## Reactions of Coordination Compounds

Coordination complexes are anionic ligands bound to a cationic metal.

### Learning Objectives

Discuss the common reaction classes of coordination compounds.

### Key Takeaways

#### Key Points

• A coordination complex, or metal complex, consists of an atom or ion (usually metallic) and a surrounding array of molecules or anions called ligands or complexing agents.
• A coordination compound is any molecule that contains a coordination complex.
• The donor atom is the atom within a ligand that is bonded to the central atom or ion.
• Coordination complexes can undergo a variety of reactions, including electron transfer, ligand exchange, and associative processes.

#### Key Terms

• coordination: The reaction of one or more ligands with a metal ion to form a coordination compound.
• redox: A reversible chemical reaction in which one reaction is an oxidation and the reverse is a reduction.
• donor atom: The atom within a ligand that is bonded to the central atom or ion within a coordination complex.

In chemistry, a coordination or metal complex consists of an atom or ion (usually metallic) and a surrounding array of bound molecules or anions known as ligands or complexing agents. Many metal-containing compounds consist of coordination complexes.

Cisplatin: This complex, PtCl2(NH3)2, is an anti-tumor drug and an example of a coordination complex.

### Donor Atom

The atom within a ligand that is bonded to the central atom or ion is called the donor atom. A typical complex is bound to several donor atoms, which can be same or different elements.

Polydentate (multiple bonded) ligands consist of several donor atoms, several of which are bound to the central atom or ion. These complexes are called chelate complexes, the formation of which is called chelation, complexation, and coordination.

EDTA chelation: The EDTA molecule has six different donor atoms that form the complex.

### Ligands

The ions or molecules surrounding the central atom are called ligands. These are generally bound to the central atom by a coordinate covalent bond (donating electrons from a lone electron pair into an empty metal orbital ). There are also organic ligands such as alkenes whose pi (π) bonds can coordinate to empty metal orbitals. An example is ethene in the complex known as Zeise’s salt, K+[PtCl3(C2H4)].

The central atom or ion, together with all ligands, comprise the coordination sphere. The central atoms or ion and the donor atoms comprise the first coordination sphere. Coordination refers to the coordinate covalent bonds (dipolar bonds) between the ligands and the central atom.

Originally, a complex implied a reversible association of molecules, atoms, or ions through such weak chemical bonds. As applied to coordination chemistry, this meaning has evolved. Some metal complexes are formed virtually irreversibly and many are bound together by bonds that are quite strong.

### Electron Transfers

A common reaction between coordination complexes involving ligands are electron transfers. There are two different mechanisms of electron transfer redox reactions: inner sphere or outer sphere electron transfer. In electron transfer, an electron moves from one atom to another, changing the charge on each but leaving the net charge of the system the same.

### Ligand Exchange

One important indicator of reactivity is the rate of degenerate exchange of ligands. For example, the rate of interchange of the coordinate water in [M(H2O)6]n+ complexes varies over 20 orders of magnitude. Complexes where the ligands are released and rebound rapidly are classified as labile. Such labile complexes can be quite stable thermodynamically. Typically they either have low-charge (Na+), electrons in d orbitals that are antibonding with respect to the ligands (Zn2+), or lack covalency (Ln3+, where Ln is any lanthanide).

The lability of a metal complex also depends on the high-spin vs. low-spin configurations when such is possible. Thus, high-spin Fe(II) and Co(III) form labile complexes, whereas low-spin analogues are inert.

### Associate Processes

Complexes that have unfilled or half-filled orbitals often show the capability to react with substrates. Most substrates have a singlet ground-state; that is, they have lone electron pairs (e.g., water, amines, ethers). These substrates need an empty orbital to be able to react with a metal center. Some substrates (e.g., molecular oxygen) have a triplet ground state. Metals with half-filled orbitals have a tendency to react with such substrates. If the ligands around the metal are carefully chosen, the metal can aid in (stoichiometric or catalytic) transformations of molecules or be used as a sensor.

## Metallurgy

Extractive metallurgy is the study of the processes used in the separation and concentration of raw materials.

### Learning Objectives

Discuss how coordination chemistry can be applied in metallurgic processes.

### Key Takeaways

#### Key Points

• Extractive metallurgy is the practice of removing valuable metals from an ore and refining the extracted raw metals into purer form.
• The field of extractive metallurgy encompasses many specialty sub-disciplines, including mineral processing, hydrometallurgy, pyrometallurgy, and electrometallurgy.
• Especially in hydrometallurgy, the coordination chemistry of the metals involved plays a large role in their solubility and reactivity as the ore is refined into precious metal.

