Why si is reluctant to form double with its identical atom.
Answers
Silicon doesn’t form a stable double bond. Si2H4
is only stable as a single molecule in high vacuum, where it has nothing to react with.
You can stabilize Si=Si bonds by putting very bulky groups on the silicon atoms, so much so that you can obtain stable crystals (but not air-stable, because of silicon’s high affinity for oxygen). See the review article by Robert West, Angew. Chem. 1987, 26, 1201–11.
Bulky groups stabilize reactive structures by making them harder for other molecules to access.
But even very bulky disilenes have defective π
bonding, as shown by the fact that they are not planar but have the silicon atoms somewhat pyramidalized. This is in contrast to carbon-carbon double bonds in which carbon is trigonal planar.
The reason for this pyramidalization is unclear to me, though it’s also seen in silyl vs. alkyl radical structures: silicon is pyramidal in a silyl radical, but carbon is trigonal planar in an alkyl radical.
Bonding in disilenes is weaker, compared to conversion to two single bonds in polysilanes, than the corresponding π bonds in alkenes, compared to single bonding in alkanes. This may or may not be due to p orbitals being more diffuse in the third row of the periodic table (silicon) than in the second row (carbon).