Low temperature, single crystal photocrystallographic studies have been carried out on four square planar Group 10 complexes Ni(PEt 3) 2(NO 2) 2] 1, [Pd(PPh 3) 2(NO 2) 2] 2, [Pd(AsPh 3) 2(NO 2) 2] 3 and [Pt(PPh 3) 2(NO 2) 2] 4 in which the two nitro groups adopt the trans configuration. Irradiation with UV light, at 100 K, of single crystals of complexes 1-3 photoisomerise from the from the η 1-NO 2 nitro form to the η 1-ONO nitrito form occurred. Complex 1 underwent 25% conversion to the nitrito form before crystal decomposition occurred. 2 and 3 underwent 46% and 39% conversion, respectively, to the nitrito form when a photostationary state was reached. While under the same experimental conditions 4 showed no isomerisation. The photocrystallographic results can be correlated with the results of DFT calculations and with the observed trends in the solution UV/visible absorption spectroscopy obtained for these complexes. The results suggest that while steric factors in the isomerization processes are important there may also be a kinetic effect relating to the lability of the metal involved.