Analogues of the Lavallo-Grubbs compound Fe3(C8H8)3: equilateral, isosceles, and scalene metal triangles in trinuclear cyclooctatetraene complexes M3(C8H8)3 of the first row transition metals (M = Ti, V, Cr, Mn, Fe, Co, and Ni)

The trinuclear derivative Fe(3)(C(8)H(8))(3) was synthesized in 2009 by Lavallo and Grubbs via the reaction of Fe(C(8)H(8))(2) with a bulky heterocyclic carbene. This fascinating structure is the first example of a derivative of the well-known Fe(3)(CO)(12) in which all 12 carbonyl groups have been replaced by hydrocarbon ligands. The density functional theory predicts a structure having a central Fe(3) equilateral triangle with ～2.9 ? Fe-Fe single bonded edges bridged by η(5),η(3)-C(8)H(8) ligands. This structure is close to the experimental structure, determined by X-ray crystallography. The related hypoelectronic M(3)(C(8)H(8))(3) derivatives (M = Cr, V, Ti) are predicted to have central scalene M(3) triangles with edge lengths and Wiberg bond indices (WBIs) corresponding to one formal single M-M bond, one formal double M═M bond, and one formal triple M≡M bond. For Mn(3)(C(8)H(8))(3), both a doublet structure with one Mn═Mn double bond and two Mn-Mn single bonds in the Mn(3) triangle, and a quartet structure with two Mn═Mn double bonds and one Mn-Mn single bond are predicted. The hyperelectronic derivatives M(3)(C(8)H(8))(3) have weaker direct M-M interactions in their M(3) triangles, as indicated by both the M-M distances and the WBIs. Thus, Ni(3)(C(8)H(8))(3) has bis(trihapto) η(3),η(3)-C(8)H(8) ligands bridging the edges of a central approximately equilateral Ni(3) triangle with long Ni···Ni distances of ～3.7 ?. The WBIs indicate very little direct Ni-Ni bonding in this Ni(3) triangle and thus a local nickel environment in the singlet Ni(3)(C(8)H(8))(3) similar to that observed for diallylnickel (η(3)-C(3)H(5))(2)Ni.