Problem 28 - Chemistry The Science in Context

Question

What are the names of the following coordination compounds? a. \(\left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{Co}(\mathrm{CN})_{6}\right]\) b. \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}\right]\left(\mathrm{NO}_{3}\right)_{2}\) c. \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] \mathrm{Cl}\)

Step-by-Step Solution

Answer

Question: Name the following coordination compounds: a. \(\left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{Co}(\mathrm{CN})_{6}\right]\), b. \(\left[\mathrm{Co}(\mathrm{en})_{2}\mathrm{Cl}\right]\left(\mathrm{NO}_{3}\right)_{2}\), c. \(\left[\mathrm{Fe}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] \mathrm{Cl}\). Answer: a. trisammonium hexacyanocobalt(III), b. bis(ethylenediamine)chloridocobalt(II) nitrate, c. tetraaquadihydroxidoiron(III) chloride.

Step 1: Identify the central metal atom

Here, the central metal atom is \(\mathrm{Co}\) (Cobalt).

Step 2: Identify the ligands

The ligands present are \(\mathrm{NH}_{4}\) (ammonium) and \(\mathrm{CN}\) (cyanide).

Step 3: Determine the coordination number and oxidation state

There are 6 cyanide ligands surrounding cobalt with a charge of \(-1\) each, so the cobalt has an oxidation state of \(+3\). The coordination number is 6.

Step 4: Arrange ligands in alphabetical order and assign prefixes

For cyanide, we use the prefix "hexa-" since there are 6 of them.

Step 5: Name the complex ion according to IUPAC rules

The full name for this compound is: trisammonium hexacyanocobalt(III) b. \(\left[\mathrm{Co}(\mathrm{en})_{2}\mathrm{Cl}\right]\left(\mathrm{NO}_{3}\right)_{2}\)

Step 1: Identify the central metal atom

Here, the central metal atom is \(\mathrm{Co}\) (Cobalt).

Step 2: Identify the ligands

The ligands present are \(\mathrm{en}\) (ethylenediamine) and \(\mathrm{Cl}\) (chloride) for the complex ion, and \(\mathrm{NO}_{3}\) (nitrate) for the counter-ion.

Step 3: Determine the coordination number and oxidation state

Cobalt is surrounded by 2 \(\mathrm{en}\) and 1 \(\mathrm{Cl}\) ligands, giving it a coordination number of 3. The nitrate ions each have a charge of \(-1\), so the cobalt has an oxidation state of +2.

Step 4: Arrange ligands in alphabetical order and assign prefixes

We have 2 ethylenediamine ligands and 1 chloride ligand, so the prefix for ethylenediamine is "bis-" and chloride has no prefix.

Step 5: Name the complex ion according to IUPAC rules

The full name for this compound is: bis(ethylenediamine)chloridocobalt(II) nitrate c. \(\left[\mathrm{Fe}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] \mathrm{Cl}\)

Step 1: Identify the central metal atom

Here, the central metal atom is \(\mathrm{Fe}\) (Iron).

Step 2: Identify the ligands

The ligands present are \(\mathrm{H}_{2}\mathrm{O}\) (aqua) and \(\mathrm{OH}\) (hydroxide) for the complex ion, and \(\mathrm{Cl}\) (chloride) for the counter-ion.

Step 3: Determine the coordination number and oxidation state

Iron is surrounded by 4 aqua ligands and 2 hydroxide ligands, giving it a coordination number of 6. The chloride ion has a charge of \(-1\), so the iron has an oxidation state of +3.

Step 4: Arrange ligands in alphabetical order and assign prefixes

We have 4 aqua ligands and 2 hydroxide ligands, so the prefix for aqua is "tetra-" and for hydroxide is "di-".

Step 5: Name the complex ion according to IUPAC rules

The full name for this compound is: tetraaquadihydroxidoiron(III) chloride.

Key Concepts

IUPAC Naming Conventions

Understanding the IUPAC naming conventions for coordination compounds is crucial for chemistry students. First and foremost, the name of the central metal atom is stated, followed by the ligands in alphabetical order. It's important to use prefixes such as mono-, di-, tri-, etc., to indicate the number of each type of ligand present. If the ligand itself contains a prefix or is a polydentate ligand, special prefixes are used (e.g., bis-, tris-, tetrakis-). Following the ligands, the central metal’s oxidation state is specified in Roman numerals within parentheses. If the compound is a complex ion, the name of the counter-ion follows last. For instance, in \(\left[\mathrm{Co}(\mathrm{en})_{2}\mathrm{Cl}\right]\left(\mathrm{NO}_{3}\right)_{2}\), the name would reflect the ethylenediamine ligands as 'bis(ethylenediamine)', while the oxidation state of cobalt is indicated as (II) and 'nitrate' appended as the counter-ion name.

Ligands Identification

Identifying ligands involves recognizing both charged and neutral molecules or ions that donate electron pairs to the central metal atom to form coordination bonds. Common ligands are named with special terms, such as 'aqua' for \(\mathrm{H}_{2}\mathrm{O}\), 'cyanido' for \(\mathrm{CN}^{-}\), and 'ammine' for \(\mathrm{NH}_{3}\). Polydentate ligands, like ethylenediamine (\mathrm{en}), which can form multiple bonds with the central atom, are noteworthy for their chelating ability. In the case of \(\left[\mathrm{Fe}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] \mathrm{Cl}\), water (aqua) and hydroxide ions are the ligands bound to iron.

Oxidation States Determination

To determine the oxidation state of the central metal in a coordination compound, sum up the charges of the ligands and counter-ions, and consider the overall charge of the complex. For neutral complexes, the sum equates to zero, while for ionic complexes, it equates to the charge of the ion. For example, in \(\left[\mathrm{Fe}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] \mathrm{Cl}\), iron must have an oxidation state of +3 to balance the -1 charge from each hydroxide and to ensure the neutrality of the complex ion, considering the single chloride counter-ion.

Coordination Number

The coordination number of a central metal atom is the total number of ligand-metal bonds within a coordination compound. It reflects the number of ligand atoms directly bonded to the metal and it is a critical factor dictating the compound's geometry. In \(\left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{Co}(\mathrm{CN})_{6}\right]\), the coordination number is 6, corresponding to six cyanide ligands bonding to a central cobalt ion. This generally leads to an octahedral geometry of the complex.