Q1. CO reacts readily with
(A) Fe
(B) Fe2+
(C) Fe4+
(D) Fe3+
CO reacts most readily with Fe²⁺ (option B).
This preference is fundamental in bioinorganic and coordination chemistry, including hemoglobin–CO binding.
Correct option and concept
Carbon monoxide is a strong π‑acceptor (π‑acid) ligand that forms very stable metal carbonyl complexes with low or moderate oxidation states of transition metals, especially Fe⁰ and Fe²⁺.
In biological systems, CO binds tightly to the Fe²⁺ center of heme in hemoglobin, forming carboxyhemoglobin, which illustrates that CO reacts most readily with Fe²⁺ rather than Fe³⁺.
Correct answer: (B) Fe²⁺.
Option‑wise explanation
Option (A) Fe (metallic iron)
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Metallic Fe⁰ can form iron carbonyls such as Fe(CO)₅ and Fe₂(CO)₉ under suitable conditions, showing that CO can react with elemental iron.
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However, in typical coordination/biological chemistry MCQs, the focus is on ionic/oxidation states relevant to complex formation in solution; Fe²⁺ is preferred over bulk Fe⁰ for “reacts readily with CO” type questions.
Option (B) Fe²⁺ (correct)
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Fe²⁺ is a d⁶ ion of moderate positive charge and suitable electron density, which supports strong σ‑donation from CO and efficient π back‑bonding into CO’s π* orbitals.
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In hemoglobin, O₂ and CO bind only when iron is in the Fe²⁺ state; CO forms an especially stable carboxyhemoglobin complex by binding to Fe²⁺, demonstrating that CO reacts most readily with Fe²⁺ among the common iron oxidation states.
Option (C) Fe⁴⁺
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Fe⁴⁺ is a highly oxidized, strongly electron‑poor state that occurs only in a few high‑valent oxo or peroxo complexes under specific conditions.
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Such a highly oxidizing center is very unfavorable for π‑acid ligands like CO, so Fe⁴⁺ does not “readily” form Fe–CO bonds in ordinary coordination or biological contexts and is not considered for standard exam MCQs.
Option (D) Fe³⁺
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Fe³⁺ is more electron‑poor than Fe²⁺ and less efficient at π back‑bonding to CO’s π* orbitals, which weakens Fe–CO bond formation compared with Fe²⁺.
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In heme systems, Fe³⁺ (methemoglobin) cannot bind O₂ properly, and CO binding is also strongly disfavored, highlighting that Fe³⁺ is not the preferred oxidation state for CO coordination in physiological or simple coordination environments.
Why CO prefers Fe²⁺ over Fe³⁺
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CO donates an electron pair from carbon to the metal (σ‑bond) and simultaneously accepts electron density back from filled metal d‑orbitals (π back‑bonding), which stabilizes the M–CO bond.
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Fe²⁺ has higher d‑electron density and lower effective nuclear charge than Fe³⁺, making back‑bonding stronger and the Fe–CO complex significantly more stable.
SEO‑friendly introduction
Carbon monoxide (CO) is a classic π‑acceptor ligand that forms strong bonds with specific oxidation states of iron, especially Fe²⁺.
Understanding why CO reacts readily with Fe2+ but not as effectively with Fe³⁺ or Fe⁴⁺ is crucial for mastering coordination chemistry, bioinorganic concepts like hemoglobin toxicity, and exam MCQs based on metal–ligand bonding.
