Correct option is (D) \([V (CO)_6]^{\ominus}\)
(A) \(\mathrm{Mn}^{+}=3 \mathrm{~d}^5 4 \mathrm{~s}^1;\) in presence of CO , effective configuration \(=3 \mathrm{~d}^6 4 \mathrm{~s}^0.\)
Three lone pair for back bonding with vacant orbital of C in CO .
(B) \(F e^0=3 d^6 4 s^2;\) in presence of CO, effective configuration \(=3 d^8\).
Four lone pair for back bonding with CO.
(C) \(C r^0=3 d^5 4 s^1;\) effective configuration \(=3 d^6.\)
Three lone pair for back bonding with CO.
(D) \(V^{-}=3 d^4 4 s^2;\) effective configuration \(=3 d^6\).
Three lone pair for back bonding with CO.
Maximum back bonding is present in \(F e(C O)_5\) therefore, CO bond order is lowest here.
