IR shows one characteristic absorption at 1979.8 cm-1
It is N=N stretch.
31P NMR shows one sharp singlet peak at -69 ppm
The question is:
There are 2 isomers, which is the correct one?
When the two N2 ligands are trans to each other, P is appeared as one peak in NMR.
However, the two N2 ligands should have symmetric and asymmetric stretches as they can couple together in linear alignment.
When the two N2 ligands are cis to each other, they are orthogonal, so they should not couple to give sym. and asym. stretches in IR spectrum. That's why there is only one IR absorption.
However, in NMR, there should be 2 absorption peak as the chemical environment of each P of the dppe are different.