Question

Write the formula for each of the following compounds, being sure to use brackets to indicate the coordination sphere: (a) hexaamminechromium(III) nitrate (b) tetraamminecarbonatocobalt(III) sulfate (c) dichlorobis(ethylenediamine) platinum(IV) bromide (d) potassium diaquatetrabromovanadate(III) (e) bis(ethylenediamine) zinc(II) tetraiodomercurate(II)

Short answer

\[[\text{Cr}(\text{NH}_3)_6] (\text{NO}_3)_3\], \[[\text{Co}(\text{NH}_3)_4 (\text{CO}_3)] (\text{SO}_4)_2\], \[[\text{Pt}(\text{en})_2 \text{Cl}_2] (\text{Br})_4\], \[\text{K}[\text{V}(\text{H}_2\text{O})_2 \text{Br}_4]\], \[[\text{Zn}(\text{en})_2] [\text{HgI}_4]\]

Step-by-step solution

(a) hexaamminechromium(III) nitrate

The compound has a chromium(III) ion in the coordination sphere, bound to six ammine ligands. The nitrate ion is the counter ion. The overall charge on the coordination sphere is +3 since chromium has a charge of +3. Thus, the formula is: \[ [\text{Cr}(\text{NH}_3)_6] (\text{NO}_3)_3 \]

(b) tetraamminecarbonatocobalt(III) sulfate

The compound has a cobalt(III) ion in the coordination sphere, bound to four ammine ligands and one carbonate ligand. The sulfate ion is the counter ion. The overall charge on the coordination sphere is +3 since cobalt has a charge of +3. Thus, the formula is: \[ [\text{Co}(\text{NH}_3)_4 (\text{CO}_3)] (\text{SO}_4)_2 \]

(c) dichlorobis(ethylenediamine) platinum(IV) bromide

The compound has a platinum(IV) ion in the coordination sphere, bound to two ethylenediamine ligands and two chloride ligands. The bromide ion is the counter ion. The overall charge on the coordination sphere is +4 since platinum has a charge of +4. Thus, the formula is: \[ [\text{Pt}(\text{en})_2 \text{Cl}_2] (\text{Br})_4 \]

(d) potassium diaquatetrabromovanadate(III)

The compound has a vanadate(III) ion in the coordination sphere, bound to two aqua (water) ligands and four bromide ligands. The overall charge on the coordination sphere is -1 since vanadate has a charge of +3 and there are four bromide ligands with a charge of -1 each. The counter ion is the potassium ion. Thus, the formula is: \[ \text{K}[\text{V}(\text{H}_2\text{O})_2 \text{Br}_4] \]

(e) bis(ethylenediamine) zinc(II) tetraiodomercurate(II)

The compound has a zinc(II) ion in the coordination sphere, bound to two ethylenediamine ligands. The overall charge on the coordination sphere is +2 since zinc has a charge of +2. The counter ion is the tetraiodomercurate(II) ion, which has a -2 charge since mercury has a charge of +2 and there are four iodide ligands with a charge of -1 each. Thus, the formula is: \[ [\text{Zn}(\text{en})_2] [\text{HgI}_4] \]

Key concepts

Coordination Sphere

In chemistry, the coordination sphere refers to the central atom/ion and its attached ligands. It represents the core part of a coordination compound. The central atom or ion in the coordination sphere is typically a metal, such as chromium in hexaamminechromium(III) nitrate or platinum in dichlorobis(ethylenediamine) platinum(IV) bromide. The ligands are the molecules or ions that are directly bonded to the metal atom. These can be neutral molecules, like ammonia in the above compounds, or negatively charged ions like chloride. The coordination sphere is enclosed in square brackets in the chemical formula, differentiating it from other components such as counter ions. By focusing on the coordination sphere, chemists can predict the geometry, reactivity, and properties of the compound. It's important to note that the overall charge of the coordination sphere results from the sum of the charges on the central metal ion and the ligands. For example, in the coordination sphere of hexaamminechromium(III), the metal, chromium, carries a +3 charge, while each ammonia ligand is neutral, giving the coordination sphere a total charge of +3.

Ligand

A ligand is a molecule or ion that binds to a central metal atom to form what is known as a coordination complex. Ligands play a key role in determining the chemical behavior and properties of coordination compounds. Ligands can be classified based on the number of donor atoms they possess:

• Monodentate Ligands: These have one donor atom that can coordinate to the metal, such as chloride ions ( ext{Cl}^-).
• Bidentate Ligands: These have two donor atoms, like ethylenediamine ( ext{en}), which can simultaneously coordinate to the metal, forming a chelate.
• Polydentate Ligands: These have more than two donor atoms and can form multiple bonds with the metal center. Examples include EDTA.

In coordination chemistry, the nature and number of ligands can influence the coordination number, geometry, and reactivity of the complex, greatly impacting its use in various chemical applications.

Coordination Number

The coordination number of a metal ion in a coordination compound is the total number of bonds formed between the metal and the ligands in its coordination sphere. This number gives insight into the spatial arrangement of the ligands around the metal center and helps predict the geometry of the compound. Common coordination numbers include:

• 4: Often leads to tetrahedral or square planar geometries.
• 6: Typically results in an octahedral geometry. For example, chromium in hexaamminechromium(III) with six ammonia ligands, exhibits an octahedral coordination.

The coordination number can be influenced by factors such as the size of the ligands, electronic configuration of the metal ion, and the overall charge of the complex. Understanding these parameters is crucial for predicting the structure and reactivity of coordination compounds.

Counter Ion

Counter ions are ions that are not part of the coordination sphere but are still essential for balancing the overall charge of a coordination compound. They often associate with the charged coordination complex to form a neutral compound. In formulas of coordination compounds:

• Counter ions help to achieve electrical neutrality in the compound.
• Their placement in the formula comes after the bracketed coordination sphere, which indicates they are not directly bonded to the central metal.

For example, in the compound hexaamminechromium(III) nitrate, nitrate ions ( ext{NO}_3^-) act as counter ions to balance the +3 charge of the coordination sphere. Similarly, in dichlorobis(ethylenediamine) platinum(IV) bromide, bromide ions ( ext{Br}^-) serve as counter ions for charge neutrality. Understanding the role of counter ions helps clarify the composition and properties of coordination compounds, especially when considering solubility and reactivity.