[GPhab]

Dihalocarbenes exist in singlet state. Wiki says that singlet carbenes can be stabilized by electron donating groups as they help in delocalizing the electrons into an empty p-orbital. So why is CF₂ the most stable

[azmanam]

It's a bit like halogen substituents for electrophilic aromatic substitution.  While electronegative (and thus electron withdrawing), the lone pairs of electrons can donate into the benzene ring through resonance and stabilize the intermediate in the substitution leading to ortho and para products.  In this sense, the halogen is more like a methoxy substituent than a nitro substituent.

Same logic applies here.

http://www.cem.msu.edu/~reusch/VirtTxtJml/benzrx1.htm#benz1