BASE-INDUCED CLEAVAGE OF OXO-BRIDGED CHROMIUM(III) DIMERS - KINETIC AND CHROMATOGRAPHIC STUDIES OF [CR(TMPA)(NCS)]2O2+, [CR(TMPA)(NCO)]2O2+, AND [CR(TMPA)(CN)]2O2+ HYDROLYSIS REACTIONS IN ALKALINE MEDIA

The reactivities of [Cr(tmpa)(NCS)]2O2+, [Cr(tmpa)(NCO)]2O2+, [Cr(tmpa)(CN)]2O2+, and [Cr(tmpa)(NCS)2]+toward base hydrolysis have been examined through kinetic and cation-exchange chromatographic determinations. The dimers undergo base-assisted oxo-bridge cleavage accompanied by loss of the pseudohalide ligand according to the overall stoichiometry: [L-(tmpa)CrOCr(tmpa)L]2++ H20 + 20H−→ 2[Cr(tmpa)(OH)2]++ 2L−. Chromatographic investigations of product mixtures from the quenching of [Cr(tmpa)(NCS)]2O2+in 0.1 M NaOH after 1 half-life of the base hydrolysis process at 45 °C demonstrated that both [(tmpa)(SCN)CrOCr(OH)(tmpa)]2+and [Cr(tmpa)(OH)]2O2+are formed as intermediates prior to cleavage of the CrOCr bridge. Rapid conversions of both intermediates to [Cr(tmpa)(OH)]24+in acidic solution suggest that oxo-bridge migration to a cis coordination position accompanies the displacement of the first thiocyanate ligand from [Cr(tmpa)(NCS)]2O2+. The base hydrolysis rate law (kobsd= k0+ kOH[OH−]) for dimers with L−= NCO−(k0= 4.1 × 10−5s−1; koH= 1.07 × 10−3M−1s−1; 40 °. © 1990, American Chemical Society. All rights reserved.