Question

Name each complex ion or coordination compound. $$\begin{array}{ll}{\text { a. }\left[\mathrm{Cu}(\mathrm{en})_{2}\right]^{2+}} & {\text { b. }\left[\mathrm{Mn}(\mathrm{CO})_{3}\left(\mathrm{NO}_{2}\right)_{3}\right]^{2+}} \\\ {\text { c. } \mathrm{Na}\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}(\mathrm{ox})_{2}\right]} & {\text { d. }\left[\mathrm{Co}(\mathrm{en})_{3}\right]\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]}\end{array}$$

Short answer

a. Diamminecopper(II) ion, b. Tricarbonyltrinitritomanganese(II) ion, c. Sodium diamminebis(oxalato)chromate(III), d. Triamminecobalt(III) hexacyanoferrate(II)

Step-by-step solution

Identify Ligands and Their Prefixes

Identify each ligand in the complex ion or coordination compound and determine the appropriate prefix based on the number of each ligand present: 1 = mono, 2 = di, 3 = tri, etc. Ligands are named in alphabetical order, with the exceptions of water, which is named 'aqua,' ammonia, which is named 'ammine,' carbon monoxide, which is named 'carbonyl,' and oxalate which is named 'oxalato'.

Name the Metal and its Oxidation State

For the metal ion, use the metal's name if the complex ion is an anion, if it is a cation or neutral complex, use the metal's Latin name if it exists and add the oxidation state of the metal in Roman numerals in parentheses.

Apply Naming to Each Complex

Use the appropriate names and prefixes for the ligands followed by the name of the metal with its oxidation state in Roman numerals to name each complex: a. The complex ion \(\left[\mathrm{Cu}(\mathrm{en})_{2}\right]^{2+}\) is named 'diamminecopper(II)', b. The complex ion \(\left[\mathrm{Mn}(\mathrm{CO})_{3}\left(\mathrm{NO}_{2}\right)_{3}\right]^{2+}\) is named 'tricarbonyltrinitritomanganese(II)', c. The complex ion \(\mathrm{Na}\left[\mathrm{Cr}\left(\mathrm{H}_2\mathrm{O}\right)_2(\mathrm{ox})_2\right]\) is named 'sodium diamminebis(oxalato)chromate(III)', d. The coordination compound \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) is named 'triamminecobalt(III) hexacyanoferrate(II)'.

Key concepts

Ligands in Coordination Chemistry

Understanding ligands is crucial in coordination chemistry, as they are the ions or molecules that bind to a central metal atom or ion within a complex. Ligands donate electron pairs to form these connections, known as coordination bonds, resulting in a coordination complex.

Ligands vary widely in their charge, size, and electron-donating ability, which can influence the properties and geometry of the complex. They are named with specific prefixes based on their quantity: 'mono' for one, 'di' for two, 'tri' for three, and so on. In nomenclature, ligands are listed in alphabetical order, with important exceptions such as 'aqua' for water, 'ammine' for ammonia, 'carbonyl' for carbon monoxide, and 'oxalato' for oxalate to account for common ligands with historical naming conventions.

Role of Ligands in Stability and Color

Ligands don't just determine the name of the complex; they're also pivotal in defining the stability and color. The nature and arrangement of ligands around the metal center influence the complex's absorption of light, contributing to the vibrant colors often observed in coordination compounds.

Oxidation State Determination

The oxidation state of a metal in a coordination compound offers insight into its electronic structure and is essential for correct nomenclature. To determine the oxidation state, follow a systematic approach that considers the overall charge of the complex and the known charges of the ligands and counterions, if present.

For example, in a complex ion like \[\left[\mathrm{Cu}(\mathrm{en})_{2}\right]^{2+}\], copper's oxidation state can be deduced by understanding the neutral charge of the ethylenediamine (en) ligand and the 2+ charge of the whole complex. The equation looks like this: oxidation state of Cu + 0 (charge of en ligands) = 2+. Hence, the oxidation state of Cu is +2.

Knowing the oxidation state is not only key to naming a complex but also to predicting its reactivity and the types of reactions it may undergo.

Complex Ion Nomenclature

Naming complex ions and coordination compounds follows a set of rules to ensure clarity and convey critical information about the structure of the compound.

The naming process begins with the ligands, followed by the central metal. If the complex is negatively charged (an anion), the metal's name is used as is, but if it is positively charged (a cation) or neutral, the Latin name of the metal is often used with the oxidation state in Roman numerals in parentheses. An anionic complex also has its metal name ending in '-ate'.

In cases where a complex forms a salt with a counterion, the cation is named before the anion, similar to conventional ionic compounds. For example, \[\mathrm{Na}\left[\mathrm{Cr}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2}(\mathrm{ox})_{2}\right]\] is named 'sodium diamminebis(oxalato)chromate(III)' where 'sodium' is the cation and the complex ion serves as the anion.

By faithfully applying these rules, students can demystify the initially complex-seeming names of coordination compounds into their component parts, gaining a deeper understanding of their nomenclature and properties.