#### Key Terms

• dissolution: Dissolving, or going into solution.
• pyrometallurgy: The thermal treatment of minerals or ores to bring about physical and chemical transformations in the materials to enable recovery of valuable metals.
• metallurgy: The science of metals; their extraction from ores, purification and alloying, heat treatment, and working.

### Extractive Metallurgy

Extractive metallurgy is the practice of removing valuable metals from an ore and refining the extracted raw metals into a purer form. The field is an applied science, covering all aspects of the physical and chemical processes used to produce mineral-containing and metallic material. The practice of extractive metallurgy almost always involves contributions from other scientific fields, such as analytical chemistry and mineralogy. Sometimes extractive metallurgy produces a finished product, but more often it produces a form that requires further physical processing.

The field of extractive metallurgy encompasses many specialty sub-disciplines, each concerned with various physical and chemical processes that are steps in an overall process to produce a particular material. These specialties are generically grouped into the categories of mineral processing:

• hydrometallurgy
• pyrometallurgy
• electrometallurgy

### Mineral processing

Mineral processing manipulates the particle size of solid raw materials to separate valuable materials from materials of no value. Usually, particle sizes must be reduced to efficiently separate valuable materials. Physical properties of valuable materials can include density, particle size and shape, electrical and magnetic properties, and surface properties.

Since many size reduction and separation processes involve the use of water, solid-liquid separation processes are part of mineral processing. In order to dissolve an ore in an aqueous solution, it is often necessary to break the large chunks into smaller pieces, thereby increasing the surface area and the rate of dissolution. It is further possible to break up only ores that are easily crushed, thus allowing them to be dissolved at a much faster rate than those remaining in large chunks.

### Hydrometallurgy

Hydrometallurgy describes the process of extracting metals from ores using aqueous solutions. The most common hydrometallurgical process is leaching, which involves the dissolution of the valuable metals into the aqueous solution. After the solution is separated from the ore solids, the solution is often subjected to various processes of purification and concentration before the valuable metal is recovered.

The solution purification and concentration processes may include precipitation, distillation, adsorption, and solvent extraction. The final recovery step may involve precipitation, cementation, or an electrometallurgical process. Sometimes, hydrometallurgical processes may be carried out directly on the ore material without any pretreatment steps. Often, the ore must be pretreated by various mineral processing steps and sometimes by pyrometallurgical processes. Here, the coordination chemistry and solubility of the compound can become very important, as one attempts to precipitate a mineral (metal) of interest over the other metals in the solution or attempts to selectively leach one desired mineral over the others present in the sample.

### Pyrometallurgy

Pyrometallurgy involves high temperature processes where chemical reactions take place among gases, solids, and molten materials. Solids containing valuable metals are reacted to form intermediate compounds for further processing or converted into their elemental or metallic state. Processes that produce molten products are collectively referred to as smelting operations. The energy required to sustain the high temperature pyrometallurgical processes may come entirely from the exothermic nature of the chemical reactions taking place, usually oxidation reactions. Often, however, energy must be added to the process by combustion of fuel or, in the case of some smelting processes, by the direct application of electrical energy.

Smelting furnace: High temperatures are often required to melt the ores and metals that come out of mining operations.

### Electrometallurgy

Electrometallurgy involves metallurgical processes that take place in some form of electrolytic cell. The most common types of electrometallurgical processes are electrowinning and electro-refining. Electrowinning is an electrolysis process used to recover metals in aqueous solution, usually as the result of an ore having undergone one or more hydrometallurgical processes. The metal of interest is plated onto the cathode, while the anode is an inert electrical conductor. Electro-refining is used to dissolve an impure metallic anode (typically from a smelting process) and produce a high purity cathode. The scope of electrometallurgy has significant overlap with the areas of hydrometallurgy and (in the case of fused salt electrolysis) pyrometallurgy.

## Chelating Agents

Chelating agents are ligands for metals that bind via multiple atoms, thus taking up several coordination sites on the metal.

### Learning Objectives

Describe the origin of the chelate effect.

### Key Takeaways

#### Key Points

• Chelation is the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central atom.
• The chelate effect describes the enhanced affinity of chelating ligands for a metal ion, compared to the affinity of a collection of similar non-chelating ( monodentate ) ligands for the same metal.
• Chelation therapy is the use of chelating agents to detoxify poisonous metal agents, such as mercury, arsenic, and lead, by converting them to a chemically inert form that can be excreted without further interaction with the body.

#### Key Terms

• chelate compound: A cyclic compound in which a metal atom is bonded to at least two other atoms.
• chelating agent: Any compound that reacts with a metal ion to produce a chelate.
• ligand: An ion, molecule, or functional group that binds to another chemical entity to form a larger complex.

