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 B 12 analog diaquacobinamide [(H 2 O) 2 Cbi(III)] with hydrogen sulfide in water from pH 3 to 11. The reaction proceeds in three steps: (1) the formation of three different complexes between cobinamide and hydrogen sulfide, viz., (HO – )(HS – )Cbi(III), (H 2 O)(HS – )Cbi(III), and (HS – ) 2 Cbi(III); (2) inner‐sphere electron transfer (ISET) in the two complexes with one coordinated HS – ligand to form the reduced cobinamide complex [(H)S]Cbi(II); and (3) the addition of a second molecule of hydrogen sulfide to the reduced cobinamide. ISET does not proceed in the (HS – ) 2 Cbi(III) complex. The final products of the reaction between cobinamide and hydrogen sulfide were found to be independent of pH, and the main product is a complex of cobinamide(II) with the anion radical SSH 2 – .