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Bonding to Hydrogen

The simplest molecule, made for connection

Roald Hoffmann

Molecular Complexes of Dihydrogen

By the 1980s there emerged evidence for weak “complexation” (binding) of hydrogen with various metal atoms. And surface scientists were piecing together the mechanism of Döbereiner’s seeming magic. Elsewhere, organometallic chemists found some reactions in which hydrogen molecules added to a metal center, the two hydrogen atoms split apart in the process. Experimentalists and theorists begin to view the seeming chemical inertness of dihydrogen as a challenge rather than dogma.

In 1984 Jean-Yves Saillard, a French postdoctoral associate (now at the University of Rennes) and I did a careful study of the interactions of hydrogen and methane with discrete transition metal centers with associated ligands. These MLn (M is a metal atom, L a ligand, say CO or PH3, n the variable number of such ligands) fragments, if carefully chosen to be good bases and acids at the same time, could, in our approximate calculations, bind dihydrogen. The molecular orbital essence of our argument is shown in the figure at lower left; a similar picture and interpretation is there in earlier work of three Alains—Dedieu, Strich and Sevin.

2012-09MargHoffmannFD.jpgClick to Enlarge ImageA small interlude here on so-called interaction diagrams, which is what you see in the figure at lower left on the previous page. These diagrams, my professional bread and butter, show the interaction of the important orbitals of two pieces of a molecule (when it can be taken apart into pieces). That’s the way we build understanding, putting together, in LEGO style, the orbitals of a more complex molecule from simpler pieces. The L5M(H2) molecule in the middle (at that time unknown, at least to us) is built from two simpler pieces—an ML5 fragment at left, and my old friend H2 at right.

The orbitals of H2 are easy—you’ve seen them above, the σg MO, with both of the 1s orbitals of the component H atoms in-phase, at low energy; the σu* MO, unfilled by electrons, at high energy.

On the other side are orbitals of the ML5 fragment, mostly on the metal. They are more complicated (the metal has important 3d orbitals), but the essential feature is that there are orbitals on the metal filled with electrons and some that are empty, and these match in symmetry and overlap reasonably well with the orbitals on the H2. The dashed lines in the figure guide us to just these stabilizing interactions.

Here’s what happens in this theoretical analysis: The acid function of the ML5 fragment (its empty orbital, called dz2) interacts with the base σg of H2, the base function of ML5 (a filled dxz orbital) interacts with the σu*, the acid function of H2. (Did I not say that there is a reason for all that seeming torture on acids and bases in first-year chemistry?) Importantly, there are consequences to the strength and length of the H2 as a function of the interaction: As a result of the mixing of MOs of ML5 with those of H2, some electrons are transferred from the σg orbital of H2, depleting its bonding density. And some electrons are transferred in the opposite direction, from ML5 to the H2 σu* orbital. Both actions—decreasing bonding, increasing antibonding—will stretch the H-H distance, even as they overall bind H2 to MLn. The figure is for ML5, but the reasoning extends to other numbers of ligands bound to the metal.

2012-09MargHoffmannFE.jpgClick to Enlarge ImageSaillard and I made no prediction of specific molecules. What we did not know when we did our work is that the first such “complex” had just been made. Greg Kubas at Los Alamos had synthesized (and with no nuclear reactions involved), the molecule shown in the figure above. It was followed over the years by a significant group of dihydrogen complexes, even ones in which the metal held more than hydrogen molecule.

In time the H-H distance in these molecules was determined accurately (one needs neutron diffraction for that; metric information also comes from nuclear magnetic resonance studies). Kubas understood very well what was going on—his qualitative thinking about what bound H2 in his molecules, quite independently conceived, was similar to ours.

But what fun for us! A theoretical idea about how a molecule could bind—and not just any molecule, but normally inert hydrogen—translated into reality! We were happy. And Kubas deserves all the credit, because science is ultimately about the reality of a compound in hand—theories come and go, the molecule is there.

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