The trinuclear derivative Fe-3(C8H8)(3) was synthesized in 2009 by Lavallo and Grubbs via the reaction of Fe(C8H8)(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structure is the first example of a derivative of the well-known Fe-3(CO)(12) in which all 12 carbonyl groups have been replaced by hydrocarbon ligands.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 similar to 2.9 angstrom Fe-Fe single bonded edges bridged by eta(5),eta(3)-C8H8 ligands. This structure is close to the experimental structure, determined by X-ray crystallography. The related hypoelectronic M-3(C8H8)(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 equivalent to M bond. For Mn-3(C8H8)(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(C8H8)(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(C8H8)(3) has bis(trihapto) eta(3),eta(3)-C8H8 ligands bridging the edges of a central approximately equilateral Ni-3 triangle with long Ni center dot center dot center dot Ni distances of similar to 3.7 angstrom. 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(C8H8)(3) similar to that observed for diallylnickel (eta(3)-C3H5)(2)Ni.