Question

Write the formula, including its correct charge, for a complex that contains \(\mathrm{Co}^{3+}\), two \(\mathrm{Cl}^{-}\), and two ethylenediamine ligands.

Short answer

The formula for the complex is \[\mathrm{[Co(en)_2Cl_2]^{+}}\].

Step-by-step solution

Identify the Cation

In the complex, the cation is cobalt (Co) with an oxidation state of +3. It is written as \(\mathrm{Co}^{3+}\).

Identify the Anionic Ligands

There are two chloride (Cl) ligands which are anions, each with a charge of -1. These will contribute a total charge of -2 to the complex.

Identify the Neutral Ligands

Ethylenediamine is a neutral bidentate ligand, which means it attaches at two coordination sites but does not contribute to the charge. There are two ethylenediamine ligands in the complex.

Determine the Overall Charge

Calculate the total charge of the complex: the cobalt cation contributes +3, and the two chloride anions contribute -2, for a net charge of +1 on the complex.

Write the Formula for the Complex

The chemical formula for a complex is written with the metal cation first, followed by the ligands in alphabetical order. Since the ethylenediamine ligand (en) is neutral, its presence does not affect the overall charge. The complex formula is \[\mathrm{[Co(en)_2Cl_2]^{+}}\].

Key concepts

Coordination Chemistry

Coordination chemistry is a branch of inorganic chemistry that focuses on the study of complex compounds formed between metal ions and organic or inorganic molecules known as ligands. These metal ions, known as central ions or atoms, typically have empty valence orbitals that can accommodate electron pairs from the ligands. The resulting structures are called coordination compounds or complex ions.

The number of ligands attached to the central metal ion is called the coordination number, and the type of bonding involves coordinate covalent bonds where both electrons in the bond come from the ligand. Coordination compounds have vast applications, including in catalysis, materials science, and as contrast agents in medical imaging.

Oxidation States

The oxidation state, often referred to as the oxidation number, is a concept used in chemistry to describe the degree of oxidation of an atom in a compound. It's a hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic. Oxidation states are important for predicting the reactivity and properties of elements within compounds.

For a coordination complex, the oxidation state of the central metal ion can give insight into the compound's structural characteristics and reactivity. In the case of \(\mathrm{Co}^{3+}\), cobalt has an oxidation state of +3, indicating it can accept three more electrons to achieve a stable electron configuration.

Ligands

Ligands are ions or molecules capable of donating a pair of electrons to a metal ion, thus forming a coordinate covalent bond. They are crucial in the formation of complex ions. Ligands can be negatively charged, neutral, or positively charged, and they influence the overall charge of the complex.

Ligands vary greatly in their properties and can be water molecules, halide ions like \(\mathrm{Cl}^{-}\), or organic molecules such as ethylenediamine. They also display different denticities - the number of donor atoms used to bind to the metal center. Knowing the charge and denticity of ligands is essential for determining the overall charge of a complex ion.

Bidentate Ligands

Bidentate ligands are versatile ligands that attach to a central metal atom or ion at two points. This 'bite' makes them particularly good at stabilizing metal complexes, as they form chelates - complexes with a ring-like structure at the binding site.

Ethylenediamine is an example of a bidentate ligand. It has two nitrogen atoms, each possessing a lone pair of electrons, allowing it to bind simultaneously with two coordination sites on a metal ion. This chelating effect often leads to complexes being more stable than those formed with monodentate ligands, which attach at only one point.

Chemical Nomenclature

Chemical nomenclature is the systematic naming of chemical compounds and is essential for clarity and precision in the communication of chemical knowledge. In coordination chemistry, the nomenclature involves naming the central metal followed by the ligands.

The ligands are named in alphabetical order, regardless of their charge, before the metal ion. If the complex ion carries a charge, it is referred to as a complex cation or a complex anion. The oxidation state of the metal is often indicated by Roman numerals in parentheses. In our example, \(\mathrm{[Co(en)_2Cl_2]^{+}}\), the cobalt ion is the central metal, and the ligands named are 'en' (short for ethylenediamine) and 'Cl' (chloride), followed by the notation of the overall charge of the complex ion.