Kinetics and mechanism of the reaction of hydrogen sulfide with diaquacobinamide in aqueous solution.

We conducted a detailed kinetic study of the reaction of the vitamin B12 analog diaquacobinamide ((H2O)2Cbi(III)) with hydrogen sulfide in water from pH 3 to 11. The reaction proceeds in three steps: (i) formation of three different complexes between cobinamide and hydrogen sulfide, viz. (HO-)(HS-)Cbi(III), (H2O)(HS-)Cbi(III), and (HS-)2Cbi(III); (ii) inner-sphere electron transfer (ISET) in the two complexes with one coordinated HS- to form the reduced cobinamide complex [(H)S]Cbi(II); and (iii) addition of a second molecule of hydrogen sulfide to the reduced cobinamide. ISET does not proceed in the (HS-)2Cbi(III) complex. The final products of the reaction between cobinamide and hydrogen sulfide were found to be independent of pH, with the main product being a complex of cobinamide(II) with the anion-radical SSH2-.