Basic Terms Questions in English

(B) According to Werner's coordination theory,the chloride ions that are ionizable are those present outside the coordination sphere (primary valency).
Since the addition of $AgNO_3$ yields $AgCl$ precipitates corresponding to two chloride ions,it implies that two $Cl^-$ ions are outside the coordination sphere.
The formula of the complex is $[Co(NH_3)_5Cl]Cl_2$.
In this complex,the $Cl^-$ ion inside the square bracket satisfies both primary and secondary valency,while the two $Cl^-$ ions outside the bracket satisfy only primary valency.
Therefore,one chlorine atom satisfies both primary and secondary valency.

(C) $1$. The complex contains $1$ $Co$ atom,$5$ $NH_3$ molecules,$1$ $NO_2$ group,and $2$ $Cl$ atoms.
$2$. The formation of $2$ moles of $AgCl$ precipitate with excess $AgNO_3$ indicates that there are $2$ ionizable $Cl^-$ ions outside the coordination sphere.
$3$. The total number of ions produced is $3$ ($1$ complex cation + $2$ $Cl^-$ anions),which matches the given information.
$4$. Therefore,the coordination sphere must be $[Co(NH_3)_5(NO_2)]$ and the counter ions are $2$ $Cl^-$.
$5$. The formula is $[Co(NH_3)_5(NO_2)]Cl_2$.

(B) The chemical formula for tetraamminedichloroplatinum $(IV)$ chloride is $[Pt(NH_3)_4Cl_2]Cl_2$.
In an aqueous solution,this coordination compound dissociates as follows:
$[Pt(NH_3)_4Cl_2]Cl_2 (aq) \rightarrow [Pt(NH_3)_4Cl_2]^{2+} (aq) + 2Cl^- (aq)$.
From the dissociation,it is clear that $1$ mole of the complex produces $2$ moles of chloride ions $(Cl^-)$.
Therefore,the number of chloride ions produced is $2$.

(D) To determine the formula,we balance the charges of the ions involved.
Iron $(III)$ is $Fe^{3+}$.
Hexacyanoferrate $(II)$ is $[Fe(CN)_6]^{4-}$.
To make the compound electrically neutral,we use the criss-cross method:
$4 \times (Fe^{3+}) + 3 \times ([Fe(CN)_6]^{4-}) = 0$.
Thus,the formula is $Fe_4[Fe(CN)_6]_3$.

(C) The chemical formula of potash alum is $K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O$,which can be written as $K_2Al_2(SO_4)_4 \cdot 24H_2O$.
In this formula,the cations are $K^+$ and $Al^{3+}$,totaling $2 + 2 = 4$ cations.
The anions are $SO_4^{2-}$,totaling $4$ anions.
The number of water molecules is $24$.
Thus,the ratio of cations : anions : water molecules is $4 : 4 : 24$.
Simplifying this ratio by dividing by $4$,we get $1 : 1 : 6$.

(C) The cyanide ion,$CN^-$,contains a triple bond between carbon and nitrogen,which consists of one $\sigma$ bond and two $\pi$ bonds.
In the complex $[Ag(CN)_2]^-$,there are two $CN^-$ ligands.
Therefore,the total number of $\pi$ bonds $= 2 \times 2 = 4$.

(A) The formation of two moles of $AgCl$ precipitate indicates that there are two ionizable chloride ions outside the coordination sphere.
Therefore,the complex can be represented as $[Co(NH_3)_5Cl]Cl_2$.
In this structure,the two chloride ions outside the square brackets satisfy the primary valency,while the one chloride ion inside the coordination sphere satisfies both the primary and secondary valency.
The reaction is: $[Co(NH_3)_5Cl]Cl_2 + 2AgNO_3 \to [Co(NH_3)_5Cl](NO_3)_2 + 2AgCl$.

