The mixed sandwich compounds C5H5MC7H7 of the first row transition metals: variable hapticity of the seven-membered ring

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The mixed sandwich compounds (η5-C5H5)M(η7-C7H7) (M = Ti, V, Cr) have been known for approximately 50 years. The vanadium derivative (‘trovacene’) has recently been shown to be a useful precursor for the preparation of a variety of paramagnetic sandwich compounds. In this connection the structures of the complete series of the first row transition metal mixed sandwich compounds C5H5MC7H7 (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been investigated by density functional theory. The experimentally known parallel ring sandwich structures (η5-C5H5)M(η7-C7H7) are found for the Ti, V, and Cr derivatives. Analysis of the frontier molecular orbitals of these complexes indicates that, for the metal d-orbitals, the z 2 orbital is essentially non-bonding, the x 2–y 2 and xy orbitals are δ bonding to the seven-membered ring, and the xz and yz orbitals are π bonding to the five-membered ring. For the later transition metals, (η5-C5H5)M(η5-C7H7) structures with pentahapto seven-membered rings are preferred for iron and manganese and (η5-C5H5)M(η3-C7H7) structures with trihapto seven-membered rings are preferred for cobalt and nickel, leading to either 17- or 18-electron complexes in all cases. A higher energy parallel sandwich structure (η5-C5H5)Co(η7-C7H7) with a quartet spin state and a formal 21-electron cobalt configuration is also found for the cobalt derivative.