Structure and reactivity of an intermediate in the two-electron reduction of dimolybdenum(III) by a hydride ligand in the MoHMo system

The raaction between the μ-hydride dimolybdenum(III) complex [Mo₂Cl₈H]^3- and carboxylic acids in aqueous solution produces a violet species which slowly decomposes to yield the quadruply bonded dimolybdenum(II) complex Mo₂(O₂CR)₄. The violet intermediate in the reaction of [Mo₂Cl₈H]^3- with glycine was isolated by ion exchange chromatography and crystallized. The crystals of [Mo₂H(OH)(gly)₂Cl₄] (1) are monoclinic, space group P2₁/n with a = 9.536(4), b = 14.792(4), c = 9.339(3) A ̊, β = 109.99(3)°, V = 1283(1) A ̊³ and Z = 4. The two Mo(III) ions in this complex are bridged by one hydride ligand one OH^- group and two glycine groups through the carboxylic end. The MoMo distance is 2.353(5) Å, the shortest found for the MoHMo system. [(H₂O)₂MoH(OH)(gly)₂Mo(H₂O)₂]⁴⁺ is the structure proposed for the violet intermediate II in the reaction 4gly+[Mo₂Cl₈H]^3-→ II→H⁺+[Mo₂(gly)₄]Cl₄+4Cl^- This structure was derived from structure 1, the high ionic charge of about 4+ and the absence of chloride in this species. The decomposition reaction of II and the relation of II with the red intermediate III of the reaction [Mo₂Cl₈H]^3-+H₂O→III→H₂+[Mo₂(OH)₂Cl₂] ²⁺+6 Cl^-] +H⁺ is discussed.