well as u go down the group the cationic size increases
so there is less polarisation (as the cation is big whereas for polarisation the cation should be small and the anion big)
less weakening of the CO or NO bond
thus the thernal stability increases
as you go down the group
the cationic size increases ( they are less polarising because the are more inner shell electrons repelling the outermost electrons therefore the positive nucleus has less control over its outermost electrons so its less likely to polarise the electrons of the anion towards)
they become less polarising...they are less likely to polarise the anion to break it apart and produce different compounds so they are more stable..less likely to break
so you need more heat to compensate for this weak polarising effect in order to break the compound apart to form other compounds..
