Amongst the following (A) $[\mathrm{Mn}(\eta^5\text{-Cp})(\mathrm{CO})_3]$ (B) $[\mathrm{Os}(\eta^5\text{-Cp})_2]$ (C) $[\mathrm{Ru}(\eta^5\text{-Cp})_2]$ and (D) $[\mathrm{Fe}(\eta^5\text{-Cp})_2]$ the compounds with most shielded and deshielded Cp protons respectively, are
(a)D and A
(b)D and B
(c)C and A
(d)C and B
Answer
Answer (as printed): A
Explanation
Here, Cp is cyclopentadienyl ligand is one of the most common and popular ligands in organometallic chemistry. It is an anionic ligand that normally coordinates in an η5 mode as a 6 electron donor. Now, 18-electron rule apply on the complexes [Mn(η5-Cp)(CO) 3] does not follow 18-electron rule. So, this complex is most deshielded compound when we are going 3d to 4d and 4d to 5d series in the transition element the shielding effect decrease because of the electron density or the distance between metal to ligand, increases. So, [Fe(η5-Cp)2] is more shielded than other. Thus, the compound D is most shielded and compound A is most deshielded.
Explanation as extracted from the printed page; notation may be imperfect.