M4X4 structures in transition metal chemistry: Cubane-like or planer? A DFT study of some clusters containing the M4X4 core (M = Ti, V, Mo; X = N, P,As)

DFT calculations have been carried out on the organometallic clusters [(eta -C5H5)TiN](4) and [Cl3MoN](4) with the aim of comparing the energies and th e electronic structures of the cubane and planar forms for these two cluste rs. Calculations indicate for the titanium cluster that the cubane conforma tion is much more stable than the structure in which the four metals and th e four nitrogens are in the Same plane, whereas the square planar structure is preferred for the molybdenum complex. The octahedral coordination of th e metal in the cubane-like form implies an important reorganization of the [Cl3MoN] unit. The high cost of this reorganization explains the relative s tability of the square planar conformation in that case. Metal-metal intera ctions have been characterized in the vanadium cubanes [(eta-C5H5)V (mu(3)- X)](4), X = N, P, and As. The distortion of the cage observed in some cuban es is discussed in relation to the metal-metal bonding interactions evidenc ed in these clusters.