Odd electrons, ions and molecules are paramagnetic. In Cr(CO)6 molecule 12 electrons are contributed by CO group and it contains no odd electron. Cr→3d54s1 Fe(CO)5 molecule also does not contain odd electron. Fe→3d64s2In[Fe(CN)6]4 ion Fe(+II)→3d64s0 ∴ No odd electrons. In [Cr(NH3)6]3+ ion Cr(+III)→3d34s0 This ion contains odd electron so it is paramagnetic.