The ion that has a spin only magnetic moment of 5.9 BM is:

Step-by-step solution

The spin-only magnetic moment is given by the formula = n(n+2) BM, where n is the number of unpaired electrons. Given = 5.9 BM, we have n(n+2) = 5.9 . Squaring both sides gives n(n+2) 35 , which corresponds to n = 5 unpaired electrons. Let us determine the number of unpaired electrons for each given ion: For Fe ³⁺ (Atomic number of Fe = 26): Electronic configuration of Fe is [ Ar ] 3d^6 4s^2 . Electronic configuration of Fe ³⁺ is [ Ar ] 3d^5 . Number of unpaired electrons, n = 5 . For Ni ²⁺ (Atomic number of Ni = 2