The complex trans-(Ph₃P)₂Ir(CO)Cl (Vaska’s complex) undergoes oxidative addition of H₂ to give a six‑coordinate dihydride iridium(III) product, where both hydrides are added cis to each other but trans to CO and Cl, corresponding to option (D).​​

Introduction

The organometallic complex trans‑(Ph₃P)₂Ir(CO)Cl, widely known as Vaska’s complex, is a square‑planar Ir(I) 16‑electron species famous for reversible oxidative addition reactions with small molecules such as H₂, O₂ and halogens. When H₂ reacts with trans‑(Ph₃P)₂Ir(CO)Cl, the metal center undergoes oxidative addition to yield an Ir(III) dihydride, increasing both coordination number and oxidation state. Understanding which structural isomer forms is essential for solving the given multiple‑choice question and for mastering oxidative addition concepts in organometallic chemistry.​

Stepwise solution and mechanism

• The starting complex trans‑(Ph₃P)₂Ir(CO)Cl is Ir(I), d⁸, square planar and 16‑electron, which makes it highly suitable for oxidative addition of H₂.​

• Oxidative addition of H₂ formally adds H⁻ and H⁻ (or H⁺/H⁻ pair) to the metal, raising Ir from +1 to +3 and increasing coordination number from 4 to 6, giving an 18‑electron octahedral Ir(III) dihydride.​

• In Vaska’s complex and its analogues, experimental and textbook discussions show that H₂ addition yields an octahedral IrH₂Cl(CO)(PPh₃)₂ complex where the two hydrides occupy cis positions, while CO and Cl remain mutually trans and each trans to one PPh₃ ligand.​

• Thus, the primary product is the octahedral dihydride shown in option (D), containing: two PPh₃ ligands, one CO, one Cl, and two hydrides arranged so that CO is trans to one PPh₃ and Cl is trans to the other PPh₃, with the two hydrides cis.​

Therefore, the correct answer is (D).​

Detailed analysis of each option

Option (A)

• Structure in (A) shows a square‑planar Ir complex with only one hydride ligand and loss of the chloride, still four‑coordinate and essentially Ir(I) or Ir(III) with 16 electrons depending on counting.

• Oxidative addition of H₂ to a d⁸ square‑planar complex should increase the coordination number by two, giving a six‑coordinate 18‑electron species rather than a four‑coordinate complex.​

• In known chemistry of Vaska’s complex, the first step with H₂ is not simple substitution of Cl⁻ but true oxidative addition to give a dihydride, so a monohydride, four‑coordinate product as in (A) is inconsistent with the typical mechanism and is not the primary product.​

Hence, option (A) is incorrect.

Option (B)

• Option (B) appears to represent a three‑center Ir–(H₂) σ‑complex or an η²‑H₂ adduct, where H₂ is bound side‑on to Ir without full cleavage of the H–H bond.

• While a σ‑H₂ complex can be a transient intermediate on the oxidative‑addition pathway, the thermodynamically favored isolated product for Vaska’s complex with H₂ is the Ir(III) dihydride, not a stable η²‑H₂ adduct under typical conditions.​

• The question asks for the species “primarily” produced, which corresponds to the dihydride oxidative‑addition product, not the short‑lived H₂ adduct, so (B) does not match the expected outcome.​

Thus, option (B) is incorrect as the main product.

Option (C)

• Option (C) shows an octahedral Ir complex with two hydride ligands, one CO, one Cl and one PPh₃ ligand, implying that one PPh₃ has been lost during reaction with H₂.

• Experimentally and in model problems, oxidative addition of H₂ to trans‑(PPh₃)₂Ir(CO)Cl does not require dissociation of a PPh₃; the dihydride product retains two PPh₃ ligands, keeping the 18‑electron count with six ligands around Ir.​

• Loss of PPh₃ would give a 16‑electron Ir(III) complex and is not the primary product under standard H₂ addition conditions, making the formulation of (C) inconsistent with the well‑established behavior of Vaska’s complex.​

Therefore, option (C) is incorrect.

Option (D)

• Option (D) depicts an octahedral Ir(III) complex IrH₂Cl(CO)(PPh₃)₂ with two hydrides, one CO, one Cl, and two PPh₃ ligands.

• This structure fits the requirements of oxidative addition: oxidation state Ir(I) → Ir(III), coordination number 4 → 6, and electron count 16 → 18 with two new hydride ligands.​

• Literature and problem sets treating trans‑Ir(PPh₃)₂(CO)Cl show this dihydride as the product of H₂ addition, matching the connectivity and ligand set in (D), which is why authoritative solutions specify (D) as the correct answer.​

Hence, option (D) is correct and represents the primary product of the reaction.

Key concepts highlighted

• Oxidative addition of H₂ to a d⁸, square‑planar metal center increases both oxidation state and coordination number, commonly giving an octahedral dihydride complex.​

• Vaska’s complex trans‑IrCl(CO)(PPh₃)₂ is a classic example that adds H₂, O₂, and other small molecules, illustrating fundamental principles of organometallic reactivity, electron counting, and ligand effects.​

• Correct identification of the product requires considering oxidation state changes, electron count, ligand retention or loss, and typical stereochemical outcomes of oxidative addition, leading to selection of option (D) in this MCQ.​