Coordination Compounds

The One Thing

Coordination compounds consist of a central metal atom or ion bonded to ligands through coordinate bonds. Their nomenclature, geometry, isomerism, magnetic behaviour, colour, stability, and applications are determined by the metal, ligands, oxidation state, coordination number, and bonding model.

Reactions, Processes and Experiments

Key Terms

• Coordination entity: The central metal atom or ion together with its attached ligands, written inside square brackets, such as [Co(NH3)6]3+.
• Central metal atom or ion: The metal species that accepts electron pairs from ligands to form coordinate bonds.
• Ligand: An ion or molecule that donates one or more lone pairs of electrons to the central metal ion.
• Coordinate bond: A covalent bond in which both bonding electrons are donated by the ligand to the metal ion.
• Coordination number: The number of donor atoms directly attached to the central metal ion; it is not always equal to the number of ligands.
• Coordination sphere: The central metal ion and the ligands directly bonded to it, represented within square brackets.
• Counter ions: Ions outside the coordination sphere that balance the charge of the coordination entity.
• Denticity: The number of donor atoms of a ligand that attach to the same metal ion.
• Monodentate ligand: A ligand that forms one coordinate bond with the metal, such as NH3, H2O, Cl−, or CN−.
• Bidentate ligand: A ligand that uses two donor atoms to attach to the metal, such as ethane-1,2-diamine, en, or oxalate, C2O4^2−.
• Polydentate ligand: A ligand with several donor atoms, such as EDTA4−, which can form multiple coordinate bonds.
• Chelate: A ring-shaped structure formed when a multidentate ligand attaches to a metal ion through two or more donor atoms.
• Ambidentate ligand: A ligand that can coordinate through either of two different donor atoms, such as NO2− or SCN−.
• Homoleptic complex: A complex containing only one type of ligand, such as [Co(NH3)6]3+.
• Heteroleptic complex: A complex containing more than one type of ligand, such as [Co(NH3)4Cl2]+.
• Oxidation state: The formal charge assigned to the central metal after treating ligand charges and the overall complex charge algebraically.
• Werner's coordination theory: A theory stating that metals show primary valency, corresponding mainly to oxidation state, and secondary valency, corresponding to coordination number and directional arrangement.
• Valence Bond Theory: A bonding model that explains complexes using hybridisation of metal orbitals and predicts geometry and, in many cases, magnetic behaviour.
• Crystal Field Theory: A model in which ligands create an electric field that removes the equal energy of metal d-orbitals, producing crystal-field splitting.
• Crystal-field splitting energy: The energy difference between sets of d-orbitals separated by the ligand field; it is represented by Δo in octahedral and Δt in tetrahedral complexes.
• Strong-field ligand: A ligand that produces large d-orbital splitting and may cause pairing of d-electrons, such as CN−, CO, and generally NH3.
• Weak-field ligand: A ligand that produces small d-orbital splitting and usually does not cause electron pairing, such as F−, Cl−, and H2O.
• High-spin complex: A complex with the maximum possible number of unpaired electrons, usually formed with weak-field ligands.
• Low-spin complex: A complex with fewer unpaired electrons because pairing occurs in lower-energy orbitals, usually with strong-field ligands.
• Geometrical isomerism: Isomerism caused by different relative positions of ligands, such as cis and trans forms of [Pt(NH3)2Cl2].
• Optical isomerism: Isomerism in which two complexes are non-superimposable mirror images, called enantiomers.
• Ionisation isomerism: Isomerism caused by exchange of a ligand inside the coordination sphere with a counter ion outside it.
• Linkage isomerism: Isomerism caused by coordination through different donor atoms of an ambidentate ligand.
• Coordination isomerism: Isomerism arising when ligands exchange between the cationic and anionic coordination entities of a compound.
• Magnetic moment: A measure of paramagnetism based mainly on the number of unpaired electrons; spin-only magnetic moment is μ = √[n(n + 2)] BM.
• Paramagnetic complex: A complex containing one or more unpaired electrons and attracted by a magnetic field.
• Diamagnetic complex: A complex containing only paired electrons and weakly repelled by a magnetic field.
• Stability constant: An equilibrium constant that measures the tendency of a metal ion and ligands to form a coordination complex.

Easily Confused

• Coordination sphere and counter ions: The coordination sphere is inside square brackets and contains directly bonded ligands; counter ions are outside the brackets and balance the charge.
• Coordination number and number of ligands: Coordination number counts directly attached donor atoms, whereas the number of ligands counts ligand species; a bidentate ligand contributes two donor atoms.
• Primary and secondary valency: Primary valency is ionisable and corresponds mainly to oxidation state; secondary valency is non-ionisable and corresponds to coordination number.
• Monodentate and bidentate ligands: A monodentate ligand forms one coordinate bond, whereas a bidentate ligand forms two through two donor atoms.
• Strong-field and weak-field ligands: Strong-field ligands produce large splitting and may cause pairing; weak-field ligands produce smaller splitting and usually give high-spin arrangements.
• High-spin and low-spin complexes: High-spin complexes have the maximum number of unpaired electrons; low-spin complexes have fewer because pairing occurs.
• Paramagnetic and diamagnetic complexes: Paramagnetic complexes contain unpaired electrons; diamagnetic complexes contain only paired electrons.
• Geometrical and optical isomerism: Geometrical isomerism involves different ligand positions, such as cis and trans; optical isomerism involves non-superimposable mirror images.
• Ionisation and linkage isomerism: Ionisation isomerism exchanges a coordinated ligand with a counter ion; linkage isomerism changes the donor atom through which an ambidentate ligand bonds.
• Coordination and linkage isomerism: Coordination isomerism involves ligand exchange between cationic and anionic coordination entities; linkage isomerism involves different donor atoms of one ambidentate ligand.
• Δo and Δt: Δo describes octahedral splitting, whereas Δt describes tetrahedral splitting and is approximately 4/9 of Δo.
• Valence Bond Theory and Crystal Field Theory: Valence Bond Theory focuses on hybridisation and approximate geometry; Crystal Field Theory explains splitting, colour, pairing, and magnetic behaviour more effectively.

What Gets Asked

• Determine the oxidation state of the metal using oxidation state of metal + sum of ligand charges = charge on the coordination entity. Marks are lost by ignoring the charge of a ligand or the charge outside the brackets.
• Name coordination compounds such as [Co(NH3)5Cl]Cl2, K4[Fe(CN)6], and [Cu(NH3)4]SO4. Marks are lost by failing to alphabetise ligand names, using incorrect ligand names, omitting the metal oxidation state, or failing to use the “-ate” ending for anionic complexes.
• Identify coordination number and geometry, using the relationships 2, linear; 4, tetrahedral or square planar; and 6, octahedral. Marks are lost by counting ligand molecules rather than donor atoms.
• Explain magnetic behaviour and calculate magnetic moment using μ = √[n(n + 2)] BM. Marks are lost by confusing the number of unpaired electrons with the total number of d-electrons or by reversing paramagnetic and diamagnetic definitions.
• Compare strong-field and weak-field complexes using the spectrochemical series and distinguish high-spin from low-spin arrangements. Marks are lost by treating all ligands as having the same field strength.
• Identify types of isomerism using [Pt(NH3)2Cl2] for cis–trans geometrical isomerism, NO2− for linkage isomerism, and [Co(NH3)5Br]SO4/[Co(NH3)5SO4]Br for ionisation isomerism. Marks are lost by confusing ligand position, donor-atom choice, and exchange with counter ions.