Question #44569

Consider three consecutive 3d transition metals $M_1$, $M_2$, and $M_3$ (where atomic numbers follow $Z_1 < Z_2 < Z_3$). The standard reduction potential $E^\circ(M^{2+}/M)$ for $M_2$ is anomalously more negative than the expected trend between $M_1$ and $M_3$. $M_1$ and $M_3$ form stable neutral mononuclear carbonyls $M_1(CO)_x$ and $M_3(CO)_y$ that strictly obey the 18-electron rule. $M_2$ forms a stable homoleptic binuclear carbonyl $(M_2)_2(CO)_z$ but no stable neutral mononuclear carbonyl.

Evaluate the following statements: \begin{enumerate} \item[I.] The anomalous $E^\circ(M^{2+}/M)$ of $M_2$ is primarily driven by the exchange energy stabilization of $M_2^{2+}$ and its low enthalpy of atomization. \item[II.] $M_2$ fails to form a stable neutral mononuclear carbonyl because its half-filled d-subshell completely inhibits $\pi$-backdonation. \item[III.] The C-O stretching frequency in the cationic complex $[M_2(CO)_x]^+$ is higher than in $M_1(CO)_x$. \item[IV.] The metal-carbon bond length in $[M_2(CO)_y]^-$ is shorter than in $M_3(CO)_y$. \end{enumerate}

How many of the above statements are CORRECT?