(D) In the given complex,there are two $en$ ligands and one $C_2O_4$ ligand. Both are bidentate ligands.
Coordination number = $(2 \times 2) + (1 \times 2) = 4 + 2 = 6$.
Let the oxidation state of $E$ be $x$.
The complex is $[E(en)_2(C_2O_4)]NO_2$. Since $NO_2$ is a counter ion with a charge of $-1$,the complex cation $[E(en)_2(C_2O_4)]^+$ has a charge of $+1$.
$x + 2(0) + 1(-2) = +1$
$x - 2 = +1$
$x = +3$.
Therefore,the coordination number is $6$ and the oxidation state is $+3$. Thus,option $(d)$ is correct.

(B) The complex is $[Co(NH_3)_5CO_3]ClO_4$.
$NH_3$ is a monodentate ligand ($5$ ligands) and $CO_3^{2-}$ is a bidentate ligand ($1$ ligand).
Coordination number $(C.N.)$ $= 5 \times 1 + 1 \times 2 = 7$.
Let the oxidation state of $Co$ be $x$.
$x + 5(0) + 1(-2) + 1(-1) = 0 \implies x - 3 = 0 \implies x = +3$.
Electronic configuration of $Co$ $(Z=27)$ is $[Ar] 3d^7 4s^2$.
Electronic configuration of $Co^{3+}$ is $[Ar] 3d^6 4s^0$.
Number of $d-$ electrons is $6$.
Since $NH_3$ and $CO_3^{2-}$ are ligands,in the presence of strong field ligands,the $6$ electrons in the $3d$ orbital are paired.
Number of unpaired $d-$ electrons $= 0$.

(B) In the qualitative analysis of nitrate,a brown ring is formed due to the formation of $[Fe(H_2O)_5(NO)]^{2+}$. The chemical reaction is: $FeSO_4 + NO + 5H_2O \to [Fe(H_2O)_5(NO)]SO_4$.
In this complex,the central metal ion is $Fe^{2+}$. The ligands are five $H_2O$ molecules and one $NO$ molecule.
The coordination number is the total number of sigma bonds formed by the ligand atoms with the central metal ion,which is $5 + 1 = 6$.
Both the Assertion and the Reason are correct,but the Reason does not explain why the complex is brown-coloured (which is due to charge transfer). Therefore,the Reason is not the correct explanation of the Assertion.

(C) The Assertion is correct because a chelating ligand requires donor atoms positioned to form stable,strain-free rings (typically $5$ or $6$ membered) with the metal ion.
The Reason is incorrect because hydrazine $(H_2N-NH_2)$ acts as a monodentate ligand. Coordination by hydrazine would result in a $3$-membered ring,which is highly unstable due to significant angle strain,thus it does not act as a chelating ligand.

(A) The $EDTA^{4-}$ ion is a hexadentate ligand,meaning it has $6$ donor atoms (two nitrogen atoms and four oxygen atoms).
These $6$ donor atoms coordinate simultaneously to the central metal ion,forming a stable octahedral complex.
Therefore,the Reason correctly explains why the Assertion is true.

(B) In the complex ion $[AlCl(H_{2}O)_{5}]^{2+}$,let the oxidation state of $Al$ be $x$.
$x + (-1) + 5(0) = +2$
$x - 1 = +2$
$x = +3$.
Thus,the oxidation state of $Al$ is $+3$.
The covalency is the total number of coordinate bonds formed by the central metal atom with the ligands.
Here,$Al$ is bonded to $1$ $Cl^-$ ion and $5$ $H_2O$ molecules,so the total number of coordinate bonds is $1 + 5 = 6$.
Therefore,the oxidation state $(+3)$ and covalency $(6)$ are not the same.

