Stable silicon-silicon triple bond discovered
Silicon isn’t supposed to form stable triple bonds with itself. Yet Akira Sekiguchi and his team at Japan’s Tsukuba University have produced the first evidence of a compound containing the silicon-silicon triple bond, as an emerald green crystalline solid, stable (albeit air and moisture sensitive) up to 127?C.
The first period main group elements (eg C, N, O) can form homonuclear multiple bonds, but those of succeeding periods (eg Si, P, S, Ge, Sn, Pb, etc) are not supposed to. The reasons are twofold. First; good p-orbital overlap to form pi bonds becomes progressively less efficient (so pi bonds are less stable) the heavier the element. Second; there is increasing electrostatic repulsion between the atoms because of intervening shells of non-bonding electrons.
But rules are meant to be broken and, since 1976, homonuclear multiple-bonded compounds of Si, Ge, Sn, and Pb have been produced. There have even been claims of triple-bonded alkyne analogues Ge, Sn, and Pb, but closer inspection of the structures of these compounds revealed bond orders closer to two (and in the case of Pb, nearer to one). In addition, groups attached to these atoms were not linear, as they are in alkynes, but bent away from the horizontal; the trans-bonding angle increasing with atomic number of the group 4 element.
The new silicon compound produced by Sekiguchi and colleagues, however, has an Si-Si bond order close to three, indicating the presence of a genuine triple bond. Also, the attached highly bulky groups that give the molecule kinetic and thermodynamic stability, though not linear as in alkynes, have a much smaller trans-bond angle.
And the green colour? Unlike the pi and pi* orbitals in alkynes, those in the new silicon compound are not energetically degenerate. A weak forbidden pi-pi* electronic transition in the visible region of the electromagnetic spectrum creates the emerald green hue.
Of these new compounds, Robert West, professor of chemistry at the University of Wisconsin, US, comments: ’The synthesis of a stable disilyne is a milestone both for silicon chemistry and for multiple-bond chemistry in general. The challenge now is to produce silicon-carbon and silicon-nitrogen triple bonds.’
1 A Sekiguchi, et al, Science, 2004, 305, 1755
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