This interesting result opens the door for a discussion that is not usually found in textbooks. Also, bond energy data for group 4 halides show that bond strength increases down the transition metal group (2). The increase in bond strength down a transition metal group, which parallels the increasing cohesive energies of metals on going from the first to the second and third transition series (3), follows the opposite trend to that in main group elements. Although inorganic chemistry textbooks do not usually explain that apparently anomalous behavior, Mingos has recently explained the vertical trends associated with transition metals (4). For the first-row transition metals, the 3d orbitals are too contracted; that is, the maximum in the radial distribution functions is significantly closer to the nucleus than the covalent or metallic radii. As a result, the overlap of 3d orbitals with ligand orbitals is not very effective at normal bonding distances. For the second- and third-row transition metals, on the other hand, the 4d and 5d orbitals are more expanded and they overlap more effectively with ligand orbitals, thus leading to stronger bonds. A well-known consequence, but often not very well explained, is that ligand field splitting energies increase on going from the first to the second and third transition series. Also, the better overlap between 4d and 5d orbitals accounts for the stronger metallic bonds formed by the second- and third-row transition metals. In conclusion, while expanding the valence orbitals of main-group elements usually leads to decreasing orbital overlapping and weaker bonds, the opposite trend is observed for d orbitals in transition metals, because orbitals that are both too contracted or too expanded, in relation to bond distances, lead to poor overlap.

Literature Cited

1. Comstock, M. G.; Gray, J. A. J. Chem. Educ. 1999, 76, 1272.
2. Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry, 4th ed.; Harper Collins: New York, 1993; p A-28.
3. Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 3rd ed.; Wiley: New York, 1995; p 253.
4. Mingos, D. M. P. Essential Trends in Inorganic Chemistry; Oxford University Press: Oxford, 1998; p 366.