The coordination chemistry of saturated molecules

Our understanding of bonding in transition metal complexes, as well as our ability to use that understanding in the synthesis and application of new species, has evolved over the last 100 years; and in some sense this special feature on the coordination chemistry of saturated molecules may be considered to represent its culmination. The nature of complexes between transition metal ions and neutral molecules such as ammonia was first correctly described by Werner around the beginning of the 20th century. Interpretations in terms of electronic bonding theories followed soon after. The key feature, of course, is the availability of a low-energy filled “lone pair” orbital available for donation to a vacant orbital on the electron-accepting metal ion.

Next, it was found that molecules containing no lone pair but having a pair of electrons in a π-bond, as in olefins and acetylenes, could also form stable complexes. The first example, Zeise's salt {[PtCl₃(C₂H₄)]⁻}, was actually synthesized more than a century earlier, in 1827, but explanations of structure and bonding in such species were only achieved around the middle of the 20th century. Because the filled orbital is bonding, and hence lower in energy than a lone pair, “back-bonding” (donation of electron density from filled metal d orbitals to vacant ligand π* orbitals) plays a crucial role.

The logical next step in this progression would be the coordination of molecules having only σ-bonded electrons. Indeed, at about the same time, Halpern and others were accounting for the activation of H₂ at metal centers in terms of interactions between the dihydrogen bonding pair and a vacant metal orbital (for example, see ref. 1). However, these were generally ascribed to transition states …

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