have similar probability of occurring. Since h'-f*y-h\y*y are all differentiable in a totally symmetric environment, detailed nmr line shape analysis may allow distinction between "helical" movement of the aliene ligand and isomerization via an intermediate configuration having coplanar allenyl methyl groups. Note that this final example has been simplified by assuming that rapid " -rotation" about the metalallene band does not occur.Should such rotation in fact occur, the distinction will be impossible.

Perhaps a better interpretation of the reduction properties, however, is one that places the added electrons in a ligand * MO of bm symmetry (Figure 7).The bathochromic shift of the M -L (b3g -* Ltt*) transition in the electronic spectra of the adducts and the energy of this transition relative to that of the d-d transition62 suggest that, in the adducts, the ligand * MO of bm symmetry is of lower energy than the dxy metal orbital. The per cent metal character of the (52) From the reported by Carlin and Canziani63 for Co(Dto)a3t he energy of the transition for the Ni(Dto)z2_ is calculated to occur at 17,700 cm-1. A shoulder found at 17,700 cm-1 in the spectrum of Ni(Dto)z2_ has been assigned43 to the transition. In agreement with the stronger M-S bonding in the adducts is the fact that, in the adducts, larger values of are generally observed.64 (53) R. L. Carlin and F. Canziani, J. Chem. Phys., 40,371 (1964). ( 54) D. Coucouvanis and D. Piltingsrud, J. Amer. Chem. Soc., in press.bm MO cannot be ascertained, although the reduction properties of the adducts and the stability of the reduction products clearly depend66 on the type of metal atom bonded to the -diketone portion of the coordinated dithiooxalate ligand and to a lesser extent on the environment about this atom.

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Ions 713 thracene is a more efficient electron scavenger than dienes and therefore the formation of diene anions is suppressed when anthracene is used as an additive. The presence of anthracene anions in such samples has been confirmed by ESR measurements as well.4 No direct proof for the existence of diene cations can be given.