Chlorination of half-sandwich carbonyl complexes: The square-pyramidal cation [(η⁶-C₆Et₆)Mo(CO)₃Cl]⁺

The molybdenum(0) half-sandwich complex tricarbonyl(η⁶-hexaethylbenzene) molybdenum (1) undergoes clean oxidation upon treatment with elemental chlorine in methylene chloride-carbon tetrachloride solution or with phenyliodo dichloride, (C₆H₅)ICl₂, in methylene chloride solution to yield the ionic complex 2 of net composition (C₆Et₆)Mo₂(CO)₃Cl₇ (Et = C₂H₅). This compound was established by a single-crystal X-ray diffraction study to contain the square-pyramidal, diamagnetic d⁴-molybdenum(II) cation [(η⁶-C₆Et₆)Mo(CO)₃Cl]⁺ and a hexachloromolybdate(V) counter anion of expected octahedral geometry (d¹; paramagnetic). The crystals are monoclinic, space group P2₁/n, with four molecules in a unit cell of dimensions a = 15.770(3), b = 19.172(5), c = 10.415(5) Å and β = 106.40(2)°. The structure was solved by the heavy-atom method and refined by full-matrix least-squares calculations to R_w = 0.037 for 1810 observed data. All molybdenum-to-carbon distances are significantly longer in the ionic compound 2 than in the neutral precursor species 1.