Coordination Compounds: IUPAC Nomenclature

• Coordination compounds consist of a central metal atom or ion, and a surrounding array of bound molecules or ions, that are known as ligands or complexing agents.

IUPAC Naming Conventions

• Nomenclature in coordination chemistry adheres to specific IUPAC rules for clarity and uniformity.
• The naming goal is to systematically describe the components and structure of the complex.
• When naming, the cation is named before the anion, regardless of whether the complex ion is the cation or the anion.
• Within the coordination sphere, ligands are named first, in alphabetical order based on the ligand name, not the ligand symbol.
• Alphabetical order does not consider prefixes indicating the number of ligands.
• Anionic ligands are named by adding the suffix "-o" to the root name of the anion, such as chloride becoming chlorido, cyanide becoming cyanido, and hydroxide becoming hydroxido
• Neutral ligands are generally referred to by their usual names, with some exceptions.
• Water is named as "aqua", ammonia is named as "ammine" (note the double "m"), carbon monoxide is named as "carbonyl," and nitric oxide is named as "nitrosyl."
• Polydentate ligands that attach through different atoms are indicated by adding italicized symbols for the donor atoms after the name.
• Glycinate ion (NH2CH2COO-) can bind through either nitrogen or oxygen.
• Prefixes indicate the number of each type of ligand in the complex ion.
• "di-" indicates two, "tri-" indicates three, "tetra-" indicates four, "penta-" indicates five, and "hexa-" indicates six.
• If the ligand name itself contains numerical prefixes (e.g., ethylenediamine), alternative prefixes are used to avoid confusion.
• "bis-" indicates two, "tris-" indicates three, "tetrakis-" indicates four, "pentakis-" indicates five, and "hexakis-" indicates six.
• After naming the ligands, the central metal ion is named.
• The oxidation state of the metal ion is indicated in Roman numerals in parentheses immediately following the name of the metal.
• If the complex ion is an anion, the name of the metal ends with the suffix "-ate".
• If the central metal is iron in an anionic complex, it is named ferrate.
• For some metals, the Latin name is used in the anionic form such as cuprate for copper, aurate for gold, stannate for tin, and plumbate for lead.
• When writing the formula of a coordination complex, the central metal ion is listed first, followed by the ligands.
• Ligands are listed in alphabetical order according to their symbols or abbreviations.
• The entire complex ion is enclosed in square brackets [ ].
• When multiple complex ions are present in a compound, the cation is written before the anion.
• If the complex ion carries a charge, the charge is indicated outside the square brackets as a superscript.
• Isomers, compounds with the same chemical formula but different arrangements of atoms, are distinguished using prefixes like cis-, trans-, mer-, or fac-.
• These prefixes indicate the spatial arrangement of ligands around the central metal atom.
• Bridging ligands, which connect two or more metal centers, are indicated by the prefix "μ-" placed before the ligand name.
• If there are multiple bridging ligands of the same type, the prefix "μn-" is used, where n is the number of metal centers bridged.

Ligand Identification

• Ligands are ions or molecules that coordinate to a central metal atom or ion to form a coordination complex.
• Ligands are classified based on the number of donor atoms through which they bind to the metal.
• Monodentate ligands bind to the metal through only one donor atom.
• Examples: halides (F-, Cl-, Br-, I-), cyanide (CN-), hydroxide (OH-), water (H2O), ammonia (NH3), and carbon monoxide (CO).
• Polydentate ligands bind to the metal through more than one donor atom.
• Examples: ethylenediamine (en), oxalate (C2O42-), and EDTA.
• Bidentate ligands bind through two donor atoms.
• Ethylenediamine (en) has two nitrogen atoms that can coordinate to the metal.
• Oxalate (C2O42-) has two oxygen atoms that can coordinate.
• Glycinate (gly) has one nitrogen and one oxygen atom for coordination.
• Tridentate ligands bind through three donor atoms.
• Diethylenetriamine (dien) coordinates through three nitrogen atoms.
• Tetradentate ligands bind through four donor atoms.
• Triethylenetetramine (trien) coordinates through four nitrogen atoms.
• Pentadentate ligands bind through five donor atoms.
• Ethylenediaminetriacetic acid (EDTA) with one proton removed can coordinate through five atoms.
• Hexadentate ligands bind through six donor atoms.
• Ethylenediaminetetraacetic acid (EDTA) coordinates through six atoms (two nitrogen and four oxygen atoms).
• Ambidentate ligands can bind through more than one type of donor atom, but only one donor atom is coordinated to the metal in a given complex.
• Thiocyanate (SCN-), can bind through either sulfur or nitrogen, and nitrite (NO2-), can bind through either nitrogen or oxygen.
• Bridging ligands can bind to two or more metal centers, linking them together.
• Common bridging ligands include: halides (Cl-), hydroxide (OH-), and oxide (O2-).
• The denticity of a ligand refers to the number of donor atoms through which it binds to the central metal.
• Monodentate ligands have a denticity of 1.
• Bidentate ligands have a denticity of 2.
• Polydentate ligands have a denticity greater than 1.
• Ligands are classified based on the magnitude and type of interaction they have with the metal center.
• Strong-field ligands cause a large splitting of the d-orbitals of the metal ion, leading to low-spin complexes.
• Examples: cyanide (CN-) and carbon monoxide (CO).
• Weak-field ligands cause a small splitting of the d-orbitals, leading to high-spin complexes.
• Examples: halides (Cl-, I-) and water (H2O).
• Spectrochemical series orders ligands based on their ability to cause d-orbital splitting.
• Ligands such as CO and CN- are high in the spectrochemical series, indicating strong-field ligands, while ligands such as I- and Br- are low in the series, representing weak-field ligands.

