Nomenclature is vital in chemistry to know the systematically named of the compounds so they may be readily identified and studied. To maintain consistency around the globe, the International Union of Pure and Applied Chemistry (IUPAC) established guidelines for the nomenclature of coordination compounds. Coordinate bonds are used to join a central metal atom or ion to a group of ligands, which might be atoms, ions, or molecules.
Basic Concepts in Coordination Compounds
The following terms must be understood in order to understand the nomenclature of coordination compounds:
1.Central Metal Atom or Ion:
It is the ion or atom that ligands are joined to. The d-block or f-block elements are often where it belongs. Cu²⁺, for example, is the primary ion in [Cu(NH₃)₄]²⁺.
2. Ligands:
Ions or molecules known as ligands provide the center metal ion two electrons to create a coordinate bond. Ligands may be:
Neutral Ligands:g., NH₃ (ammine), H₂O (aqua), CO (carbonyl)
Anionic Ligands:g., Cl⁻ (chlorido), CN⁻ (cyanido), OH⁻ (hydroxo)
Cationic Ligands: Rare but possible, such as NO⁺ (nitrosonium)
3. Coordination Sphere:
The coordination sphere, denoted by square brackets like [Fe(CN)₆]³⁻, is made up of the core metal atom or ion and the ligands that are connected to it.
4. Coordination Number:
It’s the quantity of donor ligand atoms that are directly bound to the core atom. For example, the coordination number in [Co(NH₃)₆]³⁺ is 6.
5. Oxidation State of Metal:
Metal’s name, Roman numerals in parenthesis indicate the metal ion’s oxidation state (charge). For example, iron has an oxidation state of +3, denoted as Iron(III), in [Fe(CN)₆]³⁻.
Rules for Naming Coordination Compounds
Coordination compounds are named according to certain IUPAC guidelines.:
1.Name the Cation and Anion Separately
As is the case with other ionic compounds, the cation is called first, then the anion.
For example, in K₄[Fe(CN)₆], the coordination entity [Fe(CN)₆]⁴⁻ is designated after the potassium ion (K⁺).
2.Naming the Coordination Sphere
Regardless of their charge, the ligands are listed alphabetically, with their names appearing before the central metal ion.
‘-o’ typically appears at the end of the names of anionic ligands, such as cyano for CN⁻ and chloro for Cl⁻.
• Common nomenclature for neutral ligands are used, such as H₂O for aqua, NH₃ for ammine, and CO for carbonyl.
Example:
[Cr(H₂O)₆]³⁺ – Hexaaquachromium(III) ion
[CoCl₄]²⁻ – Tetrachloridocobaltate(II)
3. Use Prefixes to Indicate the Number of Ligands
The prefixes used to denote the number of ligands are:
Mono- (1), often omitted
Di- (2)
Tri- (3)
Tetra- (4)
Penta- (5)
Hexa- (6)
For complex ligands that already contain numerical prefixes, special prefixes are used:
Bis- (2 ligands)
Tris- (3 ligands)
Tetrakis- (4 ligands)
Example:
[Ni(CO)₄] – Tetracarbonylnickel(0)
[PtCl₂(en)₂]²⁺– Dichloridobis(ethylenediamine)platinum(II)
4. Naming the Central Metal Ion
The metal’s name is followed in parenthesis by its oxidation state in Roman numerals.
The metal name is suffixed with “-ate” if the complex ion is an anion.
Some metals in anionic complexes have Latin names.
Ferrate for Fe
Cuprate for Cu
Argentate for Ag
Aurate for Au
Plumbate for Pb
Example:
[Fe(CN)₆]⁴⁻ – Hexacyanidoferrate(II)
[Ag(NH₃)₂]⁺ – Diamminesilver(I)
5. Naming Bridging Ligands (Polynuclear Complexes)
The name of the bridging ligand is preceded by the prefix “μ” (mu) when two metal ions are connected by it.
