Question

What is the coordination number of Fe in \(\left.\mathrm{FeCl}_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}(\mathrm{en})\right] ?\) What is the oxidation number of iron in this complex?

Short answer

The coordination number of Fe in the complex \(\mathrm{FeCl}_{2}(\mathrm{H}_{2}\mathrm{O})_{2}(\mathrm{en})\) is 6. The oxidation number of Fe in the complex is +2.

Step-by-step solution

Determine the coordination number

The coordination number of a central metal ion in a complex refers to the number of ligand atoms that are directly bonded to the metal. Analyze the given complex \(\mathrm{FeCl}_{2}(\mathrm{H}_{2}\mathrm{O})_{2}(\mathrm{en})\) and count the number of ligand atoms bonded to the metal ion. Chloride ligands contribute 2, water molecules contribute another 2, and ethylenediamine (en) is a bidentate ligand, so it contributes 2 more. The sum of these gives the coordination number.

Calculate the coordination number of Fe

Sum the number of atoms from each ligand type that are bonded to the Fe ion. There are 2 chlorides (each providing one bonding site), 2 water molecules (each providing one bonding site), and 1 ethylenediamine molecule (providing two bonding sites because it is bidentate). Add them to find the coordination number: 2 ([Cl] atoms) + 2 ([H2O] molecules) + 2 (from [en] molecule) = 6.

Determine the oxidation number of Fe

To find the oxidation number of iron (Fe) in the complex, first assign oxidation states to the ligands. Chloride ions have a charge of -1, and water is neutral. Ethylenediamine is also a neutral molecule. The overall complex does not have a charge (it is neutral), so the sum of the oxidation states of the ligands plus the oxidation state of iron must equal zero. With 2 chloride ions, each being -1, they contribute a total of -2. Since the other ligands are neutral, the oxidation state of Fe must be +2 to balance the charges and make the charge of the entire complex neutral.

Key concepts

Coordination Number

In coordination chemistry, the coordination number is a fundamental concept that refers to the total number of points of attachment a central metal ion has with ligands, which are the molecules or ions that donate pairs of electrons to the metal. The ligands attach to the metal ion via coordinate covalent bonds, also known as dative bonds.

For instance, in the complex \(\mathrm{FeCl}_{2}(\mathrm{H}_{2}\mathrm{O})_{2}(\mathrm{en})\), the iron (Fe) ion is the central metal, and it is surrounded by different ligands: two chloride ions (Cl), two molecules of water (H2O), and one molecule of ethylenediamine (en). Chloride ions and water molecules are each monodentate, meaning they form one bond each with the metal. Ethylenediamine is a bidentate ligand, forming two bonds with the metal. To calculate the coordination number, we count the total number of attachments: 2 from Cl, 2 from H2O, and 2 from en, totaling a coordination number of 6. The coordination number is crucial for determining the geometry and stability of the complex ion.

Oxidation Number

The oxidation number, often known as the oxidation state, is an indicator of the degree of oxidation (loss of electrons) of an atom in a chemical compound. It is an artificial charge assigned by assuming that the compound is ionic, which helps identify how electrons are distributed in a complex.

In the provided exercise, the oxidation number of iron in \(\mathrm{FeCl}_{2}(\mathrm{H}_{2}\mathrm{O})_{2}(\mathrm{en})\) is determined based on the fact that it is a neutral complex. Remember that chloride ions have an oxidation number of -1 and that neutral ligands like water and ethylenediamine have an oxidation number of 0. Since there are two chloride ions with a -1 charge each, their total contribution is -2. To balance this and achieve neutrality in the overall complex, the iron ion must have an oxidation number of +2.

Ligand Bond Calculation

Ligand bond calculation involves identifying the bonding sites of ligands in a complex ion. Ligands can be classified as monodentate, bidentate, or polydentate based on the number of donor atoms they use to link to the central metal ion. Monodentate ligands, such as chloride ions and water in our example, provide one bonding site each. By contrast, bidentate ligands like ethylenediamine offer two bonding sites.

When calculating ligand bonds, it's necessary to consider the denticity of each ligand to properly determine the number of bonds they form with the metal. Each chloride ion and water molecule forms a single bond with iron. Ethylenediamine, being bidentate, forms two bonds. Hence, we add up the number of bonds from all ligands (2 from Cl, 2 from H2O, plus 2 from en) to arrive at the total number of ligand bonds, which would also be the coordination number.

Complex Ions

Complex ions are entities formed when a central metal ion bonds with one or more ligands that can donate an electron pair. These complex ions can have a variety of shapes and charge states, and they are characterized by the particular metal ion, its oxidation state, the number and types of ligands, and their arrangement around the metal, which is influenced by the coordination number.

The example complex \(\mathrm{FeCl}_{2}(\mathrm{H}_{2}\mathrm{O})_{2}(\mathrm{en})\) illustrates a typical complex ion where iron acts as the central ion. Because of the specific arrangement of ligands around the iron ion, dictated by the coordination number and denticity of the ligands, this complex ion exhibits a particular geometry, which could be octahedral given the coordination number 6. Identifying these components helps chemists understand the reactivity, color, magnetism, and overall properties of the complex ions and their roles in various chemical contexts like catalysis, biological processes, and materials science.