Bis(methylborabenzene) sandwich compounds of the first row transition metals

The bis(methylborabenzene)metal sandwich compounds (η6-C5H5BCH3)2M of the first row transition metals from Ti to Ni have been examined by density functional theory. The four of these seven methylborabenzene sandwich compounds that have been synthesized, namely the V, Cr, Fe, and Co derivatives, are predicted to have quartet, triplet, singlet, and doublet spin states, respectively, in accord with experiment. For (η6-C5H5BCH3)2Ti the singlet spin state structure lies only ∼2 kcal/mol below the triplet spin state structure. This energetic relationship is reversed for (η6-C5H5BCH3)2Ni for which the triplet spin state structure lies ∼2 kcal/mol in energy below the singlet spin state structure. Similarly the sextet (η6-C5H5BCH3)2Mn spin state structure lies only ∼2 kcal/mol below the corresponding doublet spin state structures. These observations suggest possible spin-crossover behavior for these three still unknown (η6-C5H5BCH3)2M (M = Ti, Mn, Ni) systems. The M–C and M–B distances from the metal to the methylborabenzene ligand increase for a given (η6-C5H5BCH3)2M system as the spin multiplicity is increased. This suggests a weakening of the metal–ligand bond with an increase in the spin multiplicity. The energy surfaces of all of these (η6-C5H5BCH3)2M sandwich compounds are complicated by multiple local minima differing only by the rotation of one methylborabenzene ring relative to the other such ring. The energy differences between these rotamers for the same spin state are generally less than the energy differences between different spin states for a given (η6-C5H5BCH3)2M sandwich compound.