Notes on co-ordination compounds_050808.docx

**Coordination Compounds, Complexation, Complexes and Chelating Agents** **Introduction** - [[Define coordination compounds, complex ions, ligands, coordination number, and coordination sphere^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Pri...

**Coordination Compounds, Complexation, Complexes and Chelating Agents** **Introduction** - [[Define coordination compounds, complex ions, ligands, coordination number, and coordination sphere^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Explain the difference between coordination compounds and complex ions^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Give examples of common coordination compounds and their applications in chemistry, biology, and industry^1^](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds)[^2^](https://www.britannica.com/science/chelate)]. **Structures of Coordination Compounds** - [[Describe the common geometries of coordination compounds based on the coordination number and the type of ligands^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Use the valence shell electron pair repulsion (VSEPR) theory to predict the shapes of coordination compounds^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Draw Lewis structures and 3D representations of coordination compounds using appropriate symbols and conventions^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). **Nomenclature of Coordination Compounds** - [[Learn the rules and guidelines for naming coordination compounds, including the use of prefixes, suffixes, and parentheses^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Distinguish between the names of the cation and the anion in a coordination compound^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Name coordination compounds given their formulas and write formulas given their names^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). **Isomerism in Coordination Compounds** - [[Define isomers and identify the types of isomerism possible in coordination compounds^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Explain the difference between structural isomers and stereoisomers^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Recognize and name the geometrical and optical isomers of coordination compounds^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). **Bonding in Coordination Compounds** - [[Review the concepts of Lewis acids and bases, and apply them to the formation of coordination compounds^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Compare and contrast the two main theories of bonding in coordination compounds: the crystal field theory and the ligand field theory^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Use the crystal field theory to explain the splitting of d-orbitals, the effect of ligand strength, and the magnetic properties of coordination compounds^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Use the ligand field theory to explain the hybridization, molecular orbital diagram, and bond order of coordination compounds^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). **Stability of Coordination Compounds** - [[Define the stability constant and the formation constant of a coordination compound^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Relate the stability constant to the equilibrium constant and the Gibbs free energy of a complexation reaction^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). - [[Use the stability constants to compare the relative stabilities of different coordination compounds^1^]](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds). **Chelating Agents and Chelation Therapy** - [[Define chelating agents, chelates, and chelation](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds)[^2^](https://www.britannica.com/science/chelate)]. - [[Explain the advantages of chelating agents over monodentate ligands in terms of stability and selectivity](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds)[^2^](https://www.britannica.com/science/chelate)]. - [[Give examples of natural and synthetic chelating agents and their uses](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds)[^2^](https://www.britannica.com/science/chelate)]. - [[Describe the principles and applications of chelation therapy in medicine and environmental remediation](https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/22%3A_The_d-Block_Elements/22.04%3A_Coordination_Compounds)[^2^](https://www.britannica.com/science/chelate)]. **Summary and Review** - Summarize the main concepts and learning objectives of the lecture. - Provide some practice problems and exercises for students to test their understanding and skills. - Answer any questions or doubts from the students. INTRODUCTION Definition Coordination compounds are molecules that possess a metal