(A) The secondary valence corresponds to the coordination number of the central metal atom.
$(i) PdCl_2 \cdot 4NH_3$ gives $2$ moles of $AgCl$,meaning $2$ $Cl^-$ ions are outside the coordination sphere. The formula is $[Pd(NH_3)_4]Cl_2$. Secondary valence = $4$.
$(ii) NiCl_2 \cdot 6H_2O$ gives $2$ moles of $AgCl$,meaning $2$ $Cl^-$ ions are outside. The formula is $[Ni(H_2O)_6]Cl_2$. Secondary valence = $6$.
$(iii) PtCl_4 \cdot 2HCl$ gives $0$ moles of $AgCl$,meaning all $Cl^-$ ions are inside. The formula is $H_2[PtCl_6]$. Secondary valence = $6$.
$(iv) CoCl_3 \cdot 4NH_3$ gives $1$ mole of $AgCl$,meaning $1$ $Cl^-$ ion is outside. The formula is $[Co(NH_3)_4Cl_2]Cl$. Secondary valence = $6$.
$(v) PtCl_2 \cdot 2NH_3$ gives $0$ moles of $AgCl$,meaning all $Cl^-$ ions are inside. The formula is $[Pt(NH_3)_2Cl_2]$. Secondary valence = $4$.

(N/A) Werner's postulates explain the bonding in coordination compounds as follows:
$(i)$ $A$ metal exhibits two types of valencies,namely,primary and secondary valencies. Primary valencies are satisfied by negative ions,while secondary valencies are satisfied by both negative and neutral ions.
(In modern terminology,the primary valency corresponds to the oxidation number of the metal ion,whereas the secondary valency refers to the coordination number of the metal ion.)
$(ii)$ $A$ metal ion has a definite number of secondary valencies around the central atom. These valencies project in a specific direction in space,which determines the geometry of the coordination compound.
$(iii)$ Primary valencies are usually ionizable,while secondary valencies are non-ionizable.

(N/A) $(i)$ Coordination entity:
$A$ coordination entity is an electrically charged radical or species. In a coordination entity,the central atom or ion is surrounded by a suitable number of neutral molecules or negative ions (called ligands). For example:
Cationic complex: $[Ni(NH_3)_6]^{2+}, [Fe(H_2O)_6]^{3+}$
Anionic complex: $[PtCl_4]^{2-}, [Ag(CN)_2]^{-}$
$(ii)$ Ligands:
The neutral molecules or negatively charged ions that surround the metal atom in a coordination entity are known as ligands. For example,$NH_3$ and $Cl^-$.
$(iii)$ Coordination number:
The total number of ligand donor atoms to which the central metal atom is directly bonded is called the coordination number. For example:
$(a)$ In $[PtCl_6]^{2-}$,the coordination number of $Pt$ is $6$.
$(b)$ In $[Ni(NH_3)_4]^{2+}$,the coordination number of $Ni$ is $4$.
$(iv)$ Coordination polyhedron:
It is the spatial arrangement of the ligand atoms which are directly attached to the central atom/ion. Examples include square planar and tetrahedral geometries.
$(v)$ Homoleptic complexes:
These are complexes in which the metal ion is bound to only one kind of donor group. For example: $[Co(NH_3)_6]^{3+}, [Ni(CO)_4]$.
$(vi)$ Heteroleptic complexes:
These are complexes where the central metal ion is bound to more than one type of donor group. For example: $[Co(NH_3)_4Cl_2]^{+}, [Co(NH_3)_5Cl]^{2+}$

(N/A) ligand may contain one or more unshared pairs of electrons which are called the donor sites of ligands. Depending on the number of these donor sites,ligands can be classified as follows:
$(a)$ Unidentate ligands: Ligands with only one donor site are called unidentate ligands. For example,$NH_3$ and $Cl^-$.
$(b)$ Didentate ligands: Ligands that have two donor sites are called didentate ligands. For example,$H_2N-CH_2-CH_2-NH_2$ (ethane$-1,2-$diamine) and $C_2O_4^{2-}$ (oxalate ion).
$(c)$ Ambidentate ligands: Ligands that can attach themselves to the central metal atom through two different atoms are called ambidentate ligands. For example:
$1. NO_2^-$ (nitro,donor atom is $N$) and $ONO^-$ (nitrito,donor atom is $O$)
$2. SCN^-$ (thiocyanate,donor atom is $S$) and $NCS^-$ (isothiocyanate,donor atom is $N$)

(C) $iii.$ The given complex can be written as $[Co(NH_3)_6]Cl_2$.
In an aqueous solution,it dissociates as follows:
$[Co(NH_3)_6]Cl_2 \rightarrow [Co(NH_3)_6]^{2+} + 2Cl^-$
Thus,one $[Co(NH_3)_6]^{2+}$ ion and two $Cl^-$ ions are produced,resulting in a total of $3$ ions.

