Single-Crystal Broad-Line 59Co and 55Mn NMR in CoMn(CO)9

Abstract

The 59Co and 55Mn NMR was studied in a single crystal and solution of CoMn(CO)9. Two pairs of independent crystallographic sites were found related by the twofold axis and the mirror plane of the monoclinic crystal. The z axes of the 59Co and 55Mn electric-field-gradient tensors were found to be coaxial. The 59Co quadrupole coupling parameters were determined to be (1) e2qQ/h=129.15 MHz, η=0.1558 and (2) e2qQ/h=128.44 MHz, η=0.0937. The z component of the chemical shift was found to be identical with that for the compounds X3MCo(CO)4 (M is Si, Ge, Sn, and Pb; X is Cl, Br, I, C6H5) which have near C3ν symmetry. The 55Mn quadrupole coupling parameters were determined as (1) e2qQ/h= ∼ 20 MHz, η undetermined and (2) e2qQ/h=19.940 MHz, η=0.004. The z component of the chemical shift was found to be identical with that for the manganese pentacarbonyl halides which have approximate C4ν symmetry. The structure of CoMn(CO)9 has not been determined by x-ray crystallography as it always forms twin crystals, but the quadrupole coupling and chemical shift give evidence that there is threefold symmetry at the 59Co site and fourfold symmetry at the 55Mn site. Because the z axes of the electric-field-gradient tensors at the 55Mn and 59Co site are coaxial, they are assumed to lie along the Co–Mn bond. Information about the packing of the molecules in the crystal is therefore obtained. The bonding in CoMn(CO)9 was compared to the compounds Mn(CO)5MX3 and Co(CO)4MX3. The electrons forming the Co–Mn bond were found to be unequally shared by the two metal atoms with Co having the larger MO coefficient.