Problem 38 - Chemistry: The Central Science

Question

Write names for the following coordination compounds: (a) \(\left[\mathrm{Cd}(\mathrm{en}) \mathrm{Cl}_{2}\right]\) (b) \(\mathrm{K}_{4}\left[\mathrm{Mn}(\mathrm{CN})_{6}\right]\) (c) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{s}\left(\mathrm{CO}_{3}\right)\right] \mathrm{Cl}\) (d) \(\left[\operatorname{lr}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\left(\mathrm{NO}_{3}\right)_{3}\)

Step-by-Step Solution

Answer

The names for the given coordination compounds are: (a) Dichloro(ethylenediamine)cadmium(II) (b) Potassium hexacyanomanganese(II) (c) Pentaamminecarbonatochromium(III) chloride (d) Diamminetetraaquairidium(III) trinitrate

Step 1: (a) Naming the coordination compound: \(\left[\mathrm{Cd}(\mathrm{en}) \mathrm{Cl}_{2}\right]\)

For this compound, we have: 1. Ligands: en (ethylenediamine) and Cl (Chloride) 2. Central metal atom: Cd (Cadmium) 3. Oxidation state of Cd: Since en is neutral, and Cl has a charge of -1 each, the oxidation state of Cd is +2 4. Since there is only one ethylenediamine and two chloride ligands present, we don't need any prefixes for them. Name: \(\mathrm{Dichloro(ethylenediamine)cadmium(II)}\)

Step 2: (b) Naming the coordination compound: \(\mathrm{K}_{4}\left[\mathrm{Mn}(\mathrm{CN})_{6}\right]\)

For this compound, we have: 1. Ligands: CN (Cyanide) 2. Central metal atom: Mn (Manganese) 3. Oxidation state of Mn: Since there are 4 potassium cations with a charge of +1 each, and 6 cyanide anions with a charge of -1 each, the oxidation state of manganese is +2 4. Since there are six cyanide ligands present, we will use the prefix "hexa". Name: \(\mathrm{Potassium\ hexacyanomanganese(II)}\)

Step 3: (c) Naming the coordination compound: $\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{5}\left(\mathrm{CO}_{3}\right)\right] \mathrm{Cl}$

For this compound, we have: 1. Ligands: NH₃ (Ammine) and CO₃ (Carbonate) 2. Central metal atom: Cr (Chromium) 3. Oxidation state of Cr: Since ammine is neutral, CO₃ has a charge of -2, and Cl has a charge of -1, the oxidation state of chromium is +3 4. Since there are five ammine ligands present, we will use the prefix "penta". Name: \(\mathrm{pentaamminecarbonatochromium(III)\ chloride}\)

Step 4: (d) Naming the coordination compound: $\left[\operatorname{lr}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\left(\mathrm{NO}_{3}\right)_{3}$

For this compound, we have: 1. Ligands: NH₃ (Ammine) and H₂O (Aqua) 2. Central metal atom: lr (Iridium) 3. Oxidation state of lr: Since ammine and aqua are both neutral, the charge of the iridium complex must be +3, which means the oxidation state of iridium is also +3 4. Since there are four ammine ligands and two aqua ligands, we will use the prefixes "tetra" and "di" respectively. Name: \(\mathrm{diamminetetraaquairidium(III)\ \ trinitrate}\)

Key Concepts

Ligands

In coordination chemistry, ligands are ions or molecules that bind to a central metal atom to form a coordination complex. They act as donors, providing electrons to the central metal atom. Ligands can be classified based on their charge, size, and the number of donor atoms. They range from simple ions like chloride (Cl⁻) to more complex molecules like ethylenediamine (en).

• Monodentate ligands bind through a single donor atom, such as chloride (Cl⁻) and cyanide (CN⁻).
• Polydentate ligands, like ethylenediamine (en), bind through multiple donor atoms. Ethylenediamine, for example, has two nitrogen atoms that can attach to the metal.

Understanding the types of ligands is fundamental for determining the stability and geometry of coordination compounds.

Central Metal Atom

At the heart of every coordination compound is the central metal atom. This atom or ion holds the coordination sphere together and largely determines the compound's properties and reactivity. The metal's available orbitals participate in bonding with ligands.

• The type of metal atom often dictates the color, magnetic properties, and bioactivity of the compound.
• Common metal atoms in coordination compounds include transition metals such as chromium (Cr), manganese (Mn), cadmium (Cd), and iridium (Ir). These metals are known for their ability to form multiple oxidation states and complex geometries.

The nature of the central metal atom considerably affects the coordination complex's characteristics and applications.

Oxidation States

Oxidation states in coordination compounds refer to the total number of electrons that an atom gains or loses in forming a bond. It is a crucial component in defining the overall charge and stability of the coordination complex. Determining the oxidation state of the central metal atom is essential for naming and analyzing such complexes.

• The oxidation state is often determined by considering the charges on the ligands and any counter ions that balance the complex's charge.
• For example, in the complex \([\mathrm{Cd}(\mathrm{en})\mathrm{Cl}_{2}]\), cadmium has an oxidation state of +2 due to the neutral nature of ethylenediamine and the -1 charge of each chloride ion.

Correctly identifying the oxidation state aids in predicting the compound's reactivity and interactions with other chemical species.

Naming Coordination Compounds

Naming coordination compounds follows specific nomenclature rules to clearly convey the composition and structure of the complex. The IUPAC system provides systematic ways to name these compounds, ensuring consistency and clarity.

• The ligands are named first, in alphabetical order, using prefixes to indicate the number of each type (e.g., "di," "tri," "tetra").
• The central metal atom's name comes next, followed by the oxidation state in Roman numerals enclosed in parentheses.
• For negatively charged complexes, the metal's name is given using its Latin root, followed by "-ate."

As an example, the name for \([\mathrm{Cd}(\mathrm{en})\mathrm{Cl}_{2}]\) is "Dichloro(ethylenediamine)cadmium(II)," where the ligands "dichloro" and "ethylenediamine" precede the metal "cadmium," with its oxidation state "(II)." Understanding these rules helps chemists communicate precise information about the composition and structure of coordination compounds.