Vaska's Complex Reaction with O₂

Vaska's complex, with the chemical formula \( \text{[Ir(CO)(PPh}_3\text{)}_2\text{Cl]} \), is a well-known square planar coordination complex of iridium(I). It is famous for its ability to undergo oxidative addition reactions, including the reversible binding of diatomic oxygen (\( \text{O}_2 \)).

When Vaska's complex reacts with \( \text{O}_2 \), the \( \text{O}_2 \) molecule inserts into the Ir-Cl bond or adds across the metal center, leading to an increase in the coordination number and oxidation state of the metal.

The reaction can be represented as:

\( \text{[Ir(CO)(PPh}_3\text{)}_2\text{Cl]} + \text{O}_2 \rightleftharpoons \text{[Ir(O}_2\text{)(CO)(PPh}_3\text{)}_2\text{Cl]} \)

Oxidation State of the Metal Ion

In the reactant, Vaska's complex \( \text{[Ir(CO)(PPh}_3\text{)}_2\text{Cl]} \):

• CO is neutral (oxidation state 0).
• \( \text{PPh}_3 \) is neutral (oxidation state 0).
• Cl has an oxidation state of -1.

Let the oxidation state of Ir be x.

\( x + (0) + 2 \times (0) + (-1) = 0 \)

\( x - 1 = 0 \)

\( x = +1 \)

So, the oxidation state of iridium in Vaska's complex is +1.

In the product, the \( \text{O}_2 \) molecule binds to the iridium center. In this adduct, \( \text{O}_2 \) typically binds as a peroxide ion (\( \text{O}_2^{2-} \)), where each oxygen atom has an oxidation state of -1, making the total charge of the \( \text{O}_2 \) ligand -2.

In the product \( \text{[Ir(O}_2\text{)(CO)(PPh}_3\text{)}_2\text{Cl]} \):

• \( \text{O}_2 \) ligand has an oxidation state of -2 (as peroxide).
• CO is neutral (oxidation state 0).
• \( \text{PPh}_3 \) is neutral (oxidation state 0).
• Cl has an oxidation state of -1.

Let the oxidation state of Ir be y.

\( y + (-2) + (0) + 2 \times (0) + (-1) = 0 \)

\( y - 3 = 0 \)

\( y = +3 \)

Thus, the oxidation state of the metal ion (iridium) in the product is +3.

Coordination Geometry

In Vaska's complex \( \text{[Ir(CO)(PPh}_3\text{)}_2\text{Cl]} \), iridium is 4-coordinate with a square planar geometry.

When \( \text{O}_2 \) binds side-on (\( \eta^2 \)), it occupies two coordination sites around the metal center. In the product \( \text{[Ir(O}_2\text{)(CO)(PPh}_3\text{)}_2\text{Cl]} \), the ligands are CO, two \( \text{PPh}_3 \) molecules, Cl, and the \( \text{O}_2 \) molecule bound side-on. Counting the points of attachment to the metal:

• CO: 1 coordination site
• \( \text{PPh}_3 \): 2 coordination sites (one for each \( \text{PPh}_3 \))
• Cl: 1 coordination site
• \( \text{O}_2 \) (side-on): 2 coordination sites (one for each oxygen atom)

Total coordination sites = \( 1 + 2 + 1 + 2 = 6 \).

A metal complex with a coordination number of 6 typically adopts an octahedral geometry. The product \([Ir^{III}(O_2)(CO)(PPh_3)_2Cl]\) is expected to have an octahedral or distorted octahedral geometry around the iridium center.

Mode of Binding of O₂

Dioxygen (\( \text{O}_2 \)) can bind to transition metals in several ways. The most common modes are end-on (\( \eta^1 \)) and side-on (\( \eta^2 \)).

• End-on (\( \eta^1 \)): The \( \text{O}_2 \) molecule is bonded through only one oxygen atom, like in \( \text{V}- \text{O-O} \). This increases the coordination number by 1.
• Side-on (\( \eta^2 \)): The \( \text{O}_2 \) molecule is bonded through both oxygen atoms simultaneously, forming a three-membered ring with the metal (\( \text{M(}\eta^2\text{-O}_2\text{)} \)). This increases the coordination number by 2.

In the adduct formed by Vaska's complex and \( \text{O}_2 \), the \( \text{O}_2 \) molecule is known to bind side-on (\( \eta^2 \)). The Ir and the two oxygen atoms form a triangular arrangement.

Summary of Product Characteristics

Based on the analysis, in the product formed by the reaction of Vaska's complex with \( \text{O}_2 \), the oxidation state of the metal ion is +3, the coordination geometry is octahedral, and the mode of binding of \( \text{O}_2 \) is side-on.