### Chelation

Chelation is the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central atom. Usually these ligands are organic compounds and are called chelants, chelators, chelating agents, or sequestering agents; the resulting complexes are called chelate compounds.

Metal-EDTA chelate: Chemical structure of EDTA chelate.

Chelate complexes are contrasted with coordination complexes composed of monodentate ligands, which form only one bond with the central atom. Chelating agents, unlike the other ligands in coordination compounds, bind via multiple atoms in the ligand molecule, not just one.

### The Chelate Effect

The chelate effect describes the enhanced affinity of chelating ligands for a metal ion compared to the affinity of a collection of similar nonchelating (monodentate) ligands for the same metal. Consider the two equilibria, in aqueous solution, between the copper (II) ion (Cu2+) and ethylenediamine (en) on the one hand and methylamine (CH3NH2 (MeNH2)) on the other.

$\text{Cu}^{ 2+}+\text{en}\rightarrow [\text{Cu}(\text{en})]^{2+}$ (1)

$\text{Cu}^{ 2+ }+2\text{MeNH}_{ 2 }\rightarrow [\text{Cu}(\text{MeNH}_2)_2]^{ 2+ }$ (2)

In (1), the bidentate ligand ethylenediamine forms a chelate complex with the copper ion. Chelation results in the formation of a five-membered ring. In (2), the two monodentate methylamine ligands of approximately the same donor power (the enthalpy of formation of Cu—N bonds is approximately the same in the two reactions) forms a complex. Under conditions of equal copper concentrations and when the concentration of methylamine is twice the concentration of ethylenediamine, the concentration of the complex in (1) will be greater than the concentration of the complex in (2). The effect increases with the number of chelate rings, so the concentration of the EDTA complex, which has six chelate rings, is much higher than a corresponding complex with two monodentate nitrogen donor ligands and four monodentate carboxylate ligands. Thus, the phenomenon of the chelate effect is a firmly established empirical fact.

Ethylenediamine chelate: Ethylenediamine serves as a chelating agent by binding via its two nitrogen atoms.

### Applications of Chelating Agents

Chelation therapy is the use of chelating agents to detoxify poisonous metal agents, such as mercury, arsenic, and lead, by converting them to a chemically inert form that can be excreted without further interaction with the body. This therapy was approved by the U.S. Food and Drug Administration in 1991. Although they can be beneficial in cases of heavy metal poisoning, chelating agents also can be dangerous. Use of disodium EDTA instead of calcium EDTA has resulted in fatalities due to hypocalcemia.

Virtually all biochemicals exhibit the ability to dissolve certain metal cations. Thus, proteins, polysaccharides, and polynucleic acids are excellent polydentate ligands for many metal ions. Organic compounds such as the amino acids glutamic acid and histidine, organic diacids such as malate, and polypeptides such as phytochelatin are also typical chelators. In addition to these adventitious chelators, several biomolecules are specifically produced to bind certain metals. Virtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups. Such chelating agents include the porphyrin rings in hemoglobin and chlorophyll. Many microbial species produce water-soluble pigments that serve as chelating agents, termed siderophores. For example, species of Pseudomonas are known to secrete pyocyanin and pyoverdin that bind iron. Enterobactin, produced by E. coli, is the strongest chelating agent known.

## Chemical Analysis

Coordination complexes and their chemistry can be used to analyze the composition of a solution by precipitation or colorimetric analysis.

### Learning Objectives

Describe the application of coordination compounds in the analysis of chemical composition.

### Key Takeaways

#### Key Points

• Coordination complexes can be used to analyze chemical composition by precipitation analysis and colorimetric analysis.
• Classical qualitative inorganic analysis is a method of analytical chemistry that seeks to find the elemental compositions of inorganic compounds.
• Cations are usually classified into six groups according to their categories, while anions are classified into three groups.

#### Key Terms

• precipitation: A reaction that leads to the formation of a heavier solid in a lighter liquid; the precipitate so formed at the bottom of the container.

Coordination complexes can be used to analyze chemical composition in a variety of ways. Two common techniques are precipitation analysis and colorimetric analysis. These methods are called classical qualitative inorganic analysis.

Precipitation reaction: Difference in the visual appearance of an aggregate and a precipitate.

### Qualitative Inorganic Analysis

Classical qualitative inorganic analysis is a method of analytical chemistry that seeks to find the elemental compositions of inorganic compounds. It is mainly focused on detecting ions in aqueous solution, so materials in other forms may need to be brought into this state before standard methods can be used. The solution is then treated with various reagents to test for reactions characteristic of certain ions, which may cause color changes, solid formation, or other visible changes.