(A) Complex compounds (or coordination compounds) are formed by the coordinate bonding between a central metal atom or ion and a surrounding array of molecules or anions,which are known as $Ligands$.
These $Ligands$ donate a pair of electrons to the central metal atom or ion to form coordinate covalent bonds.

(N/A) Coordination compounds are chemical compounds formed by the transition elements of the $d$-block of the periodic table.
When the $(n-1)d, ns$,and $np$ orbitals or $ns, np$,and $nd$ orbitals of a transition metal atom or ion are vacant,these elements accept electron pairs from anions or neutral molecules to form coordination compounds.
The bond formed between the metal atom/ion and the anions/neutral molecules in these compounds is known as a coordinate covalent bond.

(N/A) The main postulates of Werner's theory are:
$1$. In coordination compounds,metals exhibit two types of linkages or valencies: primary and secondary.
$2$. Primary valencies are normally ionisable and are satisfied by negative ions.
$3$. Secondary valencies are non-ionisable.
$4$. Secondary valencies are satisfied by neutral molecules or negative ions. The secondary valency is equal to the coordination number and is fixed for a metal.
$5$. The ions/groups bound by secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers. Such a spatial arrangement is called a coordination polyhedron.
$6$. Octahedral,tetrahedral,and square planar geometrical shapes are common in coordination compounds of transition metals.
For example:
Octahedral complexes: $[Co(NH_3)_6]^{3+}, [CoCl(NH_3)_5]^{2+}, [CoCl_2(NH_3)_4]^{+}$
Tetrahedral complex: $[Ni(CO)_4]$
Square planar complex: $[PtCl_4]^{2-}$
The species within the square bracket are coordination entities or complexes,and the ions outside the square bracket are called counter ions.

(N/A) Alfred Werner prepared and characterized a large number of coordination compounds and studied their physical and chemical behavior. He was the first to formulate ideas about the structure of coordination compounds.
$1$. Primary Valency: These are ionizable valencies,generally satisfied by negative ions. They correspond to the oxidation state of the metal ion.
$2$. Secondary Valency: These are non-ionizable valencies,satisfied by neutral molecules or negative ions. They correspond to the coordination number of the metal ion.
In a series of compounds of cobalt$(III)$ chloride with ammonia,it was found that some chloride ions could be precipitated as $AgCl$ on adding excess $AgNO_{3}$ solution,while others remained bonded to the metal.
$1$ mol $CoCl_{3} \cdot 6 NH_{3}$ (Yellow) gave $3$ mol $AgCl$
$1$ mol $CoCl_{3} \cdot 5 NH_{3}$ (Purple) gave $2$ mol $AgCl$
$1$ mol $CoCl_{3} \cdot 4 NH_{3}$ (Green) gave $1$ mol $AgCl$
$1$ mol $CoCl_{3} \cdot 4 NH_{3}$ (Violet) gave $1$ mol $AgCl$

$Formula$	$Solution \text{ } conductivity$
$[Co(NH_{3})_{6}]Cl_{3}$	$1:3$ electrolyte
$[CoCl(NH_{3})_{5}]Cl_{2}$	$1:2$ electrolyte
$[CoCl_{2}(NH_{3})_{4}]Cl$	$1:1$ electrolyte

In these compounds,the atoms within the square brackets are directly bonded to the metal ion and represent the secondary valency,which is $6$ for cobalt in these examples.

