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.

Col&sion with neutral molecules is shown to provide a convenient method of adding internal energy to ions in a field-free drift region of the mass spectrometer. The effects on this process of ion accelerating potential, target gas pressure and identity, and precursor ion internal energy and mass have been investigated to optimize experimental conditions. Such collisions cause ion decompositions whose activation energies cover a broad range; for a particular ion such decompositions can be viewed as its "collisional activation (CA) spectrum." CA spectra, which can be obtained for each ion in the normal mass spectrum, and which appear to follow the predictions of the quasi equilibrium theory, show many more of the possible unimolecular ion decomposition reactions for an ion than do unimolecular metastables, and thus provide valuable information for ion reaction mechanisms and molecular structure detemhation. Collisional activation can sometimes yield ion energies which are relatively inaccessible by electron impact. The precursor ion internal energy has a negligible effect on the ion's CA spectrum except for product ions formed through the processes of lowest activation energy. Thus, CA spectra should also be valuable for the characterization of ion structures. he dissociation of a metastable organic ion in a (7) T. Wachs, P. F.

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The collisional activation (CA) spectra of gaseous ions formed by the protonation of cyclopropane and propene are identical, indicating that the isomerization c -C~H~+sec-C3H7+ occurs in < s. However, ion-molecule reactions of C3H7+ at -IO-* Torr indicate that c-C3H7+ ions can have lifetimes of s, supporting the results of equilibrium constant measurements by Chong and F r a n k h 5 They derived AHf(c-C3H7+) -AHf(sec-C3H?+) = 8 kcal/mol, consistent with the prediction of M I N D 0 / 3 calc~lations;~ however, the apparent low activation energy for c-C3H7+sec-C,H?+ indicated by the CA results is not consistent with the MIND0/3 value3 of AHf(n-C3H7+) -A H~( c -C~H~+ )