The first step is the dissociation of a PPh3 ligand. The reorganization of the structure of the intermediate from square pyramid to trigonal bipyramid, followed by entrance of the PPh3 ligand originates the trans isomer. The process is very slow even in warm, because the intermediate PPh3 in the axial position of the trigonal bipyramid is less favorable than the starting product. The trans isomer is thermodynamically more stable. The cis isomer is isolable and has kinetic stability.
The nitrite isomer-k-N (formerly called nitro), yellow colored, under light transforms into the red nitrite-K-O (formerly called nitrite). This one even in the solid state, reverts to the first one by the same concerted intramolecular mechanism.