Question

Draw the structure of \(\left[\mathrm{Re}_{2} \mathrm{Cl}_{8}\right]^{2-} ;\) discuss the metal-metal bonding in the anion and its consequences on ligand orientation.

Short answer

The complex \([\mathrm{Re}_{2} \mathrm{Cl}_{8}]^{2-}\) involves a Re-Re single bond with each Re bonded to four chloride ions, forming a square planar or bridging geometry due to metal bonding constraints.

Step-by-step solution

Understand the Composition of the Complex

The complex given is \( [\mathrm{Re}_{2} \mathrm{Cl}_{8}]^{2-} \). This means there are two rhenium (Re) atoms and eight chloride (Cl) ions, forming a complex with a net charge of \(-2\).

Determine the Oxidation State of Rhenium

Since chloride ions are each \(-1\) and the overall charge of the complex is \(-2\), the total charge contribution from chlorides is \(-8\). The total charge must be balanced by the charges on the rhenium atoms, which are the metal centers. Let \( x \) be the oxidation state of each rhenium. Therefore, \[ 2x + (-8) = -2 \]Solving for \( x \) gives:\[ 2x = 6 \ x = +3 \]Thus, each rhenium is in the +3 oxidation state.

Assess the Possible Rhenium-Rhenium Bonding

Since each rhenium is in a high +3 oxidation state, upon reduction, they can participate in metal-metal bonding. The complex likely contains a single Re–Re bond, because Re can exhibit lower oxidation states facilitating bond formation. Check literature for possible bonding modes.

Consider the Ligand Orientation

For \([\mathrm{Re}_{2} \mathrm{Cl}_{8}]^{2-}\), the coordination geometry around Re centers needs consideration. With a single Re-Re bond, each rhenium would coordinate with four chlorides, forming a square planar or bridging arrangement. Metal-metal bonding forces chlorides into a configuration consistent with maximized bonding and electronic symmetry, often retaining a close metal centre distance.

Key concepts

Oxidation States

Oxidation states are fundamental for understanding the charge flow and interactions within chemical compounds. For complex ions, knowing the oxidation states of the central metal ions can help deduce their electronic configuration and how they interact with ligands. In the complex \( [\mathrm{Re}_{2} \mathrm{Cl}_{8}]^{2-} \), calculating the oxidation state involves balancing the known charges. Chloride ions each have a \(-1\) charge, contributing a total of \(-8\) for all eight chlorides. The complex has an overall charge of \(-2\), so:

• Sum of rhenium oxidation states must cancel out residual charge from chlorides.
• The equation \( 2x - 8 = -2 \) helps us find the charge on each Re, resulting in an oxidation state of \(+3\) for each rhenium atom.

This understanding is crucial because it implies that each rhenium has three fewer electrons than the neutral atom, affecting how they bond with each other and other elements.

Coordination Chemistry

Coordination chemistry explores how metal atoms bind with ligands to form complex structures. In \( [\mathrm{Re}_{2} \mathrm{Cl}_{8}]^{2-} \), each rhenium atom coordinates with four chloride ligands, forming a square planar or bridging arrangement. The

• Metal-metal bond, a key feature in the compound, influences how chlorides arrange themselves around the rhenium atoms to enhance stability.
• Rhenium, possessing a full complement of \(d\)-orbitals, provides multiple bonding sites that support diverse coordination geometries, such as square planar.

Coordinating ligands like chloride ions dictate the spatial arrangement, accounting for ligand strength and electronic effects from the central metal atoms. This geometry allows complex ions to maintain lower energy and greater stability.

Complex Ions

Complex ions are charged species composed of a central metal ion attached to surrounding molecules or ions, called ligands. They play a significant role in chemical identity and reactivity. In the case of \( [\mathrm{Re}_{2} \mathrm{Cl}_{8}]^{2-} \), the complex ion is:

• Comprised of two rhenium metal centers bonded together.
• Surrounded by eight chloride ligands which neutralize much of the complex's inherent positive charge.

This setup creates a net charge of \(-2\), vital for the ion's interactions in solution or with other compounds. The ligands not only stabilize the complex ion but also affect its solubility, reactivity, and color. The presence of a metal-metal bond adds an intriguing dimension, showcasing how metals can interact directly within a larger structure. Complex ions like these are intriguing not only for their structure but also for their behavior in various chemical contexts.