Coordination Compounds Test - 3

Ambidentate ligands are those ligands which can ligate through two different atoms. Nitrito is an ambidentate ligand as it can coordinate to the central metal atom/ion through O in nitrito-O i.e. ONO^- and through N in nitrito-N i.e. NO₂^-.

When a di- or polydentate ligand uses its two or more donor atoms to bind to a single metal atom/ion, it is said to be a chelate ligand. Ethane-1,2-diamine (NH₂CH₂CH₂NH₂) can act as a chelate ligand as it is a didentate ligand. 

A ligand which can bind through two donor atoms is called a didentate ligand like en or ethane-1,2-diamine (NH₂CH₂CH₂NH₂) which has two donor N atoms.

Ligands are the ions/molecules bound to the central metal atom/ion in a coordination entity. These may be simple ions like Cl^-, small molecules like NH₃, large molecules like ethane-1.2-diamine or even macromolecules like proteins. A ligand must be able to donate a pair of electrons to the central metal atom/ion and thus they act as lewis bases.

Coordination number of the central metal atom/ion is equal to number of neutral molecules or negative ions bound to the central metal atom/ion by non-ionisable secondary linkages. These neutral molecules or negative ions are the ones which are shown inside the square bracket. Here there are 3 NH ₃ molecules and 3 H ₂ O molecules associated with Cr inside the bracket. So the coordination number of Cr is 6.

The ligand CN^{​​​​​-} has charge of –1. So the overall charge carried by 6 CN^{​​​​​-} ligands is -6. Each potassium ion K⁺ carries a charge of +1. So 4 potassium ions carry an overall charge of +4. This implies that the overall charge on the coordination sphere is -4 to balance the +4 charge of the potassium ions. Let the oxidation number of Fe be x. Then
x + (-6) = -4
x = -4 - (-6)
x = -4 + 6
x = +2
So, the oxidation number of Fe is +2.

In the given molecule, there are 3 Cl present, and (1/3)x3=1 Cl is precipitated per molecule. So, 1 Cl satisfies the primary valence and is ionisable. So 1 Cl will be present outside the coordination sphere. So the correct structure is [Cr(H₂O)₄Cl₂]Cl.2H₂O

Coordination number of the central metal atom/ion is equal to number of ligand donor atoms directly bonded to the central metal atom/ion. So it can be determined by the number of sigma bonds formed by the ligands with the central metal atom/ion.

White coloured BaSO_​​​4 is precipitated on reaction of the given complex with Barium chloride since SO₄^2- ion is present outside the coordination sphere and is available for reaction with Ba​​​^{​​​​​2+} ion.

In the nomenclature of coordination compounds, the ligands are named in alphabetical order followed by the central metal atom/ion with its oxidation number in roman numeral in paranthesis. The cation is named first and then the anion. Here the complex is a cation. And we can see that the ligands present are Cl ^- (chlorido), en (ethane-1,2-diamine), and NO ₂ ^- (nitrito-N). The central metal is Cobalt. The anion present is chloride and carries a -1 charge. So the overall charge on the complex cation is +1. Nitrito-N carries -1 charge and chlorido carries -1 charge. en is a neutral ligand. Let's assume the oxidation number for Co to be x. Then
x + (-1) +(-1) = +1
x - 2 = +1
x = +1 + 2 = +3
So oxidation state of Co is +3. Hence the IUPAC name for the given complex is Chloridobis(ethane-1,2-diamine)nitrito-N-cobalt(III) chloride.