(N/A) Both double salts and coordination compounds are formed by the combination of two or more stable compounds in stoichiometric ratios. However,they differ in their behavior in solution.
Double salts,such as carnallite $(KCl \cdot MgCl_2 \cdot 6H_2O)$,Mohr's salt $(FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O)$,and potash alum $(KAl(SO_4)_2 \cdot 12H_2O)$,dissociate completely into simple ions when dissolved in water.
In contrast,coordination compounds like $K_4[Fe(CN)_6]$ contain complex ions. The complex ion $[Fe(CN)_6]^{4-}$ does not dissociate into $Fe^{2+}$ and $CN^-$ ions in solution.

(B) coordination compound is a chemical structure that consists of a central metal atom or ion bonded to a surrounding array of molecules or anions,which are known as ligands.
These ligands are attached to the central metal atom or ion by coordinate covalent bonds.
For example,in the complex $[Cu(NH_3)_4]^{2+}$,the $Cu^{2+}$ ion is the central metal ion,and the four $NH_3$ molecules act as ligands.

(A) In the coordination compound $K_4[Fe(CN)_6]$,the central metal atom is $Fe$.
The primary valency corresponds to the oxidation state of the central metal atom.
Let the oxidation state of $Fe$ be $x$.
$4(+1) + x + 6(-1) = 0$
$4 + x - 6 = 0$
$x = +2$.
Thus,the primary valency is $2$.
The secondary valency corresponds to the coordination number,which is the number of ligand donor atoms directly bonded to the central metal atom.
Here,$6$ $CN^-$ ligands are bonded to $Fe$,so the coordination number is $6$.
Thus,the secondary valency is $6$.

(C) The coordination number is defined as the number of ligand donor atoms to which the metal is directly bonded.
In the complex $[CoCl_2(NH_3)_4]^+$,the central metal ion is $Co^{3+}$.
There are $2$ $Cl^-$ ligands (monodentate) and $4$ $NH_3$ ligands (monodentate).
Total coordination number = $(2 \times 1) + (4 \times 1) = 6$.

(N/A) Coordination Entity: $A$ coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules.
Example: In $\left[ CoCl_3(NH_3)_3 \right]$,the cobalt ion is surrounded by three ammonia molecules and three chloride ions.
Central Atom/Ion: The atom or ion to which a fixed number of ions or groups are bound in a definite geometrical arrangement around it in a coordination entity is called the central atom or ion.
Example: In $\left[ CoCl_3(NH_3)_3 \right]$,the central metal ion is $Co^{3+}$,which acts as a Lewis acid.

(N/A) Ligands are atoms,molecules,or ions that donate an electron pair to the central metal ion to form a coordination bond. They can be classified based on different criteria:
$1$. On the basis of charge:
- Neutral ligands: $H_2O, NH_3, CO$
- Anionic ligands: $Cl^-, CN^-, OH^-$
- Cationic ligands: $NO^+, NH_2NH_3^+$
$2$. On the basis of denticity (number of donor atoms):
- Monodentate (Unidentate): Ligands with one donor atom (e.g.,$Cl^-, H_2O$)
- Polydentate: Ligands with more than one donor atom (e.g.,$EDTA^{4-}$ is hexadentate)
Special types include:
- Ambidentate ligands: Ligands that can coordinate through two different atoms (e.g.,$SCN^-$ can coordinate through $S$ or $N$)
- Bridging ligands: Ligands that link two metal centers.

(N/A) Coordination Number: The number of ligand donor atoms to which the metal is directly bonded is called the coordination number. For example,in the complex ions $[PtCl_6]^{2-}$ and $[Ni(NH_3)_4]^{2+}$,the coordination numbers of $Pt$ and $Ni$ are $6$ and $4$ respectively. In $[Fe(C_2O_4)_3]^{3-}$ and $[Co(en)_3]^{3+}$,the coordination number of $Fe$ and $Co$ is $6$.
Coordination Sphere: The central atom/ion and the ligands attached to it are enclosed in a square bracket and are collectively termed as the coordination sphere.
The groups that ionize are written outside the bracket and are called counter ions. For example,in the complex $K_4[Fe(CN)_6]$,$[Fe(CN)_6]^{4-}$ is the coordination sphere and $K^+$ is the counter ion.
Coordination Polyhedron: The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines the geometry of the coordination polyhedron.

