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 25�C 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 pKa values of the amide complexes (I 1.0 M, 25�C) 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.