the answer is HNNH the reason for this is the double bond if you draw out the lewis structure
HNNH, it has a double bond while H2NNH2 has a single, and doubles are stronger than single bonds, just as triple is stronger than double.
HNNH (100% sure of it) :)
HNNHThe first compound(H2NNH2) has a single bong between the nitrogen atoms and follows the octet rule allowing nitrogen's orbital to hold 8 electrons. The second compound (HNNH) requires nitrogen to be double bonded. Double bonds are stronger than single bonds.Source:Yahoo Answers
In H-N=N-H are 2 sigma's (-) between N and H,and there are 1 pi bond + 1 sigma (=) between N and N
It has no other isomers, only one structure is possible, H-N=N-H