The correct pair among the following, in which each species shows orbital contribution to its magnetic moment is

Correct option is B

​The orbital contribution is possible only when an orbital will transform into an equivalent orbital by rotation. The t_2g orbitals (dxy, dyz and dzx) can be transformed into each other by rotating about an axis by 90°. The eg, orbitals (dx²-y² and dz²) can not be transformed into one another because these have different shapes. Therefore, these orbitals have no orbital contribution. If all the t_2g orbitals are singly occupied, then it is not possible to transform an orbital, say the dxy orbital into dyz or dzx orbital because they already contain an electron with same spin as the incoming electron. Similarly, it is impossible to transform a t_2g orbital into others if all the t_2g orbitals are doubly occupied. Thus, t_2g orbitals with t_2g³ and t_2g⁶ configuration have no orbital contributions. The configurations other than t_2g³ and t_2g⁶make orbital contributions to the magnetic moments of octahedral complexes. Therefore, in octahedral complexes the following configurations make orbital contributions :

The following configuration in octahedral complexes of first series transition metal ions have no orbital contributions:

In a similar way, the tetrahedral complexes with the following configurations have orbital contributions:

Option A: Co²⁺(d⁷):e⁴t₂³, Ni²⁺(d⁸): t_2g⁶e_g²​

Option B: Ni²⁺(d⁸): e⁴t₂⁴, Co²⁺(d⁷): t_2g⁵e_g²​

Option C: Co²⁺(d⁷): t_2g⁵e_g², Cr³⁺(d³): t_2g³e_g⁰​

Option D: Ni²⁺(d⁸): t_2g⁶e_g², Cr²⁺(d⁴): e²t₂²