Q7(a)(i). Vaska's compound: structure and oxidative addition of HCl and O₂

1. Compound. trans-[IrCl(CO)(PPh₃)₂], carbonylchloridobis(triphenylphosphine)iridium(I), a yellow solid. It is made from Na₂IrCl₆ and PPh₃ in diethylene glycol under CO.
2. Structure and electron count. Ir(I) is d⁸: 8 (Ir) + 2 (Cl⁻) + 2 (CO) + 2 × 2 (PPh₃) = 16 valence electrons. The two bulky phosphines are trans; Cl and CO are trans to each other. Square planar, like other d⁸ ions of the heavy metals.
3. Oxidative addition. A molecule X–Y adds across the metal and its bond breaks: the oxidation state rises by 2 (I → III), the coordination number by 2 (4 → 6) and the electron count by 2 (16 → 18). A coordinatively unsaturated, electron-rich d⁸ centre favours it.
4. With HCl: [IrCl(CO)(PPh₃)₂] + HCl → [IrHCl₂(CO)(PPh₃)₂] Ir(III), d⁶, octahedral, 18 electrons; a hydrido–chlorido complex. H and Cl occupy the two new positions above and below the original plane (cis or trans to each other, depending on the solvent and mechanism).
5. With oxygen: [IrCl(CO)(PPh₃)₂] + O₂ ⇌ [IrCl(O₂)(CO)(PPh₃)₂] O₂ binds side-on (η²); it is reduced to a peroxo ligand O₂²-, so Ir becomes Ir(III). The O–O bond lengthens. The addition is reversible: bubbling an inert gas through the solution releases O₂ (a model for oxygen carriers).
6. Other additions: H₂ (cis dihydride), CH₃I, Cl₂; the reverse step, reductive elimination, releases the product in catalytic cycles.

Application. oxidative addition followed by reductive elimination is the core of homogeneous catalysis: Wilkinson's catalyst [RhCl(PPh₃)₃] hydrogenates alkenes this way, and the Monsanto acetic acid process starts with oxidative addition of CH₃I to [RhI₂(CO)₂]⁻.