We know that there are silicon analogs of alkanes, but according to Wikipedia, silicon analogs of alkenes or alkynes are virtually unknown. How come?
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2 Answers
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The typical Si–Si single bond length in a silane is around 2.33 Å. This is much longer than a typical C–C single bond (~1.53 Å) and helps explain why silicon-silicon single bonds are so much weaker than carbon-carbon single bonds (bond dissociation energy: ~53 kcal/mol for silicon-silicon vs. ~83 kcal/mol for carbon-carbon single bonds). When we try to form a silicon-silicon double or triple bond, the large separation between the two silicon atoms results in even less effective p-orbital overlap and therefore even weaker π-bonds. The result is that while disilenes and disilynes are known (see the linked SE Chem posts), they are extremely reactive. Only when bulky substituents are placed around the double or triple bonds (to provide some "steric protection" against further reaction) can the molecules be isolated and characterized.
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Just to add to ron's answer with some information on the properties of the silicon analogues.
- Silicon "alkynes"
These exotic compounds were first reported on in 2004 in this paper. Typically show bond order somewhat less than 3; the HOMO has some non-bonding/lone pair character which is also manifest in a small deviation from linearity. The silicon-silicon "triple" bond is pretty reactive and most examples have very bulky alkyl substituents to create a steric barrier to reaction.
- Silenes
These have been known since 1981; the first example being tetramesitylsilene. Similar to the "alkynes", (trans) deviation from planarity and bond order less than 2 due to HOMO lone pair character observed. Decrease in Si-Si bond length observed but it is smaller than that typically seen for C-C bonds.
