Question

Give the IUPAC name of ${\mathrm{K}}_3\left[\mathrm{Cr}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]$

Answer 1

Coordination compounds:

• Coordination compounds can be defined as a compound that results from the combination of two or more stable chemical species and retains their identity in the solid as well as the dissolved state.
• A water-soluble coordination compound almost invariably contains a complex ion.
• A complex ion may be defined as an electrically charged ion that consists of a central metal atom or ion surrounded by a group of ions or neutral molecules.
• The cation to which one or more neutral molecules or ions are coordinated is called the central ion.
• The molecules or ions so attached are called ligands.
• The atom in the ligand which can donate the electron pair is called the donor atom and coordinating atom.
• The total number of ligands attached to a central ion is called the coordination number of that ion.
• The central ion together with molecules or ions coordinated to it constitutes what is termed a coordination sphere.

IUPAC naming rule for Coordination compounds:

• In ionic coordination complexes, the cation is named first, and then the anion and in non-ionic coordination complexes, that is neutral complexes
• In naming the coordination sphere, the ligands are named first and then the central metal ion.
• The names of negative ligands end in ‘o’ and of positive ligands end in ‘ium’.
• The ligands are named in alphabetical order irrespective of their being neutral, negatively, or positively charged.
• When more than one simple ligands of a particular kind are present in the complex, the prefixes di, tri, tetra, penta, hexa ..etc, are used to indicate their number but if the ligands have complex names such as Ethylenediamine, which themselves contains the affixes di, tri.. etc are indicated by prefixes such as bis, tris, tetrakis, etc.
• When the complex is anionic, the name of the central metal atom ends in ‘ate’ but in the cationic and neutral complexes, the name of the metal is given without any characteristic ending.
• The oxidation state of the central ion is designated by a Roman numeral in parentheses at the end of the name of the complex without a space between the two.

IUPAC name of ${\mathrm{K}}_3\left[\mathrm{Cr}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]$

• In ${\mathrm{K}}_3\left[\mathrm{Cr}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]$, the $\mathrm{Cr}$ is the central metal atom.
• The central metal atom is bonded to three Oxalate ions${\mathrm{C}}_2{\mathrm{O}}_4$ which is a bi-dentate ligand.
• The coordination number will be 6 (3 Potassium ions and 3 Oxalate ions).
• The oxidation number of the central metal atoms should be calculated by subtracting the overall charge of the ligands from the overall charge of the coordinate compound.
• The charge of one Oxalate ion is$-2$ and the charge of one Potassium ion is$+1$.
• Here the coordinate compound is $[\mathrm{Cr}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3{]}^{3-}$(since the charge of 3 Potassium ions is $+3$, the coordinate compound will get $-3$)
• Let $x$ be the oxidation number of the central metal atom, then

$$\begin{gathered} x+\left(3\times -2\right)=-3 \\ x+(-6)=-3 \\ x=-3+6 \\ x=3 \end{gathered}$$

• Therefore, the name of ${\mathrm{K}}_3\left[\mathrm{Cr}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]$ is Potassium trioxalato chromate (III).