<unk>  is the chemical compound CH2N2.  In the pure form at room temperature, it is a yellow gas, but it is almost universally used as a solution in diethyl ether.  It is one of the more common diazo compounds. It is also toxic and potentially explosive.
Diazomethane
Bromomethane
Iodomethane
Chlorodifluoromethane
Chloroethane
Chloromethane
Chlorofluoromethane
Dichloromethane