Question

Total number of electrons donated by ligands to platinum ion in $\left[ {Pt{{\left( {en} \right)}_2}C{l_2}} \right]$ :
A.8
B.10
C.12
D.14

Answer 1

We need the concept of ligand in a coordination compound. The basic formula for writing a coordination compound is [M(Ligand)x]+ ion-. Ligands are Lewis bases since they can donate electrons to the central metal atom. The number of ligands that attach to a metal depends on whether the ligand is monodentate, bidentate or polydentate. In the given coordination compound, the ligands are $en$ which is basically ethane-1,2-diamine and chlorine.

Complete step by step answer:
In order to calculate the number of electrons donated by the ligands, we need to briefly study the types of ligands which influence its electron donation ability. There are three types of ligands- monodentate, bidentate and polydentate ligands.
-Monodentate ligands: These ligands can donate a single pair of electrons to the metal atom in a coordination compound. $Cl$ is an example of a monodentate ligand.
-Bidentate ligands: These ligands can donate two pairs of electrons to the metal atom in a coordination compound. $en$ is an example of bidentate ligands.
-Polydentate ligands: Ligands which can donate more than two pairs of electrons to the central metal atom are known as polydentate ligands. EDTA is an example of polydentate ligand.
-Now we know that the two ligands in the given compound $\left[ {Pt{{\left( {en} \right)}_2}C{l_2}} \right]$ are $Cl$ and $en$ which are monodentate and bidentate ligands respectively. Hence, $Cl$ donates a single pair of electrons to the platinum ion and $en$donates two pairs of electrons.
-In the compound $\left[ {Pt{{\left( {en} \right)}_2}C{l_2}} \right]$ : There are 2 $en$ and 2 $Cl$ molecules. Therefore each $en$ contributes 4 electrons and 2 $en$ contributes a total of 8 electrons. In addition, each $Cl$ donates 2 electrons and 2 $Cl$ donates a total of 4 electrons .Therefore ,the total number of electrons donated by both the ligands are 8+4=12 electrons.
Therefore, the correct option is option (C).

Note:
We must be noted that ligands are actually Lewis bases which can donate electrons. Also, bidentate ligands and polydentate ligands are also referred to as chelating ligands which bring about chelating effects .This effect explains the ability of chelating ligands towards the central metal atom or ion compared to the affinity of non chelating ligands (monodentate ligands) for the same metal atom or ion.