20.2
Coordination Compounds and Nomenclature
In most main group element compounds, the valence electrons of the isolated atoms combine to form chemical bonds that satisfy the octet rule. For inst…
Coordination compounds are neutral species consisting of either a neutral complex or charged complex ions and oppositely-charged counter ions. The complex ion is a central metal ion bound to molecules or ions, known as ligands.
Ligands are compounds with one or more electron pairs. They act as Lewis bases by donating a pair of electrons to the metal ion, which acts as a Lewis acid to form a coordinate covalent bond. Together, they constitute a coordination sphere.
In 1893, Alfred Werner proposed a coordination theory to explain why specific stable molecules react and violate the valence theory, to form coordination compounds. By studying various cobalt-ammonia complexes, Werner postulated that the central metal atom exhibits two types of valence interactions.
The primary valence is the oxidation number of the central metal, while the secondary valence is the number of ligands bound to it, also called the coordination number. Thus, if the number of bounded ligands decreases, counter ions can bind directly to the metal ion, maintaining the secondary valence.
Werner’s coordination theory is nowadays used to write chemical formulas of coordination compounds. Start with the symbol of the metal ion, then list all ligands in alphabetical order. Put square brackets around the complex ion, and list counter ions last. Use numerical subscripts to indicate the number of ligands and the counterions involved.
For a systematic nomenclature of coordination compounds, first, name the complex ions. Start by enlisting the ligands alphabetically. An anionic ligand ends with the letter -o, while a neutral ligand is denoted by its molecular name with a few exceptions such as aqua and ammine.
Use Greek-prefixes such as di-, tri- or tetra- to denote the number of ligands. If a prefix is present in the ligand name already, alternate prefixes to bis- tri- or tetrakis with the ligand name in parentheses.
Then name the metal name and its oxidation state using Roman numerals in parentheses. In the case of an anionic complex, the metal name is suffixed with -ate.
Lastly, the coordination compound is named by adding the cation name first, followed by the anion. Thus, the two coordination compounds are called amminechlorobis(ethylenediammine)cobalt(III) bromide and ammonium diaquatetrachlorocobaltate(III).
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Q1: What is a ligand in a coordination compound?
A ligand is a molecule or ion with one or more electron pairs that acts as a Lewis base, donating electrons to a central metal ion. Ligands form coordinate covalent bonds with the metal, which acts as a Lewis acid. Common ligands include ammonia, water, and chloride ions, each contributing electron pairs to stabilize the complex.
Q2: How do you write the chemical formula for a coordination compound?
Start with the central metal ion symbol, then list all ligands alphabetically. Enclose the complex ion in square brackets, and place counter ions outside the brackets last. Use numerical subscripts to indicate the number of each ligand and counter ion. For example, [Co(NH3)5Cl]Br2 shows cobalt with five ammonia ligands and one chloride, balanced by two bromide counter ions.
Q3: What is the difference between primary and secondary valence in coordination theory?
Primary valence is the oxidation number of the central metal ion, while secondary valence is the coordination number—the total number of ligands bonded to the metal. Alfred Werner's coordination theory explains that if fewer ligands bind to the metal, counter ions can attach directly to maintain the secondary valence constant.
Q4: How do you name anionic ligands in coordination compounds?
Anionic ligands are named by adding the suffix -o to the stem name of the group. For example, chloride becomes chloro, and hydroxide becomes hydroxo. These are listed alphabetically before neutral ligands when naming the complex, and their count is indicated using Greek prefixes like di-, tri-, or tetra-.
Q5: What are the exceptions to neutral ligand naming in coordination complexes?
Four common neutral ligands have special names: aqua for water (H2O), ammine for ammonia (NH3), carbonyl for carbon monoxide (CO), and nitrosyl for nitrogen monoxide (NO). These exceptions are used instead of the molecular names. When multiple identical ligands are present and the ligand name contains di-, tri-, or tetra-, use bis-, tris-, or tetrakis- prefixes instead.
Q6: How do you determine the oxidation state of the central metal in a coordination compound?
Set the overall charge of the coordination compound equal to the sum of the ligand charges plus the metal's oxidation state. Water and neutral molecules contribute zero charge, while anionic ligands contribute their negative charge. For example, in [Cr(H2O)4Cl2]Br, the complex has a +1 charge: +1 = 0 − 2 + x, so the metal's oxidation state is +3.
Q7: What is the coordination sphere and how is it represented in formulas?
The coordination sphere consists of the central metal ion plus all attached ligands. It is enclosed in square brackets in the chemical formula, while counter ions remain outside the brackets. The coordination number is the total number of donor atoms bonded to the metal. For instance, in [Co(H2O)6]2+, the coordination sphere contains cobalt and six water molecules with a coordination number of six.