The binuclear cyclooctatetraene sandwich compounds (C8H8)2M2 of the first row transition metals: Analogues of the perpendicular dimetallocenes

Abstract

Cyclooctatetraene has been found experimentally to form the perpendicular binuclear sandwich compounds (η3,η3-C8H8)2Ni2 and [η5,5-(Me3Si)2C8H6]2Ti2. The entire series of such cyclooctatetraene sandwich compounds (C8H8)2M2 has now been examined by density functional theory. Bis(pentahapto) folded η5,η5-C8H8 rings are found in the lowest energy (C8H8)2M2 derivatives of the early transition metals Ti, V, Cr, and Mn. The lowest energy Ti derivative is the singlet spin state structure (η5,η5-C8H8)2Ti2 containing two such rings with geometry close to the experimental structure for the known silylated derivative [η5,5-(Me3Si)2C8H6]2Ti2. The lowest energy (C8H8)2M2 structures for V, Cr, and Mn are (η5,η5-C8H8)M2(η3,η3-C8H8) having metal–metal multiple bonds of various orders. For (η5,η5-C8H8)V2(η3,η3-C8H8) singlet and triplet spin state structures are approximately equal in energy. For (η5,η5-C8H8)M2(η3,η3-C8H8) (M=Cr, Mn) the triplet state structures are the lowest energy structures. The lowest energy (C8H8)2M2 structures for the later transition metals Fe, Co, Ni are triplet spin state structures for Fe and Co and a singlet spin state structure for Ni. All three such (η3,η3-C8H8)2M2 (M=Fe, Co, Ni) structures have two bis(trihapto) η3,η3-C8H8 rings containing uncomplexed CC double bonds.