(N/A) Homoleptic complexes: These are coordination complexes in which the metal atom or ion is bonded to only one type of donor group (ligand).
Example: $[Co(NH_3)_6]^{3+}$.
Heteroleptic complexes: These are coordination complexes in which the metal atom or ion is bonded to more than one type of donor group (ligand).
Example: $[Co(NH_3)_4Cl_2]^+$.

(N/A) Hexadentate ligands: These are ligands that possess $six$ donor atoms and are capable of forming $six$ coordinate bonds with a single metal ion simultaneously.
Example: Ethylenediamine tetraacetate $(EDTA^{4-})$ is a classic example of a hexadentate ligand. It contains $two$ nitrogen atoms and $four$ oxygen atoms as donor sites,as shown in the structure:
(Structure of $EDTA^{4-}$ with $six$ donor sites labeled $1$ to $6$)

(B) Ligands are classified based on the number of donor atoms they use to coordinate with the central metal atom or ion. This property is known as the $denticity$ of the ligand. Depending on the number of donor atoms,ligands can be classified as monodentate,bidentate,polydentate,etc.

(N/A) The following rules are applied for writing the formulas of coordination compounds:
$(i)$ The central atom is written first.
$(ii)$ The ligands are then written in alphabetical order.
$(iii)$ Polydentate ligands are also listed in alphabetical order. In the case of abbreviated ligands,the first letter of the abbreviation is used to determine the alphabetical order.
$(iv)$ The formula for the entire coordination entity,whether charged or not,is enclosed in square brackets. When ligands are polyatomic,their formulas are enclosed in parentheses. Abbreviations are also enclosed in parentheses.
$(v)$ There should be no space between the ligands and the metal within the coordination sphere.
$(vi)$ When the formula of a charged coordination entity is written,the charge is indicated outside the square brackets as a superscript on the right side,without the counter-ion charge.
Example: $[Co(CN)_6]^{3-}$.
$(vii)$ The charge of the cation is balanced by the charge of the anion.

(A) An ambidentate ligand is a ligand that can coordinate to a central metal atom through two different donor atoms.
Examples include:
$1$. $NO_2^-$: It can coordinate through the nitrogen atom (nitro,$M-NO_2$) or through the oxygen atom (nitrito,$M-ONO$).
$2$. $SCN^-$: It can coordinate through the sulfur atom (thiocyanato,$M-SCN$) or through the nitrogen atom (isothiocyanato,$M-NCS$).

(N/A) The compound gives a white precipitate of $AgCl$ when treated with $AgNO_{3}$,which indicates that at least one $Cl^{-}$ ion is present in the ionization sphere.
Since the molar conductance corresponds to a total of two ions,the complex must dissociate into two ions: one cation and one anion.
Therefore,the complex must be $[Cr(H_{2}O)_{4}Cl_{2}]Cl$,which dissociates as $[Cr(H_{2}O)_{4}Cl_{2}]Cl \rightarrow [Cr(H_{2}O)_{4}Cl_{2}]^{+} + Cl^{-}$.
The structural formula is $[Cr(H_{2}O)_{4}Cl_{2}]Cl$ and its $IUPAC$ name is Tetraaquadichloridochromium$(III)$ chloride.

Coordination entity: $A$ coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example,$[CoCl_{3}(NH_{3})_{3}]$ is a coordination entity in which the cobalt is surrounded by three ammonia and three chloride ions. Other examples are $[Ni(CO)_{4}]$,$[PtCl_{2}(NH_{3})_{2}]$,$[Fe(CN)_{6}]^{4-}$,and $[Co(NH_{3})_{6}]^{3+}$.
Central metal ion/atom: In a coordination entity,the atom or ion to which a fixed number of ions or groups are bound in a definite geometrical arrangement is called the central metal ion or atom. For example,in the coordination entities $[NiCl_{2}(H_{2}O)_{4}]$,$[CoCl(NH_{3})_{5}]^{2+}$,and $[Fe(CN)_{6}]^{3-}$,the central ions are $Ni^{2+}$,$Co^{3+}$,and $Fe^{3+}$ respectively. These central ions or atoms act as electron pair acceptors and are also referred to as Lewis acids.

