Abstract

The C5H5MC8H8 complexes (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been investigated by density functional theory. Only for titanium is the octahapto parallel ring sandwich (η5-C5H5)Ti(η8-C8H8) the lowest energy structure; this 17-electron complex has been synthesized and characterized structurally by X-ray crystallography. For vanadium, chromium, and manganese the (η5-C5H5)M(η6-C8H8) (M = V, Cr, Mn) structures with hexahapto C8H8 rings are the lowest energy structures. However, for the vanadium compound the octahapto parallel ring singlet spin state structure (η5-C5H5)V(η8-C8H8) lies only ∼6 kcal/mol above the hexahapto triplet spin state (η5-C5H5)V(η6-C8H8) structure. The chromium and manganese (η5-C5H5)M(η6-C8H8) complexes have been synthesized. For iron the 17-electron tetrahapto structure (η5-C5H5)Fe(η4-C8H8) with four adjacent C8H8 carbons bonded to the iron is the lowest energy structure. For cobalt, two isomeric 18-electron tetrahapto structures, namely, (η5-C5H5)Co(η4-C8H8), with four adjacent C8H8 carbons bonded to the cobalt, and (η5-C5H5)Co(η2,2-C8H8), with two nonadjacent C8H8 double bonds bonded to the cobalt, lie within 5 kcal/mol in energy. Experimental work indicates the existence of both isomers in solution, but only (η5-C5H5)Co(η2,2-C8H8) has been isolated in the crystalline state. The lowest energy structure for the nickel complex is the 17-electron complex (η5-C5H5)Ni(η2-C8H8), with a dihapto C8H8 ring containing three uncomplexed conjugated C═C double bonds.

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