Q.10 In compounds K4[Fe(CN)6] (P) and Fe(CO)5 (Q), the iron metal centre is bonded to
(A) C of CN− in P and C of CO in Q
(B) N of CN− in P and C of CO in Q
(C) C of CN− in P and O of CO in Q
(D) N of CN− in P and O of CO in Q

Potassium ferrocyanide K4[Fe(CN)6] (P) and iron pentacarbonyl Fe(CO)5 (Q) feature iron centers that bond specifically through carbon atoms in both ligands. The correct answer is option (A): C of CN⁻ in P and C of CO in Q.

Correct Answer

Option (A) states that the iron metal center bonds to the carbon (C) of cyanide (CN⁻) in K4[Fe(CN)6] and the carbon (C) of carbonyl (CO) in Fe(CO)5. Cyanide acts as CN⁻, where the carbon end donates electrons to form a sigma bond with Fe²⁺ (d⁶ low-spin configuration), confirmed by structural studies showing Fe-C distances around 1.93 Å. Carbonyl ligands in Fe(CO)5 bond via the lone pair on carbon to neutral Fe(0), enabling synergic σ-donation and π-backbonding, with Fe-C bond lengths near 1.81 Å.

Option Analysis

• (A) Correct: Matches standard coordination chemistry; M←C≡N for CN⁻ and M←C≡O for CO due to better orbital overlap and polarity.

• (B) Incorrect: N-bonding for CN⁻ (isonitrile mode, M←N≡C) is rare and weaker; occurs only under specific conditions like steric hindrance. CO never bonds via N.

• (C) Incorrect: CO bonding through oxygen (M-O≡C) is unstable and uncommon; oxygen lacks suitable lone pair for metal donation.

• (D) Incorrect: Both N of CN⁻ and O of CO contradict established structures; isocyanide and acyl modes do not apply here.

Bonding Mechanism

In K4[Fe(CN)6], CN⁻ is ambidentate but prefers C-end binding (hard-soft acid-base theory: soft Fe²⁺ matches soft C donor). Fe(CO)5 features trigonal bipyramidal geometry with all CO ligands σ-donating from C and accepting π-backbonding to CO π* orbitals, strengthening Fe-C bonds. These facts appear in exams like GATE Chemistry for testing ligand field knowledge.