Coordination Chemistry by Owen Borville 11.6.2025
Coordination compounds contain coordinate covalent bonds formed by the reactions of metal ions with groups of anions or polar molecules.
A coordinate covalent bond is a covalent bond in which one of the atoms donates both of the electrons that constitute the bond.
Coordination compounds can consist of a complex ion and one or more counter ions. The compound consists of the complex ion and two counter ions.
Such as K2[PtCl6] contains the (PtCl6)2- ion and two K+ ions.
However, some coordination compounds do not contain complex ions. A coordination compound without a complex ion is a neutral, uncharged molecule, which has no overall charge and is not ionic.
Most of the metals in coordination compounds are transition metals, located in the central section of the Periodic Table of the Elements (Groups 3-11) (the "d" block).
Properties of the transition metals include (1) incompletely filled d subshells (2) reacts to form ions with incompletely filled d subshells (3) distinctive colors (4) para-magnetism (5) catalytic activity (6) tendency to form complex ions (7) exhibit variable oxidation state.
Ligands are the molecules or ions that surround the metal in a complex ion. Ligands must contain at least one unshared pair of valence electrons. The donor atom is the atom in the ligand directly bonded to the metal atom. Examples of ligands are water H2O, ammonia NH3, carbon monoxide CO, chloride ion Cl-, and cyanide ion CN-.
The coordination number in a coordination compound refers to the number of donor atoms surrounding the central metal atom in a complex ion.
A chelating agent is a compound that binds to metal ions, forming a stable, water soluble complex that can be excreted from the body. Chelating agents are used in medicine to treat heavy metal poisoning and there are also industrial uses. Ethylenediamine, a bidentate ligand, is also called a chelating agent. EDTA, a polydentate ligand, is also called a chelating agent. Chelating agents have ring-like molecular structures.
Nomenclature Rules of Coordination Compounds: (1) the cation is named before the anion, as in other ionic compounds (2) Within a complex ion, the ligands are named first, in alphabetical order, and the metal ion is named last (3) the names of anionic ligands end with the letter o, whereas neutral ligands are usually called by the names of the molecules. (4) when two or more of the same ligand are present use Greek prefixes to specify their number: di, tri, tetra, penta, and hexa. (5) the oxidation number of the metal is indicated in Roman numerals immediately following the name of the metal. (6) the names of anions end in -ate.
Common ligands in coordination compounds are: (Br-), (Cl-), (CN-), (OH-), (O2-), ((CO3)2-), ((NO2)-), ((C2O4)2-), (NH3), (CO), (H2O)
Names of anions containing metal atoms and ending in (-ate): Al, Chromium Cr, Cobalt Co, Copper Cu, Gold Au, Iron Fe, Lead Pb, Manganese Mn, Molybdenum Mo, Nickel Ni, Silver Ag, Tin Sn, Tungsten W, Zinc Zn
Structure of Coordination Compounds: The geometry of coordination compounds play a significant role in determining its properties:
Linear coordination compound=coordination number=2
Tetrahedral or square planar compound=coordination number=4
Octahedral compound=coordination number 6
Stereoisomers are compounds that differ in the arrangement of ligands around the central atom. Stereoisomers have different chemical and physical properties. Stereoisomers can have two types of stereoisomerisms: geometric and optical.
Geometrical isomers are stereoisomers that cannot be interconverted without breaking chemical bonds. Geometric isomers come in pairs.
Optical isomers are nonsuperimposable mirror images. Termed chiral (lacks a plane of symmetry). Central atom is bonded to four different groups. Rotate polarized light in different directions: to the right: dextrorotatory (d isomer) rotation to the left: levorotatory (l isomer).
Enantiomers (non-superimposable mirror images of each other) are a pair of d and l isomers.
Racemic mixture=equimolar mixture of two enantiomers (50-50 blend). Rotate plane polarized light in equal and opposite directions, and is optically inactive. Net rotation of polarized light is zero.
Bonding in Coordination Compounds: Crystal field theory explains the bonding in complex ions purely in terms of electrostatic forces. Attraction between the metal ion (atom) and the ligands: Repulsion between the lone pairs on the ligands and the electrons in the d orbitals of the metal. In the absence of ligands, the d orbitals are degenerate.
Crystal field splitting is the separation of d-orbitals into different energy levels in a transition metal complex due to the electrostatic interactions between the metal ion and the surrounding ligands. Splitting affects the complex's color, magnetic properties, and stability, and the energy difference is important for understanding electronic transitions in inorganic chemistry.
Crystal Field Splitting in Octahedral Complexes: In the presence of ligands, electrons in d orbitals experience different levels of repulsion for the ligand lone pairs. As a result (depending on the geometry) some d orbitals attain higher energy and others lower energy.