Oxidation States of Metal Centers

• The oxidation state of the metal center in a coordination compound is the charge it would have if all the ligands were removed along with their electron pairs.
• To determine the oxidation state of the metal, one must know the overall charge of the complex and the charges of the ligands.
• The sum of the charges of the metal and all the ligands must equal the overall charge of the complex.
• Common oxidation states for transition metals vary widely, but many exhibit multiple stable oxidation states.
• Iron can exist as Fe(II) or Fe(III), and copper can exist as Cu(I) or Cu(II).
• The charge of the metal ion is represented in Roman numerals in the name of the coordination compound.
• The charge of common ligands include:
• Halides (F-, Cl-, Br-, I-) have a charge of -1.
• Cyanide (CN-) has a charge of -1.
• Hydroxide (OH-) has a charge of -1.
• Oxide (O2-) has a charge of -2.
• Water (H2O) has a charge of 0.
• Ammonia (NH3) has a charge of 0.
• Carbon monoxide (CO) has a charge of 0.
• Ethylenediamine (en) has a charge of 0.
• Oxalate (C2O42-) has a charge of -2.
• EDTA has a charge of -4.
• The oxidation state helps predict the electronic configuration of the metal center, which in turn affects the magnetic properties and color of the complex.
• High oxidation states stabilize anionic ligands, while low oxidation states stabilize π-acceptor ligands.
• Redox reactions can change the oxidation state of the metal center while keeping the ligands intact.
• These reactions are essential in many catalytic processes.

Complex Ion Structure

• Coordination complexes have a central metal atom or ion bonded to a group of ligands.
• The arrangement of these ligands in three-dimensional space defines the structure of the complex.
• Coordination number refers to the number of ligands directly attached to the central metal ion.
• Common coordination numbers are 2, 4, and 6, but others are also possible.
• Linear complexes have a coordination number of 2.
• The metal is bonded to two ligands, with a bond angle of 180°.
• Example: [Ag(NH3)2]+
• Tetrahedral complexes have a coordination number of 4.
• The metal is at the center of a tetrahedron, and the four ligands are at the vertices.
• The bond angles are approximately 109.5°.
• Example: [NiCl4]2-
• Square planar complexes also have a coordination number of 4.
• The metal and the four ligands lie in the same plane, with bond angles of 90°.
• Common for metal ions with a d8 electronic configuration, such as Pt(II) and Pd(II).
• Example: [PtCl4]2-
• Octahedral complexes have a coordination number of 6.
• The metal is at the center of an octahedron, and the six ligands are at the vertices.
• The bond angles are 90°.
• Example: [Co(NH3)6]3+
• Trigonal bipyramidal complexes have a coordination number of 5.
• The metal is at the center, with three ligands in a trigonal plane and two ligands axial positions.
• Example: [Fe(CO)5]
• Square pyramidal complexes also have a coordination number of 5.
• The metal is at the center, with four ligands forming a square base and one ligand at the apex.
• Isomers are compounds with the same chemical formula but different arrangements of atoms.
• Structural isomers have different connectivity between the metal and ligands.
• Ionization isomers: Differ in which ions are inside the coordination sphere versus outside.
• Hydrate isomers: Differ in whether water molecules are inside the coordination sphere or act as crystal water.
• Coordination isomers: Occur when both cation and anion are complex ions, and the ligands are distributed differently between the two metal centers.
• Linkage isomers: Occur when an ambidentate ligand binds to the metal through different donor atoms.
• Stereoisomers have the same connectivity but different spatial arrangements of ligands.
• Geometric isomers are stereoisomers that are not mirror images of each other.
• Cis-trans isomers: Occur in square planar and octahedral complexes, indicating ligands on the same side (cis) or opposite sides (trans) of the metal center.
• Fac-mer isomers: Occur in octahedral complexes with three identical ligands. Fac (facial) means the three ligands are on one face of the octahedron, while mer (meridional) means they are around the meridian.
• Optical isomers (enantiomers) are stereoisomers that are non-superimposable mirror images of each other.
• Chiral complexes: Lack an internal plane of symmetry and exist as a pair of enantiomers.
• Jahn-Teller distortion occurs in complexes where certain electronic configurations lead to unequal occupation of d-orbitals, resulting in a distortion of the complex geometry.
• Common in octahedral copper(II) complexes, leading to elongation or compression along one axis.
• Crystal field theory and ligand field theory explain how the electronic structure of the metal ion is affected by the ligands, influencing the geometry and properties of the complex.