(N/A) Ligands: The ions or molecules bound to the central atom/ion in the coordination entity are called ligands. These may be simple ions such as $Cl^{-}$,small molecules such as $H_{2}O$ or $NH_{3}$,larger molecules such as $H_{2}NCH_{2}CH_{2}NH_{2}$ or $N(CH_{2}CH_{2}NH_{2})_{3}$,or even macromolecules such as proteins. Ligands are electron pair donors and are therefore referred to as Lewis bases.
Coordination number $(CN)$: The coordination number of a metal ion in a complex is defined as the number of ligand donor atoms to which the metal is directly bonded. The coordination number depends on the denticity of the ligands. For example,in the complexes $[PtCl_{6}]^{2-}$ and $[Ni(NH_{3})_{4}]^{2+}$,the coordination numbers of $Pt$ and $Ni$ are $6$ and $4$,respectively. Similarly,in the complex ions $[Fe(C_{2}O_{4})_{3}]^{3-}$ and $[Co(en)_{3}]^{3+}$,the coordination number of both $Fe$ and $Co$ is $6$ because $C_{2}O_{4}^{2-}$ and $en$ (ethane-$1,2$-diamine) are bidentate ligands. The coordination number is determined only by the number of sigma bonds formed between the ligand donor atoms and the central metal ion.

(N/A) Coordination sphere: The central metal atom or ion and the ligands directly attached to it are enclosed within a square bracket,which is collectively referred to as the coordination sphere. For example,in the complex $K_{4}[Fe(CN)_{6}]$,the coordination sphere is $[Fe(CN)_{6}]^{4-}$,while $K^{+}$ acts as the counter ion. Counter ions are ionizable groups that are written outside the square bracket.

(N/A) Homoleptic and heteroleptic complexes:
$(a)$ Homoleptic Complexes: Complexes in which a metal is bound to only one kind of donor groups are known as homoleptic.
Example: $[Co(NH_3)_6]^{3+}, [Ni(CO)_4]$ etc.
$(b)$ Heteroleptic Complexes: Complexes in which a metal is bound to more than one kind of donor groups are known as heteroleptic.
Example: $[Co(NH_3)_4Cl_2]^{+}, [Co(CN)_2Cl_2]^{+}$ etc.

(N/A) Homoleptic and heteroleptic complexes:
$(a)$ Homoleptic Complexes: Complexes in which a metal is bound to only one kind of donor groups are known as homoleptic.
Example: $[Co(NH_{3})_{6}]^{3+}, [Ni(CO)_{4}]$.
$(b)$ Heteroleptic Complexes: Complexes in which a metal is bound to more than one kind of donor groups are known as heteroleptic.
Example: $[Co(NH_{3})_{4}Cl_{2}]^{+}, [Pt(NH_{3})_{2}Cl_{2}]$.

(N/A) $i$. Anionic ligands: Ligands that carry a negative charge are called anionic ligands. Examples include $Cl^{-}, Br^{-}, I^{-}, C_{2}O_{4}^{2-}, SO_{4}^{2-}, NO_{3}^{-}, NO_{2}^{-}$.
$ii$. Cationic ligands: Ligands that carry a positive charge are called cationic ligands. Examples include $NO^{+}, NH_{2}NH_{3}^{+}$.
$iii$. Neutral ligands: Ligands that carry no charge are called neutral ligands. Examples include $H_{2}O, CH_{3}OH, NH_{3}, CO$,and pyridine.