The electrons in the d orbitals located along the coordinate axes experience stronger repulsions and increase in energy. The electrons in the d orbitals 45 degrees from the coordinate axes experience weaker repulsions and decrease in energy.
The energy difference between the two sets of orbitals is the crystal field splitting: depends on the nature of metals and ligands and also determines color and magnetic properties.
Color: As with reflected light, transmitted light of selected wavelengths is responsible for color. The color of observed light is the complementary color to the light absorbed.
Photoabsorption process: In the absorption spectrum of a compound, the energy of the incoming photon is equal to the crystal field splitting.
E = hv =hc/λ
𝛥 = hv =hc/λ
Spectroscopic measurements allow an ordering of ligands ability to split the d orbitals called a spectrochemical series, which shows the field strength of the ligand in order of increasing strength.
The magnitude of the crystal field splitting also determines the magnetic properties of a complex ion. The electron configuration of the ion is a balance between: the energy to promote an electron to a higher energy d orbital and stability gained by maximum number of unpaired spins.
Tetrahedral and Square Planar Field Complexes: Proximity of the ligands to d orbitals changes with the geometry of the complex: d electrons in orbitals more closely associated with the lone pairs of ligand electrons attain higher energies. Splitting patterns reflect this repulsion.
Reactions of Coordination Compounds: Complex ions undergo ligand exchange (or substitution) reactions in solution (where compounds exchange molecules or ions during chemical reaction in solution). Rates of exchange reactions vary widely. Ex. A solution of hexaaquacobalt (II) exchanges with chloride ions to form tetrachloroaquacobalt (II) ions.
Exchange reactions are characterized by (1) Thermodynamic stability, measured by Kf: Large Kf values indicate stability. Small Kf values indicate instability. (2) Kinetic lability-is the tendency to react: Labile complexes undergo rapid exchange. Inert complexes undergo slow exchange. Thermodynamically stable complexes can be labile or inert.
Applications of Coordination Compounds: (1) Metallurgy in the extraction by complex formation (2) Chelation therapy is the removal of toxins by chelation (3) Chemotherapy is the use of complexes to inhibit the growth of cancer cells. (4) Chemical analysis is used in both qualitative and quantitative analysis. (5) Detergents: Chelating agents (tripolyphosphates) to complex divalent ions associated with water hardness. (6) Environmental impact: eutrophication from phosphates. (7) Sequestrants: agents to complex metal ions that catalyze oxidation reactions in foods.
Coordination compounds contain coordinate covalent bonds formed by the reactions of metal ions with groups of anions or polar molecules.
A coordinate covalent bond is a covalent bond in which one of the atoms donates both of the electrons that constitute the bond.
Coordination compounds can consist of a complex ion and one or more counter ions. The compound consists of the complex ion and two counter ions.
Such as K2[PtCl6] contains the (PtCl6)2- ion and two K+ ions.
However, some coordination compounds do not contain complex ions. A coordination compound without a complex ion is a neutral, uncharged molecule, which has no overall charge and is not ionic.
Most of the metals in coordination compounds are transition metals, located in the central section of the Periodic Table of the Elements (Groups 3-11) (the "d" block).
Properties of the transition metals include (1) incompletely filled d subshells (2) reacts to form ions with incompletely filled d subshells (3) distinctive colors (4) para-magnetism (5) catalytic activity (6) tendency to form complex ions (7) exhibit variable oxidation state.
Ligands are the molecules or ions that surround the metal in a complex ion. Ligands must contain at least one unshared pair of valence electrons. The donor atom is the atom in the ligand directly bonded to the metal atom. Examples of ligands are water H2O, ammonia NH3, carbon monoxide CO, chloride ion Cl-, and cyanide ion CN-.
The coordination number in a coordination compound refers to the number of donor atoms surrounding the central metal atom in a complex ion.
A chelating agent is a compound that binds to metal ions, forming a stable, water soluble complex that can be excreted from the body. Chelating agents are used in medicine to treat heavy metal poisoning and there are also industrial uses. Ethylenediamine, a bidentate ligand, is also called a chelating agent. EDTA, a polydentate ligand, is also called a chelating agent. Chelating agents have ring-like molecular structures.
Nomenclature Rules of Coordination Compounds: (1) the cation is named before the anion, as in other ionic compounds (2) Within a complex ion, the ligands are named first, in alphabetical order, and the metal ion is named last (3) the names of anionic ligands end with the letter o, whereas neutral ligands are usually called by the names of the molecules. (4) when two or more of the same ligand are present use Greek prefixes to specify their number: di, tri, tetra, penta, and hexa. (5) the oxidation number of the metal is indicated in Roman numerals immediately following the name of the metal. (6) the names of anions end in -ate.
