Question

The homogeneous catalyst whose metal ion does NOT undergo either oxidation or reduction in any of the steps during the hydrogenation of terminal olefins is:

• A. RhCl(PPh$_3$)$_3$
• B. HRuCl(PPh$_3$)$_3$
• C. [Ir(COD)(PCy$_3$)(Py)]$^+$ PF$_6^-$   (COD = cyclooctadiene)
• D. [Rh(COD)(PPh$_3$)$_2$]$^+$ PF$_6^-$    (COD = cyclooctadiene)

Hint:

When analyzing catalytic hydrogenation mechanisms, check if the complex already contains a metal-hydride and whether it avoids oxidative addition or reductive elimination steps—this often implies no redox change at the metal center.

Answer 1

The Correct Option is B

In hydrogenation reactions catalyzed by transition metal complexes, the metal center may or may not undergo a change in oxidation state depending on the mechanism.

Let’s examine each complex:

• (A) RhCl(PPh₃)₃ is the Wilkinson's catalyst. It operates via oxidative addition of H₂ (Rh(I) to Rh(III)) and reductive elimination—so the metal undergoes redox changes. WRONG
• (B) HRuCl(PPh₃)₃ is a ruthenium(II) complex that already contains a hydride. It operates via associative substitution and migratory insertion, but does not change oxidation state during the hydrogenation process. CORRECT
• (C) [Ir(COD)(PCy₃)(Py)]⁺ undergoes oxidative addition of H₂ to form Ir(III), followed by reductive elimination. WRONG
• (D) [Rh(COD)(PPh₃)₂]⁺ also follows a similar redox cycle involving Rh(I) to Rh(III) transition. WRONG

Thus, the correct complex where the metal ion remains in the same oxidation state throughout is:

\[ \boxed{(B)} \]