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) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{CO}_{3}\right] \mathrm{Cl}\) (d) \(\left[\operatorname{Ir}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\left(\mathrm{NO}_{3}\right)_{3}\)

Step-by-Step Solution

Answer

The correct names for the coordination compounds are: (a) dichlorobis(ethylenediamine)cadmium(II) (b) potassium hexacyanomanganate(II) (c) pentaamminecarbonatochromium(III) chloride (d) tetraammineaquodichloro-iridium(III) nitrate

Step 1: Identify central metal, ligands and their count

In this compound, the central metal is Cd. The ligands are en (ethylenediamine) and two Cl atoms.

Step 2: Name the compound

The correct name for this compound is: \(\textbf{dichlorobis(ethylenediamine)cadmium(II)}\) (b)

Step 3: Identify central metal, ligands and their count

In this compound, the central metal is Mn. The ligands are six CN groups. The counter ion is K.

Step 4: Name the compound

The correct name for this compound is: \(\textbf{potassium hexacyanomanganate(II)}\) (c)

Step 5: Identify central metal, ligands and their count

In this compound, the central metal is Cr. The ligands are five NH3 molecules and one CO3 group. The counter ion is Cl.

Step 6: Name the compound

The correct name for this compound is: \(\textbf{pentaamminecarbonatochromium(III) chloride}\) (d)

Step 7: Identify central metal, ligands and their count

In this compound, the central metal is Ir. The ligands are four NH3 molecules and two H2O groups. The counter ion is NO3.

Step 8: Name the compound

The correct name for this compound is: \(\textbf{tetraammineaquodichloro-iridium(III) nitrate}\)

Key Concepts

Central Metal Identification

Understanding the central metal in a coordination compound is foundational in its chemical identity. The central metal atom or ion acts as the hub around which ligands (molecules or ions that surround the central atom in a complex) are arranged.

For example, in the compound \(\left[\mathrm{Cd}(\mathrm{en}) \mathrm{Cl}_{2}\right]\), the central metal is cadmium (Cd). Identifying the central metal is the first step in understanding the structure of a coordination compound, as its properties significantly influence the overall characteristics of the compound. Therefore, when analyzing such a compound, one should always start by looking for the central metallic element, which is usually located inside the square brackets.

Ligands and Their Count

Ligands are atoms, ions, or molecules that can donate a pair of electrons to the central metal to form a coordination bond. The number and type of ligands are critical for the compound's structure and properties. Organic ligands often have specific names, such as 'en' for ethylenediamine seen in the compound \(\left[\mathrm{Cd}(\mathrm{en}) \mathrm{Cl}_{2}\right]\), where 'en' acts as a bidentate ligand, meaning it has two points of attachment to the central metal.

Ligand count facilitates the correct formatting of the compound's name. For instance, 'bis' is prefixed before 'ethylenediamine' in the given cadmium complex due to the presence of two of these ligands. Counting ligands correctly is essential for accurate naming. For example, hexa-, penta-, and tetra- prefixes are used for six, five, and four ligands, respectively.

Nomenclature Rules for Coordination Chemistry

The nomenclature of coordination compounds follows specific rules set by the International Union of Pure and Applied Chemistry (IUPAC). Knowing these rules is vital for generating the correct names of coordination compounds. The rules include sequence of naming where the cation is named before the anion, and within the complex ion, ligands are named before the central metal.

Additionally, the number of each type of ligand in the complex is indicated by Greek prefixes, and the ligands themselves are named in a specific order: neutral ligands, followed by anionic ones, and then the central metal. Modification of the central metal's name is often necessary to reflect the oxidation state, which is indicated by Roman numerals in parentheses—for example, chromium(III). In the case of anionic complexes, the metal name ends with 'ate', such as in hexacyanomanganate(II) from the potassium hexacyanomanganate(II) compound.

Oxidation State of Metal Ions

The oxidation state of a metal ion in a coordination compound signifies the number of electrons it has donated or accepted when forming bonds with ligands. It's a critical aspect of the compound's chemistry and is included in its name. Oxidation states are represented by Roman numerals in parentheses immediately following the metal's name within the complex.

As an example, in the compound name potassium hexacyanomanganate(II), the (II) indicates that the manganese ion has an oxidation state of +2. Determining the correct oxidation state can involve calculating the total charge of the ligands and any counter ions present, to ensure charge balance with the central metal ion. It's important to include the correct oxidation state to avoid confusion about a compound's identity, as different oxidation states typically lead to different properties.