Kinetics and mechanism of base hydrolysis of mer-[Cr(pic)3]0 and [Cr(ox)2(pic)]2− (pic = picolinate, ox = oxalate)

Mer-[Cr(pic)3]0 and [Cr(ox)2(pic)]2− undergo successive base hydrolysis to give chromates(III). Dissociation of the first ligand, pic from [Cr(pic)3]0 and ox from [Cr(ox)2(pic)]2−, proceeds in two stages, namely initial chelate-ring opening followed by slower liberation of the monodentate ligand. Kinetics of both the stages were studied spectrophotometrically in 0.2–0.9 M NaOH solution, under pseudo-first-order conditions. The calculated values of kobs were independent of [OH−]. A mechanism is proposed, where the formation of intermediates in the hydroxo form prevents the monodentate ligand from undergoing chelate-ring closure. Evidence for the formation of an intermediate with O-bonded picolinate is given. The effects of pH and the complex composition on the reactivity are discussed.