(1)

Ethylene diamine tetraacetate ion $\left({\mathrm{EDTA}}^{4-}\right)$ is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion.

(1)

The oxalate $\left({\mathrm{C}}_2{\mathrm{O}}_4^{2-}\right)$ ligand is said to be a didentate ligand as it can bind through two donor atoms.
 

(1)

Complexes in which a metal is bound to only one kind of donor groups e.g., ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}$ are known as homoleptic. Complexes in which a metal is bound to more than one kind of donor groups, e.g., ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]}^{+}$ are known as heteroleptic.

(1)

Chelating ligands (e.g., en) form more stable complexes than unidentate ligands.

(2)

Ethylenediaminetetraacetate ion $\left({\mathrm{EDTA}}^{4-}\right)$ is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion.

(2)

The intensity of the trans-effect (as measured by the increase in rate of substitution of the trans ligand) follows the sequence: 
${\mathrm{CN}}^{-}>{\mathrm{C}}_6{\mathrm{H}}_5^{-}>{\mathrm{Br}}^{-}>{\mathrm{NH}}_3$

(4)

$\left[M{\left(en\right)}_2\right(C_2{O}_4\left)\right]Cl$ : 

Oxidation number of metal = +3

Coordination number of metal = 6

$\therefore$ Sum of oxidation number and coordination number = 3 + 6 = 9