Common ligands in coordination compounds are: (Br-), (Cl-), (CN-), (OH-), (O2-), ((CO3)2-), ((NO2)-), ((C2O4)2-), (NH3), (CO), (H2O)
Names of anions containing metal atoms and ending in (-ate): Al, Chromium Cr, Cobalt Co, Copper Cu, Gold Au, Iron Fe, Lead Pb, Manganese Mn, Molybdenum Mo, Nickel Ni, Silver Ag, Tin Sn, Tungsten W, Zinc Zn
Structure of Coordination Compounds: The geometry of coordination compounds play a significant role in determining its properties:
Linear coordination compound=coordination number=2
Tetrahedral or square planar compound=coordination number=4
Octahedral compound=coordination number 6
Stereoisomers are compounds that differ in the arrangement of ligands around the central atom. Stereoisomers have different chemical and physical properties. Stereoisomers can have two types of stereoisomerisms: geometric and optical.
Geometrical isomers are stereoisomers that cannot be interconverted without breaking chemical bonds. Geometric isomers come in pairs.
Optical isomers are nonsuperimposable mirror images. Termed chiral (lacks a plane of symmetry). Central atom is bonded to four different groups. Rotate polarized light in different directions: to the right: dextrorotatory (d isomer) rotation to the left: levorotatory (l isomer).
Enantiomers (non-superimposable mirror images of each other) are a pair of d and l isomers.
Racemic mixture=equimolar mixture of two enantiomers (50-50 blend). Rotate plane polarized light in equal and opposite directions, and is optically inactive. Net rotation of polarized light is zero.
Bonding in Coordination Compounds: Crystal field theory explains the bonding in complex ions purely in terms of electrostatic forces. Attraction between the metal ion (atom) and the ligands: Repulsion between the lone pairs on the ligands and the electrons in the d orbitals of the metal. In the absence of ligands, the d orbitals are degenerate.
Crystal field splitting is the separation of d-orbitals into different energy levels in a transition metal complex due to the electrostatic interactions between the metal ion and the surrounding ligands. Splitting affects the complex's color, magnetic properties, and stability, and the energy difference is important for understanding electronic transitions in inorganic chemistry.
Crystal Field Splitting in Octahedral Complexes: In the presence of ligands, electrons in d orbitals experience different levels of repulsion for the ligand lone pairs. As a result (depending on the geometry) some d orbitals attain higher energy and others lower energy.
The electrons in the d orbitals located along the coordinate axes experience stronger repulsions and increase in energy. The electrons in the d orbitals 45 degrees from the coordinate axes experience weaker repulsions and decrease in energy.
The energy difference between the two sets of orbitals is the crystal field splitting: depends on the nature of metals and ligands and also determines color and magnetic properties.
Color: As with reflected light, transmitted light of selected wavelengths is responsible for color. The color of observed light is the complementary color to the light absorbed.
Photoabsorption process: In the absorption spectrum of a compound, the energy of the incoming photon is equal to the crystal field splitting.
E = hv =hc/λ
𝛥 = hv =hc/λ
Spectroscopic measurements allow an ordering of ligands ability to split the d orbitals called a spectrochemical series, which shows the field strength of the ligand in order of increasing strength.
The magnitude of the crystal field splitting also determines the magnetic properties of a complex ion. The electron configuration of the ion is a balance between: the energy to promote an electron to a higher energy d orbital and stability gained by maximum number of unpaired spins.
Tetrahedral and Square Planar Field Complexes: Proximity of the ligands to d orbitals changes with the geometry of the complex: d electrons in orbitals more closely associated with the lone pairs of ligand electrons attain higher energies. Splitting patterns reflect this repulsion.
Reactions of Coordination Compounds: Complex ions undergo ligand exchange (or substitution) reactions in solution (where compounds exchange molecules or ions during chemical reaction in solution). Rates of exchange reactions vary widely. Ex. A solution of hexaaquacobalt (II) exchanges with chloride ions to form tetrachloroaquacobalt (II) ions.
Exchange reactions are characterized by (1) Thermodynamic stability, measured by Kf: Large Kf values indicate stability. Small Kf values indicate instability. (2) Kinetic lability-is the tendency to react: Labile complexes undergo rapid exchange. Inert complexes undergo slow exchange. Thermodynamically stable complexes can be labile or inert.
Applications of Coordination Compounds: (1) Metallurgy in the extraction by complex formation (2) Chelation therapy is the removal of toxins by chelation (3) Chemotherapy is the use of complexes to inhibit the growth of cancer cells. (4) Chemical analysis is used in both qualitative and quantitative analysis. (5) Detergents: Chelating agents (tripolyphosphates) to complex divalent ions associated with water hardness. (6) Environmental impact: eutrophication from phosphates. (7) Sequestrants: agents to complex metal ions that catalyze oxidation reactions in foods.