Question

Identify the paramagnetic outer orbital complex ion from the following.

• A. \([Co(C_2O_4)_3]^{3-}\)
• B. \([CoF_6]^{3-}\)
• C. \([Fe(CN)_6]^{3-}\)
• D. \([Mn(CN)_6]^{3-}\)

Hint:

Weak-field ligands: \[ F^-,\; Cl^-,\; Br^-,\; I^- \] generally form \[ sp^3d^2 \] outer orbital complexes. Strong-field ligands: \[ CN^-,\; CO,\; NO_2^- \] usually form inner orbital complexes.

Answer 1

The Correct Option is B

Concept: Outer orbital complexes are formed when weak-field ligands do not cause electron pairing in \((n-1)d\) orbitals. Such complexes use outer \(d\)-orbitals and are generally high-spin.

Step 1: Determine oxidation state and \(d\)-electron configuration. \[ \begin{aligned} [Co(C_2O_4)_3]^{3-} &:\quad Co^{3+},\ d^6 \\ [CoF_6]^{3-} &:\quad Co^{3+},\ d^6 \\ [Fe(CN)_6]^{3-} &:\quad Fe^{3+},\ d^5 \\ [Mn(CN)_6]^{3-} &:\quad Mn^{3+},\ d^4 \end{aligned} \]

Step 2: Identify ligand strength. \[ \begin{aligned} F^- &:\quad \text{Weak-field ligand} \\ CN^- &:\quad \text{Strong-field ligand} \\ C_2O_4^{2-} &:\quad \text{Intermediate/strong-field ligand} \end{aligned} \]

Step 3: Determine magnetic behaviour. For \[ [CoF_6]^{3-} \] \(F^-\) is a weak-field ligand. No pairing occurs and the complex becomes high-spin. Hybridization: \[ sp^3d^2 \] using outer \(d\)-orbitals. Hence it is an outer orbital paramagnetic complex.

Step 4: Eliminate remaining options. \(CN^-\) produces low-spin inner orbital complexes. Therefore options (C) and (D) are inner orbital complexes. \[ \boxed{[CoF_6]^{3-}} \] Hence, option \(\mathbf{(B)}\) is correct.