Among the following complexes, the one showing the highest rate of substitution of a CO ligand on heating with one equivalent of $\mathrm{PPh_3}$ in decalin, is
(a)$\mathrm{(\eta^{5}\text{-}C_5H_5)Mn(CO)_3}$
(b)$\mathrm{(\eta^{5}\text{-}C_5Ph_5)Mn(CO)_3}$
(c)$\mathrm{(\eta^{5}\text{-}C_5Me_5)Mn(CO)_3}$
(d)$\mathrm{(\eta^{5}\text{-}indenyl)Mn(CO)_3}$
Answer
Answer (as printed): D
Explanation
The highest rate of substitution of a CO ligand on heating with $\mathrm{PPh_3}$ is easy where slippage of haptacity is easy.For slippage is easy because it is not at the cost of aromatic character ($\eta^5 \rightarrow \eta^3$).But $\eta^5\text{-Cp*}$ complexes, rate decreases because with respect Cp slippage of haptacity is due to at the cost of aromatic character.