[Kenny]

Hello all,

Background: Carbenes can exist as singlet species, with both nonbonding electrons in an sp2-hybridized orbital. In this case there will be a vacant p-orbital on the carbon atom. Carbenes can also exist as triplet species, in which one electron will reside in the sp2-hybridized orbital, and the other in the p-orbital. However, triplet configurations are not observed for the heavier analogues of carbenes (silylenes, germylenes, stannylenes, and plumbylenes). In all cases they exist as singlet species.

The geometry of these species is also different - in a heavier analogue, the angle between the substituents is closer to 90 degrees than 120 degrees (the approximate bond angle in sp2-hybridized systems). This is because the tendency to form hybridized orbitals decreases on descending group 14...I accept this fact, but I don't fully understand it. I was told it had to do with the promotion energy, but none of the textbooks I own provide an adequate discussion.

Could someone explain why the heavier group 14 elements do not form hybridized orbitals?

[FeLiXe]

Hi, I'll tell you a few thoughts about it. Actually I just wrote something longer, but then the whole thing was erased when I tried to upload a file.

Triplet carbenes are rather sp hybridized and therefore linear.

The bonding angle in sp2 carbenes decreases with higher EN difference. That is explained through VSEPR rules.

Triplet carbenes are in general more stable because of spin coupling energy (cf. high spin complexes).

It seems that with increasing EN difference the singlet carbene is more stable (also in Dichloro-carbene). That probably comes from the fact that the sp2 orbital can bond better to a highly electronegative atom. In an sp2 orbital the highest electron density is farther away from the nucleus.

I added an image that shows the energy levels of the different species. But it does not show as much as I thought it would. you find it here: http://stud4.tuwien.ac.at/~e0425252/Carbenes.skc

Hybridisation in higher elements normally looks different because d-orbitals are also used.

hope that helps a little bit