Effects of Increased Ligand Fields of the Second- and Third-Row Transition Metals on the Preferred Structures of Sandwich Compounds of Stoichiometry C12H12M.

The lowest-energy sandwich structures for the (C n H n )-M-(C m H m ) (n = 5, 6; m = 7, 6; m + n = 12; M = Zr to Pd and Hf to Pt) systems are all singlet or doublet spin state structures, reflecting the increased ligand-field strengths of second- and third-row transition metals relative to first-row transition metals. For Groups 7 and 8, this differs from the first-row transition metals, for which the lowest energy C12H12M sandwich structures for iron and manganese are high-spin triplet and quartet-spin state dibenzene-metal (η6-C6H6)2M (M = Fe, Mn) structures, respectively. For palladium and platinum, the lowest-energy C12H12M (M = Pd, Pt) structures have two dihapto benzene ligands coordinating to the central metal atom, each through only one of their three CC double bonds. This contrasts with the nickel C12H12Ni system, in which a singlet spin state (η6-C6H6)-Ni-(η2-C6H6) structure having one hexahapto and one dihapto benzene ligand is strongly energetically preferred. Similar doublet-spin state (η6-C6H6)-M-(η2-C6H6) (M = Co, Rh, Ir) structures are energetically preferred for the three Group 9 metals. The energetically preferred structures for all of the early transition metals are low-spin (η6-C6H6)2M and (η5-C5H5)-M-(η7-C7H7) structures, in which all 12 ring carbon atoms are bonded to the central metal atom.