constants and pK_a values of the amide complexes (I 1.0 M, 25ºC) are respectively: 37.0 ± 1.5 dm³ mol^-1 s^-1 and 11.72 ± 0.06 for formamide; 0.324 ± 0.004 dm³ mol^-1 s^-1 and l3.50 ± 0.05 for urea; 0.0375 ± 0.0020 dm³ mol^-1 s^-1 and 14.0 ± 0.2 for N,N'-dimethylurea. The observations for chromium(III) parallel similar behaviour in cobalt(III) and rhodium(III) analogues, but are unusual for chromium(III) chemistry.