Question

Write the formula and the name of each complex ion. \begin{equation}\begin{array}{l}{\text { a. a complex ion with } \mathrm{Co}^{3+} \text { as the central ion and three } \mathrm{NH}_{3} \text { mole- }} \\ {\text { cules and three } \mathrm{CN}^{-} \text { ions as ligands }} \\ {\text { b. a complex ion with } \mathrm{Cr}^{3+} \text { as the central ion and a coordination num- }} \\ {\text { ber of } 6 \text { with ethylenediamine ligands }}\end{array}\end{equation}

Short answer

[Co(NH_{3})_{3}(CN)_{3}] is hexaamminecobalt(III), [Cr(en)_{3}]^{3+} is tris(ethylenediamine)chromium(III) ion.

Step-by-step solution

Identify the Components for the First Complex Ion

The first complex ion has a central metal ion, Cobalt Co^{3+}, and its ligands which are three ammonia molecules, NH_{3}, and three cyanide ions, CN^{-}. Cobalt has a +3 charge, ammonia is a neutral ligand, and cyanide has a -1 charge.

Write the Formula for the First Complex Ion

To write the formula of the complex, place the metal ion first. This is followed by ligands in alphabetical order. Use parentheses to group the ligands and apply subscripts for the number of each type of ligand. Since the complex has an overall neutral charge, the charges of the metal and ligands must balance out. The formula is [Co(NH_{3})_{3}(CN)_{3}]^{0}, which simplifies to [Co(NH_{3})_{3}(CN)_{3}].

Name the First Complex Ion

The complex ion is named by first naming the ligands in alphabetical order followed by the metal with its oxidation state in Roman numerals. Since NH_{3} is called 'ammine' in coordination chemistry, and CN^{-} is called 'cyanido', the name of the complex is hexaamminecobalt(III).

Identify the Components for the Second Complex Ion

The second complex ion has Chromium Cr^{3+} as the central ion, and its ligands are six ethylenediamine molecules, which is a bidentate ligand (each ethylenediamine can bind twice to the metal ion).

Write the Formula for the Second Complex Ion

For Chromium with a coordination number of 6, the formula involves 3 ethylenediamine molecules since each ethylenediamine molecule binds twice to fill the six coordination sites. The formula is [Cr(en)_{3}]^{3+}.

Name the Second Complex Ion

To name this complex, first, name the ligands and then the metal with its oxidation state. Ethylenediamine is 'ethylenediamine' in the name of complexes. Thus, the name of the complex is tris(ethylenediamine)chromium(III) ion.

Key concepts

Coordination Chemistry

Coordination chemistry is a branch of inorganic chemistry that revolves around the study of coordination compounds, where a set of ligands which are molecules or ions, surround a central metal atom or ion. This system is known as a coordination complex. The ligands are attached to the metal atom or ion via coordinate covalent bonds, which involves the sharing of a pair of electrons where one atom provides both electrons for the bond.

Ligands can be anions, neutral molecules, or cations, and they have a significant influence on the properties of the metal at the center, including its reactivity, color, and magnetic properties. Coordination chemistry finds extensive applications across various scientific fields, such as biochemistry, materials science, and catalysis.

Ligands in Coordination Compounds

Ligands are integral components of coordination compounds, and they can vary greatly in size, charge, and the number of atoms used to bind to the central metal atom or ion. A key concept in ligand behavior is the 'dentate' nature of the ligand, which refers to how many points of attachment the ligand has to the metal center. Monodentate ligands, such as ammonia (NH₃) and cyanide (CN^-), bind through a single site.

Some ligands can bind through two or more atoms; these are known as polydentate ligands. An example of this is ethylenediamine, which is a bidentate ligand, meaning it can form two bonds with a central metal. Ligands can also be characterized by the 'chelate effect', where bidentate or polydentate ligands form more stable complexes due to the formation of ring structures with the metal ion.

Transition Metal Complexes

Transition metals are the metals found in the center of the periodic table and are characterized by their ability to form a wide variety of complex ions. These complexes have a transition metal ion at the center surrounded by ligands. A key feature of transition metal complexes is their ability to have different oxidation states, which can affect the color, magnetism, and reactivity of the complex.

For instance, when naming the complex ions mentioned in the exercise - [Co(NH₃)₃(CN)₃] and [Cr(en)₃]³⁺ - the oxidation states of the central ions (cobalt and chromium) are indicated in Roman numerals as cobalt(III) and chromium(III). This is crucial for understanding the behavior of these complexes in various chemical reactions and is an essential aspect of coordination chemistry nomenclature.