Elsevier

Polyhedron

Volume 6, Issue 4, 1987, Pages 705-714
Polyhedron

δ→δ* Revisited: What the energies and intensities mean

https://doi.org/10.1016/S0277-5387(00)86878-3Get rights and content

Abstract

Attempts to extract estimates of the δ-bond strength of quadruply metal-metal-bonded molecules from their δ→δ* electronic transitions have been hindered in the past by limited understanding of the origins of the energies and intensities of these transitions. We show that the energies of the δ→δ* transitions of a wide variety of these molecules are adequately interpreted in terms of a simple zero-differential-overlap model that yields one-electron δ-δ* splittings of 5000–10,000 cm−1 and two-electron exchange terms [K(δ,δ*)] of 5000–8000 cm−1. Because of the magnitude of K, singlet-triplet δ→δ* splittings are very large, and configuration interaction is important for the correct description of the ground state. The intrinsic intensity of 1(δ→δ*) is estimated to be quite low. The considerable intensities observed in many cases do not correlate with δ-bond strength, but instead reflect intensity stealing from charge-transfer excited states as a result of δ,δ*-orbital mixing with ligand orbitals. The intrinsic δ-bond stabilization is estimated to be on the order of 10 kcal mol−1.

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