Question #44337

In transition metal coordination chemistry, thermodynamic stability is classically predicted by Fajans' rules, where a higher central cation charge enhances electrostatic polarization and metal-ligand attraction. However, for $\pi$-acceptor ligands, synergic orbital-overlap often dictates stability. Consider the isoelectronic octahedral complexes \ce{[V(CO)6]^-} (I), \ce{Cr(CO)6} (II), and \ce{[Mn(CO)6]^+} (III). Let $x$, $y$, and $z$ represent their respective \ce{C-O} stretching frequencies ($\nu_{\ce{CO}}$). Furthermore, let $y'$ be the average $\nu_{\ce{CO}}$ of the remaining carbonyl ligands when one \ce{CO} in \ce{Cr(CO)6} is substituted by \ce{PMe3} (trimethylphosphine). Which of the following correctly establishes the relationship between these frequencies, demonstrating the dominance of synergic back-bonding over simple electrostatic polarization?