An element M belongs to the 3d transition series. It forms an octahedral complex [M(NH ₃ ) ₆ ] ³⁺ which is diamagnetic in nature. Another octahedral complex of the same metal, [M(F) ₆ ] ³⁻ , is found to be paramagnetic with a spin-only magnetic moment of approximately 4.9 BM . The atomic number of the metal M is

Step-by-step solution

In both complexes, the oxidation state of metal M is +3 . The complex [M(NH ₃ ) ₆ ] ³⁺ is diamagnetic. In an octahedral field, a diamagnetic complex must have all its electrons paired. For a +3 ion of the 3d series with NH ₃ (which acts as a strong field ligand for most +3 ions), a diamagnetic configuration is only possible for a d^6 system, where the electrons pair up in the lower energy t_ 2g orbitals to give a t_ 2g ^6 e_g^0 configuration. Let us verify this with the second complex. If M ³⁺ is 3d^6 , then in [M(