Example:
[Co₂(μ-OH)₂(en)₄]Cl₂ – μ-hydroxo-bis(ethylenediamine)cobalt(III) chloride
6. Oxidation State of the Central Metal
The charge on the ligands and the total charge of the complex are used to infer the oxidation state of the central metal ion, which is displayed in Roman numerals right below the metal’s name.
Example:
[Mn(CN)₆]³⁻ – Hexacyanomanganate(III)
[Pt(NH₃)₂Cl₂] – Diamminedichloridoplatinum(II)
7. Isomerism in Coordination Compounds
Isomers in coordination compounds are named according to their type:
Geometrical Isomers: cis- and trans-
Optical Isomers: Prefixes like (+) and (–) or Λ and Δ.
Example:
cis-[PtCl₂(NH₃)₂] – cis-Diamminedichloridoplatinum(II)
trans-[PtCl₂(NH₃)₂] – trans-Diamminedichloridoplatinum(II)
Examples and Their Nomenclature
[Cr(NH₃)₆]Cl₃ – Hexaamminechromium(III) chloride
K₄[Fe(CN)₆] – Potassium hexacyanidoferrate(II)
[Co(en)₃]Cl₃ – Tris(ethylenediamine)cobalt(III) chloride
[PtCl₄]²⁻ – Tetrachloridoplatinate(II) ion
[Ni(CO)₄] – Tetracarbonylnickel(0)
Summary
To understand and convey the structure and composition of coordination compounds, we must know their nomenclature. Chemists can appropriately name and identify coordination compounds by following IUPAC criteria. Gaining a knowledge of these guidelines is essential for understanding more complex coordination chemistry.
A coordination compound’s name is written as follows:
The cation’s name appears first, then the anion’s name (if any).
The names of the core metal ions are presented after the ligands in alphabetical order.
Roman numerals in parenthesis represent the metal’s oxidation state.
The suffix “-ate” is appended to the metal’s name if the complex is an anion.
Prior to the central metal ion, ligands are named alphabetically.
• Anionic ligands terminate in “-o” (for example, CN⁻ becomes cyanido and Cl⁻ becomes chlorido).
Common names for neutral ligands include NH₃ for ammine, H₂O for aqua, and CO for carbonyl.
• The complete names of polydentate ligands, such ethylenediamine, are used in their names.
The following prefixes are used to indicate the number of ligands:
Mono- (1, often omitted)
Di- (2)
Tri- (3)
Tetra- (4)
Penta- (5)
Hexa- (6)
For complex ligands with numerical prefixes, special prefixes such as bis-, tris-, tetrakis- are used.
The charges of the ligands and the total charge of the complex are taken into account when determining the oxidation state of the central metal ion.
• The metal’s name is followed by Roman numerals enclosed in parenthesis. For example, [Fe(CN)₆]³⁻ is known as hexacyanidoferrate(III) since the oxidation state of Fe is +3.
The suffix ‘-ate’ is added to the name of the metal if the complex is anionic.
In such cases, Latin names are used for some metals:
Ferrate for Fe
Cuprate for Cu
Argentate for Ag
Aurate for Au
Plumbate for Pb
Example:
[CoCl₄]²⁻ is named tetrachloridocobaltate(II).
Geometrical Isomers: Use cis- and trans- to indicate the relative positions of ligands.
cis-[PtCl₂(NH₃)₂] – cis-Diamminedichloridoplatinum(II)
trans-[PtCl₂(NH₃)₂] – trans-Diamminedichloridoplatinum(II)
Optical Isomers: Use (+)- or (−)- or symbols Λ and Δ to indicate the chirality.
In polynuclear coordination compounds, a bridge ligand that joins two or more metal atoms or ions is denoted by the prefix “μ” (mu). It comes before the bridging ligand’s name.
Example:
[Co₂(μ-OH)₂(en)₄]Cl₂ – μ-hydroxo-bis(ethylenediamine)cobalt(III) chloride.