center that is bound to ligands (atoms, ions, or molecules that donate electrons to the metal). Complex ions are a type of coordination compounds that carry a net electric charge. Ligands are the atoms, ions, or molecules that are attached to the metal center by coordinate covalent bonds. Coordination number is the number of donor atoms bonded to the central metal atom or ion. Coordination sphere is the central metal atom or ion plus its attached ligands. The coordination sphere is usually enclosed in brackets when written in a formula. **Differences between co-ordination compounds and complex ions** Coordination compounds and complex ions are both substances that have a central metal atom or ion surrounded by ligands. However, the difference is that coordination compounds are neutral, while complex ions are charged. For example, hexaamminecobalt(III) chloride. \[Co(NH~3~)~6~\]Cl~3~ is a coordination compound that contains the complex ion \[Co(NH~3~)~6~\]^3+^ and three Cl^-^ counterions. The complex ion has a net charge of +3, while the coordination compound has no net charge. Another difference is that coordination compounds can exist as solids, liquids, or gases, while complex ions only exist in solution **APPLICATIONS OF CO-ORDINATION COMPOUNDS** Coordination compounds have many applications in chemistry, biology, and industry, such as: Coloration: Coordination compounds have specific colors due to the interaction of light with the d-orbitals of the metal. They are used to make dyes and pigments for fabrics, paints, and printing. Catalysis: Coordination compounds can act as catalysts, which speed up chemical reactions without being consumed. They are used to produce polymers, fuels, fertilizers, and pharmaceuticals. Biomolecules: Coordination compounds are essential for many biological processes, such as oxygen transport, photosynthesis, enzyme activity, and drug action. Examples of biomolecules that contain coordination compounds are hemoglobin, chlorophyll, vitamin B12, and cisplatin. **STRUCTURES OF CO-ORDINATION COMPOUNDS** The number of ligands attached to the metal is called the coordination number (CN), and the spatial arrangement of the ligands is called the coordination geometry. The coordination geometry depends on the number and type of ligands, as well as the repulsion between the electron pairs in the valence shell of the metal. The valence shell electron pair repulsion (VSEPR) theory is a model that can be used to predict the shapes of molecules and polyatomic ions based on the assumption that electron pairs repel each other and adopt the geometry that minimizes the repulsion. According to this theory, the electron pairs around the metal can be classified as bonding pairs (BP) or lone pairs (LP), and the shape of the complex is determined by the number of effective electron pairs (EEP), which is the sum of BP and LP. The EEP can be different from the CN if the metal has multiple bonds or unpaired electrons. To draw Lewis structures and 3D representations of coordination compounds, the following steps can be followed: - Write the formula of the complex ion, using square brackets to indicate the coordination sphere and the charge on the complex. - Determine the oxidation state of the metal and the charge on each ligand. - Draw the Lewis structure of the metal, showing the number of valence electrons and the number of unpaired electrons. - Draw the Lewis structures of the ligands, showing the lone pairs and the atoms that coordinate to the metal. - Arrange the ligands around the metal according to the VSEPR theory, using dashed lines to indicate the dative bonds and wedges and dashes to indicate the 3D geometry. - Indicate the overall charge on the complex and the oxidation state of the metal using Roman numerals. For example, consider the complex ion \[Cr(NH3)5Cl\]2+. The formula shows that the CN is 6 and the charge on the complex is 2+. The oxidation state of Cr is 3+ and the charge on each ligand is 0 for NH3 and -1 for Cl. The Lewis structure and 3D representation of the complex are shown below: H H \| \| H - N - Cr - N - H \| /\| \| H / \| H / \| Cl N - H \| H \[Cr(NH3)5Cl\]2+ Cr(III) **NOMENCLATURE OF CO-ORDINATION COMPOUNDS** The coordination compounds have a specific nomenclature that depends on the number, type, and charge of the ligands and the metal. Here are some rules and guidelines for naming coordination compounds: - If the coordination compound is an ion, name the cation before the anion, just like in ionic compounds. For example, \[Co(NH3)6\]Cl3 is hexaamminecobalt(III) chloride. - If the coordination compound is neutral, name the complex as a single word. For example, \[Ni(CO)4\] is tetracarbonylnickel(0). - Use square brackets to enclose the coordination sphere, which consists of the metal and the ligands. For example, \[Cr(NH3)6\]3+ is the