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The characteristics of back-bonding in metal–olefin complexes have been studied by using the NBO analyses. In metal–olefin complexes, the evidence for back-bonding is only possible from the d M–C and/or d CC bond length variations. However, these bond lengths might be ambiguous in explaining the extents of back-bonding, since both bond lengths are dependent upon the extents of the σ-bonding as well as the π-back-bonding. Instead, the explanation and prediction using the NBO analyses of electron populations in π * -orbitals of olefin ligands and of the second-order charge transfer interactions between metal d-orbitals and olefins π * -orbitals could be more efficient and correct.

Characterization of the stationary points in a flat transition structure region of the potential energy surface for the HCOCl+Cl − reaction has been carried out at the MP2/6-311+G∗∗ level. Two stationary points are found: an intermediate with C S symmetry and all positive eigenvalues in the Hessian and a transition state with C 1 symmetry and only one negative eigenvalue in the Hessian. The MP2/6-311+G∗∗ electronic energies of these two only differ by δΔ E (=Δ E TS −Δ E Int )=0.01 kcal mol −1 . The zero-point and thermal energy corrections to the electronic energy lead to an inversion of levels: δΔ E ZPE =−0.06 and δΔ H =−0.59 kcal mol −1 , which becomes normal when the − T Δ S term is added to obtain δΔ G =+0.90 kcal mol −1 (+0.80 kcal mol −1 at the G2(+)MP2 level). This inversion arises solely from one less vibrational mode in the TS on the flat transition structure region.

Relative electron affinities have been determined for a series of acetophenones and methyl benzoates by measurement of electron transfer equilibria in the gas phase. The LArSR analysis showed that the substituent effects on the electron affinities of these systems are characterized by r − values of 1.18 and 1.31 for acetophenones and methyl benzoates, respectively. These values are significantly higher than the corresponding value for nitrobenzenes.