(N/A) The total number of donor atoms in a ligand is called its denticity.
$(i)$ Monodentate ligands: These ligands have only one donor atom or are coordinated through a single electron pair with a metal ion. They are also known as unidentate ligands. They may be neutral or anionic.
Example:
$(a)$ Neutral: $H_{2}O, CO, NH_{3}, C_{6}H_{5}N$ (pyridine) etc.
$(b)$ Anionic: $Cl^{-}, Br^{-}, I^{-}, NO_{2}^{-}, SCN^{-}$ etc.
$(ii)$ Polydentate ligands: These ligands may be bidentate,tridentate,tetradentate,pentadentate,and hexadentate if the number of donor atoms are $2, 3, 4, 5$ and $6$ respectively.
Examples:
$C_{2}O_{4}^{2-}$ (bidentate),$PO_{4}^{3-}$ (tridentate),$N(CH_{2}CH_{2}NH_{2})_{3}$ (tetradentate),ethylene diamine triacetate ion (pentadentate),and ethylene diamine tetraacetate ion (hexadentate).

(N/A) Ambidentate ligands are a type of monodentate ligand that has more than one potential donor atom and can coordinate to the metal ion through only one donor atom at a time.
Examples: $NO_{2}^{-}$,$SCN^{-}$,$CN^{-}$,$S_{2}O_{3}^{2-}$ etc.
$(i)$ $NO_{2}^{-}$ (nitrite ion):
- $M \leftarrow NO_{2}$ (nitrito-$N$,coordinating through nitrogen)
- $M \leftarrow ONO$ (nitrito-$O$,coordinating through oxygen)
$(ii)$ $SCN^{-}$ (thiocyanate ion):
- $M \leftarrow SCN$ (thiocyanato-$S$,coordinating through sulphur)
- $M \leftarrow NCS$ (isothiocyanato-$N$,coordinating through nitrogen)

Bidentate ligands are ligands that possess two donor atoms and can form two coordinate bonds with a single metal ion simultaneously.
Examples include:
$1$. Ethane$-1,2-$diamine $(NH_2CH_2CH_2NH_2)$: $A$ neutral bidentate ligand where both nitrogen atoms act as donor sites.
$2$. Oxalate ion $(C_2O_4^{2-})$: An anionic bidentate ligand where two oxygen atoms act as donor sites.

(N/A) Tridentate ligands: These are ligands that possess $3$ donor atoms and form $3$ coordinate bonds with a central metal ion.
Example: Diethylenetriamine $(NH_2CH_2CH_2NHCH_2CH_2NH_2)$ is a common tridentate ligand. It has $3$ nitrogen atoms with lone pairs that can coordinate to the metal ion.
$(a)$ Triaminopropane: $CH_2(NH_2)-CH(NH_2)-CH_2(NH_2)$
$(b)$ Diethylenetriamine: $H_2N-CH_2-CH_2-NH-CH_2-CH_2-NH_2$

(N/A) Tetradentate ligands: These are ligands that possess four donor atoms and can form four coordinate bonds with a single metal ion simultaneously.
Example: Triethylenetetramine (trien),which has the formula $NH_2-CH_2-CH_2-NH-CH_2-CH_2-NH-CH_2-CH_2-NH_2$. It contains four nitrogen atoms with lone pairs that can act as donor sites.

(N/A) Pentadentate ligands: These are ligands that possess five donor atoms and form five coordinate bonds with a central metal ion.
Example: Ethylene diamine triacetate $(EDTA^{3-})$. The structure shows five donor sites (three oxygen atoms from carboxylate groups and two nitrogen atoms from amine groups) capable of coordinating with a metal ion.

(N/A) Hexadentate ligands: These ligands have $six$ donor atoms and form $six$ coordinate bonds with a metal ion.
Example: Ethylenediamine tetraacetate $(EDTA^{4-})$. The structure shows the $six$ donor atoms (two nitrogen atoms and four oxygen atoms) labeled $1$ to $6$.

(N/A) The chemical formulas are as follows:
$(i)$ $Na_{2}[Fe(CN)_{5}NO]$
$(ii)$ $[Fe(CN)_{5}NOS]^{4-}$
$(iii)$ $Na_{4}[Fe(CN)_{6}]$
$(iv)$ $Fe_{4}[Fe(CN)_{6}]_{3}$
$(v)$ $Fe_{4}[Fe(CN)_{6}]_{3}$
$(vi)$ $NaSCN$