hexaamminechromium(III) ion. - Name the ligands in alphabetical order, regardless of their charge or number. Use prefixes to indicate the number of each ligand: mono-, di-, tri-, tetra-, penta-, hexa-, etc. For example, \[Pt(NH3)2Cl4\] is diamminetetrachloroplatinum(IV). - If the ligand is an anion, change its ending to -o. For example, Cl- is chloro, O2- is oxo, CN- is cyano, etc. - If the ligand is a neutral molecule, use its common name. For example, NH3 is ammine, H2O is aqua, CO is carbonyl, etc. - If the ligand is a polyatomic ion or molecule with more than one donor atom, use parentheses to enclose its formula and a prefix to indicate the number of such ligands. For example, \[Co(en)3\]3+ is tris(ethylenediamine)cobalt(III) ion, where en is H2NCH2CH2NH2. - Indicate the oxidation state of the metal by a Roman numeral in parentheses after its name. For example, \[Fe(CN)6\]4- is hexacyanoferrate(II) ion, where Fe is in the +2 oxidation state. - If the complex has geometric or optical isomers, use special prefixes or suffixes to specify the isomer. For example, cis- and trans- for geometric isomers, and - and - for optical isomers. For example, \[Co(NH3)4Cl2\]+ has two geometric isomers: cis-tetraamminedichlorocobalt(III) ion and trans-tetraamminedichlorocobalt(III) ion. To write the formulas of coordination compounds given their names, follow the reverse process: - Identify the metal and its oxidation state from the name. Write the symbol of the metal and enclose it in square brackets. - Identify the ligands and their number from the name. Write the formulas of the ligands and use subscripts to indicate their number. If the ligand has parentheses, multiply the subscript outside the parentheses by the number of atoms inside the parentheses. - Balance the charge of the complex by adding or subtracting electrons from the metal. If the complex is an ion, write the charge as a superscript after the square brackets. If the complex is neutral, omit the charge. - If the complex is an ion, write the formula of the counterion and use subscripts to balance the charge of the compound. If the complex is neutral, omit the counterion. Some examples of naming and writing formulas of coordination compounds are: - \[Cr(H2O)6\]Cl3 is hexaaquachromium(III) chloride. - Potassium hexacyanoferrate(III) is K3\[Fe(CN)6\]. - \[Co(NH3)5(NO2)\]Cl2 is pentaamminenitrito-N-cobalt(III) chloride. - Diamminedichloroplatinum(II) is \[Pt(NH3)2Cl2\]. ISOMERISM Isomers are compounds that have the same molecular formula but different arrangements of atoms or groups. Coordination compounds can exhibit two main forms of isomerism: structural isomerism and stereoisomerism. Structural isomers differ in the bonding patterns or types of bonds among the atoms. For example, they may have different ligands attached to the central metal atom, or different atoms of the ligands coordinated to the metal. Structural isomers have different chemical formulas and properties. There are three types of structural isomers: ionization, coordination, and linkage. Stereoisomers have the same bonding patterns and types of bonds, but differ in the spatial orientation of the atoms or groups. They have the same chemical formula and similar properties, but may interact differently with other molecules or light. There are two types of stereoisomers: geometrical and optical. Geometrical isomers have different relative positions of the ligands around the central metal atom. They are possible for square planar and octahedral complexes, but not for tetrahedral complexes. Geometrical isomers are named using prefixes such as cis- and trans-, or fac- and mer-, to indicate the arrangement of the ligands. Optical isomers are non-superimposable mirror images of each other. They have the same relative positions of the ligands, but different orientations in space. They are possible for both tetrahedral and octahedral complexes, but not for square planar complexes. Optical isomers can rotate the plane of polarized light in different directions, and are named using prefixes such as D- and L-, or + and -- BONDING Lewis acids and bases are species that can accept or donate an electron pair, respectively. Coordination compounds are formed by the interaction of a Lewis acid (a metal ion) and a Lewis base (a ligand) to form a coordinate covalent bond. Crystal field theory (CFT) and ligand field theory (LFT) are two models that describe the bonding and properties of coordination compounds. CFT is based on the electrostatic interaction between the metal ion and the ligands, and assumes that the ligands are point charges or dipoles. LFT is a more advanced model that considers the covalent bonding and molecular orbital interactions between the metal and the ligands34. CFT explains the splitting of d-orbitals by the effect of the ligands on the energy of the metal orbitals. Depending