[allthingschm]

Hi, could you please advise on whether this explanation for the rarity of Si multiple bonds is correct?

The pi bonding in Si uses 3p orbitals and pi bonding is weak because of the increased inner electronic repulsion from 2s, 2p orbitals and also because of low electron density on the 3p orbitals due to presence of inner orbital lobes that causes loss of electron density for bonding as the smaller orbital lobes are too small to overlap resulting in weak overlap and hence weak multiple bonds.

Also, when should inert pair effect be used in explanation of weak bonding or why some compounds cannot form? this is because sometimes I'm not sure whether to use this explanation or to use the reasoning that overlap of orbitals are poor due to different sizes in atoms. is there specific information i should look out for in a question to be able to answer with the appropriate explanation?

[allthingschm]

I understand that transition metals can activate small molecules like H2, and this was explained as such: the H2 donates electrons from its sigma orbital to the empty d orbital of the transition metal and the transition metal donates electrons via pi-back bonding into the sigma* orbital of H2. As the sigma* orbital is antibonding, the H-H bond in H2 is weakened and hence breaks.

However in the case of Si Si triple bonds, how do they break the H2 bond? i.e. what is the orbital the electrons from H2 are donated to and where do the electrons donated from Si complex come from? I'm unsure of this as I understand that Si Si triple bonds are not formed from the usual overlap of 3p orbitals but rather from a donor-acceptor interaction.