Coordination Compounds

Nomenclature of Coordination Compounds

Just like we know that nomenclature of organic compounds is important, naming the various coordination or complex compounds is also very important. Complex nomenclature is an extensive study and as chemistry students, you must be well-versed with the same. In this chapter, we will see how the various complex compounds get their names. We will look at the few simple rules to follow for the same.

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Complex Nomenclature

Why do we need to name the compounds? How would your teacher call you and your best friend if you both didn’t have any names? Similarly, the naming of coordination compounds(complex nomenclature) is important to provide an unambiguous method to represent and describe formulas and names of coordination compounds systematically.

Also, it becomes very important while you deal with isomers. We follow a few rules of the International Union of Pure and Applied Chemistry (IUPAC) system while naming these compounds. In a nutshell, the rules are as follows:

In Coordination Compounds,

  • Within the coordination entities, we list down the central atom/ion first followed by the ligands.
  • After this, we list down the ligands in alphabetical order.
  • We write the formula of coordination entity in square brackets.
  • Ligand abbreviations are to be enclosed in parentheses.
  • We must not keep any space between the ligands and the metal within a coordination sphere.
  • The charge on the cation(s) should be equal to the charge of the anion(s).

Now, let us look at these rules of complex nomenclature in greater details, along with suitable examples.

Complex nomenclature

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Bonding in Metal Carbonyls

Rules of Complex Nomenclature

Rule 1

While naming a coordination compound, we always name the cation before the anion. This rule does not count the fact of whether the complexion is cation or anion. Let us understand this with the help of an example.

Na[Co(NH)4 (Cl)2]   →  Na is to be named first followed by [Co(NH)4 (Cl)2]

Sodium tetramminedichlorocoblatate(I)

[Co(NH)4 (Cl)2]SO4   →  [Co(NH)4 (Cl)2] is to be named first followed by SO4

Tetraamminedichlorocobalt(0) sulphate.

Rule 2

When we see that there are multiple types of ligands present in any coordination compound, we name the ligands in alphabetic order after by the name of central metal atom/ion.

  • Name of the anionic ligands ends with ‘o’.
    For example, Chloride    →   Chlorido,    Nitrate   →   Nitrito
  • For neutral ligands, their common name is used as such e.g.

H2NCH2CH2NH2   →  ethylenediamine
H2O  → aqua
NH3  → ammine
CO   → carbonyl
N2  → dinitrogen
O2   → dioxygen

Rule 3

If the names of the ligands have a numerical prefix, then we use the terms like bis, tris, tetrakis. This will represent the ligand to which they refer being placed in parentheses. For example, we name [NiCl2(PPh3)2] as dichloridobis(triphenylphosphine)nickel(II)

Rule 4

After naming the ligand in alphabetic order, we name the central metal atom/ion.

  • If the complex ion is a cation, we name the metal same as the element.
  • If the complex is an anion, the name of metal ends with the suffix -ate for Latin name.

Rule 5

We give the oxidation state of the metal in the complex as a roman numeral in parentheses.

Rule 6

We name the neutral complex molecule similar to that of the complex cation.

Rule 7

There are some ligands like NO2, CN that are attached to the central metal atom/ion through different atoms. We name them as follows:

Thus, M-NO2 → nitro
M-ONO           →  nitrito
M-SCN            →  thiocyanato
M-NCS            →  isothiocyanato

Solved Example for You

Q: Write the IUPAC name of [Fe(NH3)4O2C2O4]Cl.

Ans: In this complex part has a charge of +1. The ligand oxalato has a charge of –2, so iron should be in  +3 state meaning O2 to be neutral. O2 behaves as a neutral ligand and the IUPAC name is Tetraammineoxalatodioxygeniron (III) chloride.

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