Select correct order of C − O bond length:

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Question Explanation: Determine which species has the longest $C - O$ bond length (compare $Cr (CO)_{6}$ , CO , and $CO^{+}$ ).

Concept: This question is based on Metal-CO $π$ -backbonding versus bond order trends. Solution: In free CO , the $C - O$ bond is short $(\sim 1.128 \overset{˚}{A})$ due to its triple bond character. In $CO^{+}$ , the bond order increases (around 3.5), shortening the bond further. In a metal carbonyl like $Cr (CO)_{6}$ , strong $π$ backbonding from the metal into CO's antibonding orbitals lowers the bond order and weakens the C-O bond, making it longer. Therefore, ranking bond lengths from longest to shortest gives: ( $C - O$ ) in $Cr (CO)_{6}$ is largest, then $(C - O)$ in CO , and $(C - O)$ in $CO^{+}$ is shortest .

The correct order is $(C - O) (Cr (CO)_{6}) > (C - O) (CO) > (C - O) (CO^{+})$ .

Final Answer: Option (A)

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