Identify the electron configuration

The given crystal field configuration is: $t_{2g}^3 e_g^1$

Total number of $d$-electrons: $3+1=4$ So this is a $d^4$ configuration in an octahedral field.

Recall octahedral splitting

In an octahedral complex, the five $d$-orbitals split into:

• lower energy: $t_{2g}$
• higher energy: $e_g$

The way electrons occupy these depends on the competition between:

• crystal field splitting energy, $\Delta_o$
• pairing energy, $P$

Case for weak field ligands

If the ligand is weak field, then: $\Delta_o < P$ So electrons prefer to occupy the higher-energy $e_g$ orbital rather than pair up in $t_{2g}$.

For a $d^4$ ion, the electron arrangement becomes: $t_{2g}^3 e_g^1$ This has maximum unpaired electrons, so it is a high spin complex.

Case for strong field ligands

If the ligand is strong field, then: $\Delta_o > P$ Electrons pair in the lower $t_{2g}$ set before going to $e_g$.

For a $d^4$ ion, the configuration becomes: $t_{2g}^4 e_g^0$ This is a low spin complex.

Match with options

The configuration $t_{2g}^3 e_g^1$ corresponds to:

• weak field ligand
• high spin complex

Therefore, the correct option is: $\boxed{\text{C}}$

Compare with stored correct answer

Stored correct answer: C

My derived answer: C

They match.