### Precipitation Analysis

Cations are usually classified into one of six groups according to their properties, while anions are classified into one of three groups. The details of classification vary slightly from one source to another. Each group has a common reagent that can be used to separate the cation or anion from a solution. To obtain meaningful results, the separation must be done in a specific sequence, as some ions of an earlier group may also react with the reagent of a later group, leading to ambiguity as to which ions are present. This happens because cationic analysis is based on the solubility products of the ions. As the cation achieves the optimum concentration needed, it precipitates, allowing detection.

Precipitation analysis: Copper from a wire displaces silver in a silver nitrate solution it is dipped into, and solid silver precipitates out. Precipitation analysis can be used to determine the chemical makeup of the solution.

The groups of cations include:

1. Those that form insoluble chlorides, such as lead, silver, and mercury.
2. Those that form acid-insoluble sulfides, such as cadmium, bismuth, copper, antimony, and tin.
3. Those that form insoluble hydroxide complexes, such as iron, aluminum, and chromium.
4. Zinc, nickel, cobalt, and manganese are determined by the addition of ammonium chloride, ammonium hydroxide, and hydrogen sulfide gas. The color of the precipitate will indicate the metal.
5. A group of insoluble carbonate salts. (While many of the earlier cations will precipitate with carbonate, they will have been detected prior to this point if the steps have been followed in order.) Barium, calcium, and strontium will precipitate at this point, but not before.
6. Magnesium, lithium, sodium, potassium, and ammonium are difficult to precipitate and are usually detected by flame color.

There are three groups of anions, and their detection methods vary widely. However, precipitation methods similar to those mentioned above are often used.

### Colorimetric Analysis

Colorimetric analysis is a method of determining the concentration of a chemical element or chemical compound in a solution with the aid of a color reagent. It is applicable to both organic compounds and inorganic compound. The method is widely used in medical laboratories and for industrial purposes, such as the analysis of water samples in connection with industrial water treatment.

## Coloring Agents

The electronic configuration of some metal complexes gives them important properties, such as color in coordination compounds.

### Learning Objectives

Discuss the relationship between charge transfer and the color of a metal complex.

### Key Takeaways

#### Key Points

• Metal complexes often have spectacular colors caused by electronic transitions by the absorption of light.
• Most transitions that are related to colored metal complexes are either d–d transitions or charge transfer bands.
• A charge transfer band entails promotion of electrons from metal-based orbitals and ligand -based orbitals.

#### Key Terms

• inorganic chemistry: The chemistry of the elements (including carbon) and those compounds that do not contain carbon.
• ligand: An ion, molecule, or functional group that binds to another chemical entity to form a larger complex.

### Bonding in Coordination Compounds

Many of the properties of metal complexes are dictated by their electronic structures. The electronic structure can be described by a relatively ionic model that ascribes formal charges to the metals and ligands. This approach is the essence of crystal field theory (CFT), which is a core concept in inorganic chemistry.

More sophisticated models (relative to crystal field theory) embrace covalency. This approach is described by the ligand field theory (LFT) and the molecular orbital theory (MO). Ligand field theory, introduced in 1935 and built from molecular orbital theory, can handle a broader range of complexes. It can explain complexes in which the interactions are covalent. The chemical applications of group theory can aid in the understanding of crystal or ligand field theory, by allowing simple, symmetry-based solutions to the formal equations.

Chemists tend to employ the simplest model required to predict the properties of interest. For this reason, CFT has been a favorite for the discussions when possible. MO and LF theories are more complicated but provide a more realistic perspective.

### Color in Coordination Compounds

Metal complexes often have spectacular colors caused by electronic transitions due to the absorption of light. For this reason, they are often applied as pigments. Most transitions that are related to colored metal complexes are either d–d transitions or charge transfer bands. In a d–d transition, an electron in a d orbital on the metal is excited by a photon to another d orbital of higher energy. However, the electron remains centered on the metal.

Color of various Ni(II) complexes in aqueous solutions: From left to right, [Ni(NH3)6]2+, [Ni3]2+, [NiCl4]2-, [Ni(H2O)6]2+. Reactions starting from NiCl2·6H2O can be used to form a variety of nickel coordination complexes because the H2O ligands are rapidly displaced by ammonia, amines, thioethers, thiolates, and organophosphines.

A charge transfer band entails promotion of an electron from a metal-based orbital into an empty ligand-based orbital (Metal-to-Ligand Charge Transfer or MLCT). Conceptually, one can imagine the oxidation state of the metal increasing by one (losing on electron), while the oxidation state of the ligand decreases by one (becomes anionic). The overall charge of the system remains the same, but the localization of the electron changes.

