\(Co(O_x)_3]^{3-}\)
\([Fe(CN)_6]^{3-}\) has magnetic moment of a single unpaired electron whereas \([FeF_6]^{3-}\) has a magnetic moment of five unpaired electrons.
\([CoF_6]^{3-}\) is paramagnetic with four unpaired electrons while \([Co(C_2O_4)_3]^3\) is diamagnetic. This anomaly is explained by valence bond theory in terms of formation of inner and outer orbital coordination entities. \([Co(C_2O_4)_3]^{3-}\) is an inner orbital complexes having \(d^2sp^3\) hybridization.
So, the correct option is (B): \(Co(O_x)_3]^{3-}\)


In the first configuration (1) as shown in the figure, four identical charges \( q_0 \) are kept at the corners A, B, C and D of square of side length \( a \). In the second configuration (2), the same charges are shifted to mid points C, E, H, and F of the square. If \( K = \frac{1}{4\pi \epsilon_0} \), the difference between the potential energies of configuration (2) and (1) is given by:
A coordination compound holds a central metal atom or ion surrounded by various oppositely charged ions or neutral molecules. These molecules or ions are re-bonded to the metal atom or ion by a coordinate bond.
A coordination entity composes of a central metal atom or ion bonded to a fixed number of ions or molecules.
A molecule, ion, or group which is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond is commonly called a ligand. It may be either neutral, positively, or negatively charged.