6.1: Introduction to Coordination Chemistry

Coordination chemistry is the study of the compounds that form between metals and ligands, where a ligand is any molecule or ion that binds to the metal. A metal complex is the unit containing the metal bound to its ligands. For example, [PtCl₂(NH₃)₂] is the neutral metal complex where the Pt⁺² metal is bound to two Cl^- ligands and two NH₃ ligands. If a complex is charged, it is called a complex ion (ex. [Pt(NH₃)₄]⁺² is a complex cation). A complex ion is stabilized by formation of a coordination compound with ions of opposite charge (ex. [Pt(NH₃)₄]Cl₂). It is convention to write the formula of a complex or complex ion inside of square brackets, while counterions are written outside of the brackets. In this convention, it is understood that ligands inside the brackets are bound directly to the metal ion, in the metal's first coordination sphere (a.k.a inner coordination sphere). Ions written outside of the brackets are assumed to be in the second coordination sphere, and they are not directly bound to the metal.

• Neutral Complex: \([CoCl_3(NH_3)_3]\)
• Complex Cation: \([CO(NH_3)_6]^{3+}\)
• Complex Anion: \([CoCl_4(NH_3)_2]^-\)
• Coordination Compound: \(K_4[Fe(CN)_6]\)

A common metal complex is Ag(NH₃)₂⁺, formed when Ag⁺ ions are mixed with neutral ammonia molecules.

\[Ag^+ + 2 NH_3 \rightarrow Ag(NH_3)_2^+ \nonumber \]

A complex Ag(S₂O₃)₂^3- is formed between silver ions and negative thiosulfate ions:

\[Ag^+ + 2 S_2O_3^{2-} \rightarrow Ag(S_2O_3)_2^{3-} \nonumber \]

The geometry and arrangement of ligands around the metal center affect the properties of a coordination compounds. Compounds with the same molecular formula can appear as isomers with very different properties. Isomers are molecules that have identical chemical formulas, but have different arrangements of atoms in space. Isomers with different geometric arrangements of ligands are called geometric isomers. Isomers whose structures are mirror images of each other are called optical isomers.

How did the study of coordination compounds started?

The coordination chemistry was pioneered by Nobel Prize winner Alfred Werner (1866-1919). He received the Nobel Prize in 1913 for his coordination theory of transition metal-amine complexes. At the start of the 20th century, inorganic chemistry was not a prominant field until Werner studied the metal-amine complexes such as \([Co(NH_3)_6Cl_3]\). Werner recognized the existence of several forms of cobalt-ammonia chloride. These compounds have different color and other characteristics. The chemical formula has three chloride ions per mole, but the number of chloride ions that precipitate with Ag⁺ ions per formula is not always three. He thought only ionized chloride ions will form precipitate with silver ion. In the following table, the number below the Ionized Cl^- is the number of ionized chloride ions per formula. To distinguish ionized chloride from the coordinated chloride, Werner formulated the Complex formula and explained structure of the cobalt complexes.

Figure 1: Proposed Structure of Cobalt Ammonia Complexes from Number of Ionized Chloride

Solid	Color	Ionized Cl^-	Complex formula
CoCl36NH3	Yellow	3	[Co(NH3)6]Cl3
CoCl35NH3	Purple	2	[Co(NH3)5Cl]Cl2
CoCl34NH3	Green	1	trans-[Co(NH3)4Cl2]Cl
CoCl34NH3	Violet	1	cis-[Co(NH3)4Cl2]Cl

The structures of the complexes were proposed based on a coordination sphere of 6. The 6 ligands can be ammonia molecules or chloride ions. Two different structures were proposed for the last two compounds, the trans compound has two chloride ions on opposite vertices of an octahedral, whereas the the two chloride ions are adjacent to each other in the cis compound. The cis and trans compounds are known as geometric isomers.

Other cobalt complexes studied by Werner are also interesting. It has been predicted that the complex Co(NH₂CH₂CH₂NH₂)₂ClNH₃]²⁺ should exist in two forms, which are mirror images of each other. Werner isolated solids of the two forms, and structural studies confirmed his interpretations. The ligand NH₂CH₂CH₂NH₂ is ethylenediamine (en) often represented by en.

Properties of Coordination Complexes

Some methods of verifying the presence of complex ions include studying its chemical behavior. This can be achieved by observing the compounds' color, solubility, absorption spectrum, magnetic properties, etc. The properties of complex compounds are separate from the properties of the individual atoms. By forming coordination compounds, the properties of both the metal and the ligand are altered.

Metal-ligand bonds are typically thought of Lewis acid-base interactions. The metal atom acts as an electron pair acceptor (Lewis acid), while the ligands act as electron pair donors (Lewis base). The nature of the bond between metal and ligand is stronger than intermolecular forces because they form directional bonds between the metal ion and the ligand, but are weaker than covalent bonds and ionic bonds.

Common Ligands

Monodentate ligands donate one pair of electrons to the central metal atoms. An example of these ligands are the haldide ions (F^-, Cl^-, Br^-, I^-). Polydentate ligands, also called chelates or chelating agents, donate more than one pair of electrons to the metal atom forming a stronger bond and a more stable complex. A common chelating agent is ethylenediamine (en), which, as the name suggests, contains two ammines or :NH₂ sites which can bind to two sites on the central metal. An example of a tridentate ligand is bis-diethylenetriammine. An example of such a coordination complex is bis-diethylenetriamine cobalt III.

Complex compound/ion	Coordination number	Oxidation State of Metal Atom
[Fe(CN)6]^4-	6	2+
[Co(NH3)4SO4]^-	5	1+
[Pt(NH3)4]^2+	4	2+
[Ni(NH2CH2CH2NH2)3]^2+	6	2+

Complex ions can form many compounds by binding with other complex ions in multiple ratios. This leads to many combinations of coordination compounds. The structures of certain coordination compounds can also have isomers, which can change their interactions with other chemical agents. The binding between metal and ligands is studied in metals, tetrahedral, and octahedral structures. There are many pharmaceutical and biological applications of coordination complexes and their isomers.

Contributors and Attributions

• Chung (Peter) Chieh (Professor Emeritus, Chemistry @ University of Waterloo)

• Anushweta Asthana (UCD)