The converse will also occur: excitation of an electron in a ligand-based orbital into an empty metal-based orbital (Ligand to Metal Charge Transfer or LMCT). These phenomena can be observed with the aid of electronic spectroscopy (also known as UV-Vis).

It is the relative energetics of these electronic transitions that allows for them to have absorbencies in the visible region. Since the nature of the ligands and the metal can be tuned extensively, a variety of colors can be obtained.

Colors of various coordination complexes: Changing the metal or the ligand can change the color of the coordination complex.

## Biomolecules

Coordination complexes are found in many biomolecules, especially as essential ingredients for the active site of enzymes.

### Learning Objectives

Discuss the application of coordination complexes in biomolecules.

### Key Takeaways

#### Key Points

• Coordination complexes and transition metals are an integral component of proteins, especially the class of proteins that can perform chemical reactions, called enzymes.
• It is estimated that approximately half of all proteins contain a metal.
• The transition metals, particularly zinc and iron, are most often found at the active sites of enzymes.
• In metalloenzymes, the metal ion is bound to the protein with one labile coordination site.

#### Key Terms

• enzyme: A globular protein that catalyses a biological chemical reaction.
• cofactor: A substance, especially a coenzyme or a metal, that must be present for an enzyme to function.
• labile: A property of transient chemical species that enables rapid synthesis and degradation of substrates in biological systems.
• Metalloprotein: A protein that contains a metal ion cofactor.

### Coordination Complexes in Biology

Coordination complexes (also called coordination compounds) and transition metals are widespread in nature. They are an integral component of proteins, especially the class of proteins that can perform chemical reactions, called enzymes. The transition metals, particularly zinc and iron, are often key components of enzyme active sites. While there are other biologically relevant molecules that also contain metals, coordination complexes contain a central metal ion and are important in many biological processes.

Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large fraction of all proteins are metalloenzymes, and they have many diverse functions including transport, storage, and signal transduction.

### Coordination Chemistry Principles

In metalloproteins, metal ions are usually coordinated by nitrogen, oxygen, or sulfur centers belonging to amino acid residues of the protein. These donor groups are often provided by side-chains on the amino acid residues. Important donor groups include:

• imidazole (a nitrogen atom donor) substituents in histidine residues
• thiolate (sulfur atom) substituents in cysteinyl residues
• carboxylate groups (oxygen atom) provided by aspartate

Given the diversity of metalloproteins, virtually all amino acid residues have been shown to bind metal centers. The peptide backbone also provides donor groups; these include deprotonated amides and the amide carbonyl oxygen centers (oxygen and nitrogen atoms as ligands ).

In addition to donor groups that are provided by amino acid residues, a large number of organic cofactors function as ligands. Perhaps most famous are the tetradentate N4 macrocyclic ligands incorporated into the heme protein (most commonly seen as part of hemoglobin). Inorganic ligands such as sulfide and oxide are also common.

Heme B: Heme B is a porphyrin (four linked pyrrole rings) that readily binds iron, as shown. This is an example of a biomolecule that contains non-protein ligands for a transition metal.

### Metalloenzymes

Metalloenzymes contain a metal ion bound to the protein with one labile coordination site. As with all enzymes, the shape of the active site is crucial. The metal ion is usually located in a pocket whose shape fits the substrate. The metal ion catalyzes reactions that are difficult to achieve in organic chemistry. Consider the following reaction:

$\text{CO}_2+\text{H}_2\text{O}\rightleftharpoons \text{H}_2\text{CO}_3$

This reaction is very slow in the absence of a catalyst, but quite fast in the presence of the hydroxide ion.

$\text{CO}_2+\text{OH}^-\rightleftharpoons \text{HCO}_3^-$

A reaction similar to this is almost instantaneous with carbonic anhydrase. The structure of the active site in carbonic anhydrases is well known from a number of crystal structures. It consists of a zinc ion coordinated by three imidazole nitrogen atoms from three histidine units. The fourth coordination site is occupied by a water molecule.

The positively charged zinc ion polarizes the coordinated water molecule, and the nucleophilic attack by the negatively charged portion on carbon dioxide proceeds rapidly. The catalytic cycle produces the bicarbonate ion and the hydrogen ion as the equilibrium favors dissociation of carbonic acid at biological pH values.

$\text{H}_2\text{CO}_3\rightleftharpoons \text{HCO}_3^-+\text{H}^+$

Carbonic anhydrase active site: Active site of carbonic anhydrase. The three coordinating histidine residues are shown in green, hydroxide in red and white, and the zinc in gray.