Nomenclature is important in Coordination Chemistry because of the need to have an unambiguous method of describing formulas and writing systematic names, particularly when dealing with isomers. The formulas and names adopted for coordination entities are based on the recommendations of the International Union of Pure and Applied Chemistry (IUPAC).
Formulas of Mononuclear Coordination Entities:
The formula of a compound is a shorthand tool used to provide basic information about the constitution of the compound in a concise and convenient manner. Mononuclear coordination entities contain a single central metal atom.
The following rules are applied while writing the formulas:
- Central atom is listed first.
- Ligands are then listed in alphabetical order. The placement of a ligand in the list does not depend on its charge.
- Polydentate ligands are also listed alphabetically. In case of abbreviated ligand, the first letter of the abbreviation is used to determine the position of the ligand in the alphabetical order.
- The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligand abbreviations are also enclosed in parentheses.
- There should be no space between the ligands and the metal within a coordination sphere.
- When the formula of a charged coordination entity is to be written without that of the counterion, the charge is indicated outside the square brackets as a right superscript with the number before the sign. For example, [Co(CN)6]3-, [Cr(H2O)6]3+, etc.
- The charge of the cation(s) is balanced by the charge of the anion(s).
The naming of Mononuclear Coordination Compounds
Step to name a complex compound is:
- Cations are always named before the anions.
- The ligands are then listed in alphabetical order.
- In case of polydentate ligands, ligands are named alphabetically using a prefix di, tri, tetra, penta etc, to indicate the number of ligands of that type present.
‘ide’ are changed to ‘o’
‘ite’ are changed to ‘ito’
‘ate’ are changed to ‘ato’
- Then the name the metal atom is written followed by its oxidation state in Roman numerals.
- Finally, the anion is named.
For example: [Cu(NH3)4]SO4 Tetra ammine copper (II) sulphate
If the ligands itself include di, tri etc. then we use bis – (for two), tris (for three) as prefix. For example:
If the complex ion is an anion, the name of the metal ends with the suffix – ate.
For example: [Cr(C2O4)3]3‒ Trioxalatochromate (III) ion.
- If anion is a complex, then metal ends with ‘ate’
[Ni(CN)4]2- : tetracyanonickelate (II) ion
- When writing (not naming) the formula of the complex:
- Complex-ion should be enclosed by square brackets.
- Ligands are placed after metal in the alphabetical order but first negative ligands, then neutral, then positive.
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