Question

What is the formula, including the charge, for each of the following complexes? (a) An iridium(III) complex with three ammonia and three chloride ligands (b) A chromium(III) complex with two water and two oxalate ligands (c) A platinum(IV) complex with two ethylenediamine and two thiocyanate ligands

Short answer

(a) [Ir(NH₃)₃Cl₃] (neutral), (b) [Cr(H₂O)₂(C₂O₄)₂]⁻, (c) [Pt(en)₂(SCN)₂]²⁺.

Step-by-step solution

Identify the central metal and oxidation state

To find the formula and charge of each complex, start by identifying the central metal ion and its oxidation state. This information is given: (a) Iridium(III) indicates Ir with a +3 oxidation state. (b) Chromium(III) indicates Cr with a +3 oxidation state. (c) Platinum(IV) indicates Pt with a +4 oxidation state.

Determine the ligands and their charges

Next, list the ligands and their charges: (a) Ammonia (NH₃) is a neutral ligand and Chloride (Cl⁻) is a monodentate ligand with a -1 charge. (b) Water (H₂O) is a neutral ligand and Oxalate (C₂O₄²⁻) is a bidentate ligand with a -2 charge. (c) Ethylenediamine (en) is a neutral bidentate ligand and Thiocyanate (SCN⁻) is a monodentate ligand with a -1 charge.

Calculate the total charge of the complexes

For each complex, calculate the total charge: (a) Iridium has a +3 charge, 3 chloride ligands contribute -3. Total Charge = +3 - 3 = 0. (b) Chromium has a +3 charge and 2 oxalate ligands contribute -4. Total Charge = +3 - 4 = -1. (c) Platinum has a +4 charge, and 2 thiocyanate ligands contribute -2. Total Charge = +4 - 2 = +2.

Write the formulas including the charges

Construct the formula for each complex including charges: (a) [Ir(NH₃)₃Cl₃], with a net charge of 0. (b) [Cr(H₂O)₂(C₂O₄)₂]⁻ due to net charge of -1. (c) [Pt(en)₂(SCN)₂]²⁺ due to net charge of +2.

Key concepts

Oxidation State

The oxidation state refers to the hypothetical charge a central metal ion would have if all bonds to ligands were purely ionic. It’s a crucial detail in coordination chemistry, helping us determine the overall charge of the complex ion.
For instance, in our examples, the oxidation states were clearly specified as Iridium(III), Chromium(III), and Platinum(IV). The Roman numerals indicate that Iridium and Chromium each carry a charge of +3, while Platinum carries a +4 charge. These values are crucial, as they allow us to calculate the overall charge by balancing the charges from the metal ions and the ligands.
To determine the oxidation state, follow these steps:

• Identify the central metal ion and its given oxidation state from the problem statement.
• Use this information to balance the charges from the ligands and the central metal ion.

Remember, the sum of the charges from the central metal ion and ligands must equal the overall charge of the complex ion, as shown in the examples.

Complex Ions

Complex ions consist of a central metal ion bonded to one or more ligands, forming a stable complex with a specific geometry or shape. The entire set, including the metal and its attached ligands, forms a complex ion.
Complex ions can be either negatively charged, positively charged, or neutral, depending on the sum of the charges from the central metal ion and the ligands. In our problems, each complex has its unique composition and charge:

• Iridium complex is neutral: [Ir(NH₃)₃Cl₃]
• Chromium complex has a -1 charge: [Cr(H₂O)₂(C₂O₄)₂]⁻
• Platinum complex has a +2 charge: [Pt(en)₂(SCN)₂]²⁺

You can see that a careful selection and combination of ligands, along with knowing the oxidation state of the metal, lets us predict the overall behavior and charge of the complex ion. This understanding is critical for solving coordination chemistry problems efficiently.

Ligands

Ligands are ions or molecules that can donate a pair of electrons to the central metal ion, forming coordinate covalent bonds. They can drastically influence the chemistry, shape, and properties of complex ions.
Ligands can be categorized based on their denticity, or the number of donor atoms present:

• Monodentate ligands: Bind through a single donor atom, such as Chloride (Cl⁻) and Thiocyanate (SCN⁻). These contribute one bond to the central metal ion.
• Bidentate ligands: Bind through two donor atoms, such as Oxalate (C₂O₄²⁻) and Ethylenediamine (en), forming two bonds.

Neutral ligands like Ammonia (NH₃) and Water (H₂O) can also act as electron pair donors without adding additional charge to the complex. Understanding the type and charge contribution of each ligand is crucial for deducing the overall charge of the complex ion.

Central Metal Ion

The central metal ion is at the heart of a coordination complex, serving as the site for ligand attachment. It dictates the complex’s properties and plays a significant role in determining the geometry and charge of the complex.
In our examples:

• Iridium (Ir) in the first complex has an oxidation state of +3, contributing to a neutral overall charge when combined with its ligands.
• Chromium (Cr) also has an oxidation state of +3, but due to more negatively charged ligands, the overall charge becomes -1.
• Platinum (Pt), with an oxidation state of +4, results in a +2 overall charge when balanced against its ligands.

The type and oxidation state of the central metal ion greatly influence the chemical behavior and stability of the coordination compound. As you study coordination chemistry, pay attention to how different metals and oxidation states affect the resulting complexes.