It was not made adequately clear that the simple model in ref 1 includes only the leading “exchange-overlap” contribution to bond energy (2), whereas other quasi-classical electrostatic effects and higher-order Coulombic distortion and dispersion effects were ignored.

1. Deviations from the expected trend suggested in the left column of page 430 of ref 1 for Li₂⁺ and Li₂ [|D_e| is 29.7 kcal mol^–1 for Li₂⁺, 24.2 kcal mol^–1 for Li₂ (3)] are mostly due to these second-order distortion effects and to the dependence of the βs on the overlap S, as a deeper analysis shows (3), so that may not be unexpected to occur at this level of the theory.
   
2. We observe that there is no relationship whatever between bond energies and experimentally observed UV photoelectron spectra. The βs of ref 1 are purely empirical parameters (different for 1σ or 2σ or 1π), assumed equal along the series,¹ in terms of which the bond energy² is simply assumed to depend on the relative number of electrons filling bonding and antibonding relevant MOs either σ or π. So, the bottom part of Figure 6 must be interpreted only in relation to the dependence of the bond energy on the number of π electrons along the series. It is not claimed to have any spectroscopic significance, when often small energy differences between low lying electronic states [about 2 kcal mol^–1 for C₂ between ¹Σ_g⁺ and ³Π_u (4)] may be of difficult evaluation even for sophisticated ab initio calculations.
   
3. The experimental bond energy results |D_e| of the bottom part of Table 1 in ref 1 must be updated as follows with values taken from ref 4:

The corresponding bond lengths are unchanged, to the figures reported in Table 1, except for F₂⁺ = 1.32 Å and F₂ = 1.31 Å. The substance of the discussion in ref 1 is left essentially unaltered: it is observed that there is a strict correlation between bond lengths and bond energies and the number of π electrons along the series. This is an experimental observation, which is qualitatively followed by the simple model proposed in ref 1.

Acknowledgments

Support by the Italian Ministry of Education University and Research (MIUR) under Grant no. 2004 03 4838 009 and by the University of Genoa is gratefully acknowledged.

Notes

1. Therefore, the βs do not depend in any way on the kind of atom or on the internuclear distance R.
2. Bond energy is a complicated, small difference between very large quantities such as the molecular energy and the sum of the atomic energies of the constituent atoms.

Literature Cited

1. Magnasco, V. J. Chem. Educ. 2004, 81, 427–435.
2. Magnasco, V.; McWeeny, R. In Theoretical Models of Chemical Bonding, Part 4; Maksic, Z. B., Ed.; Springer Verlag: Berlin, 1991; p 133–169.
3. Kutzelnigg, W. In Theoretical Models of Chemical Bonding, Part 2; Maksic, Z.B., Ed.; Springer Verlag: Berlin, 1990; p 1–43.
4. Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979.