on the geometry of the complex, the d-orbitals are split into two or three sets of different energies. The energy difference between the sets is called the crystal field splitting energy (Δ). The ligand strength or field strength is a measure of how much a ligand can split the d-orbitals. Strong field ligands cause a large Δ, while weak field ligands cause a small Δ. The magnetic properties of a complex depend on the number of unpaired electrons in the d-orbitals, which is determined by the value of Δ and the electron configuration of the metal56. LFT explains the hybridization, molecular orbital diagram, and bond order of coordination compounds by using the principles of molecular orbital theory. The metal orbitals (s, p, and d) and the ligand orbitals (usually p or π) are combined to form bonding, nonbonding, and antibonding molecular orbitals. The hybridization of the metal orbitals depends on the symmetry and overlap of the ligand orbitals. The molecular orbital diagram shows the relative energies and occupations of the molecular orbitals. The bond order is calculated by subtracting the number of electrons in the antibonding orbitals from the number of electrons in the bonding orbitals, and dividing by two Hybridization of Orbitals - Chemistry Topics STABILITY The stability constant (Ks) and the formation constant (Kf) of a coordination compound are two terms for the same equilibrium constant that describes the formation of the complex from the metal ion and the ligands in solution. For example, for the reaction: M\^n+ + nL \ \[ML\_n\]\^m+ , Ks = Kf = \[ML\_n\]\^m+ / (\[M\^n+\] \[L\]\^n) 12 The stability constant is related to the equilibrium constant (K) and the Gibbs free energy (ΔG) of the complexation reaction by the equation: ΔG = -RT ln K = -RT ln Ks , where R is the gas constant, T is the temperature, and ln is the natural logarithm. This equation shows that a more negative ΔG corresponds to a larger K and Ks, which means a more stable complex 34. The stability constants can be used to compare the relative stabilities of different coordination compounds by looking at their magnitude and sign. A larger and positive Ks indicates a more stable complex, while a smaller and negative Ks indicates a less stable complex. The stability of a complex also depends on the nature of the metal ion, the type and number of ligands, and the solvent CHELATING AGENTS AND CHELATION THERAPY \ Chelating agents are ligands that can form two or more coordinate bonds with a metal ion, forming a ring-like structure called a chelate. [[Chelation is the process of forming such bonds]](https://www.britannica.com/science/chelate).  Chelating agents have advantages over monodentate ligands in terms of stability and selectivity. Stability means that chelating agents bind more strongly to a metal ion than monodentate ligands, making the complex less likely to dissociate. [[Selectivity means that chelating agents can bind preferentially to certain metal ions over others, depending on their size, shape, charge, and donor atoms]](https://www.britannica.com/science/chelate).  Examples of natural and synthetic chelating agents and their uses are: - Ethylenediaminetetraacetic acid (EDTA): a synthetic chelating agent that can bind to almost any metal ion. [[It is used for chelation therapy, water treatment, food preservation, and analytical chemistry]](https://www.britannica.com/science/chelate). - Porphyrins: natural chelating agents that contain four nitrogen atoms in a cyclic structure. They are found in biological molecules such as hemoglobin, chlorophyll, and vitamin B12. [[They are involved in oxygen transport, photosynthesis, and enzyme activity]](https://www.britannica.com/science/chelate). - Amino acids: natural chelating agents that contain an amino group and a carboxyl group. They are the building blocks of proteins and can bind to metal ions in various ways. [[They are used for nutrition, agriculture, and biotechnology]](https://www.britannica.com/science/chelate).  Chelation therapy is a medical treatment that uses chelating agents to remove toxic metals from the body, such as lead, mercury, arsenic, and iron. It can be used to treat metal poisoning, blood disorders, cardiovascular diseases, and neurological disorders. [[Chelation therapy can be administered intravenously, orally, or topically]](https://www.britannica.com/science/chelate).  Chelation technology is an environmental remediation technique that uses chelating agents to extract metals from contaminated soils, waters, and wastes. It can be used to recover valuable metals, reduce metal toxicity, and improve soil quality. [[Chelation technology can be applied in situ or ex situ, using various methods such as washing, leaching, extraction, and precipitation]](http://turfcare.eu/wp-content/uploads/2018/08/Chelating-Agents-Organic-V-Synthetic.pdf)