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We know that all the known elements are arranged in the periodic table in the increasing order of their atomic number. Out of these all the elements, 75% of the elements are metallic n nature while few are non-metallic and metalloids in nature. The metallic elements have 1, 2 or 3 electrons in their valance shell and can easily donate to form cations. The complete s, d and f-block elements are metallic in nature while p-block elements are mixture of metals, non-metals and metalloids.

The d and f-block elements have incomplete orbitals which can accept electrons from the donor atom/molecule/ ion to form coordinate or dative bond. The transition elements have incomplete d-orbitals which tend to accept electrons and form coordination bonds with donor atom.

These donor atoms are called as ligands. They must have some extra pairs of electrons in the form of lone pair or negative charge. They form coordination bonds with transition metals to complex compounds. The formation of coordination compound is a unique feature of transition metals.

Usually the coordination compounds have intense color such as Cu (NH3)4SO4 is a blue color solid that is formed with the reaction of copper sulphate and excess of concentrated ammonia solution. Here Cu(NH3)42+ is complex ion and SO42- is counter ion that balance the positive charge of coordination entity.

Complex Ion Definition

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  1. The coordination compounds are composed of one acceptor and donor atom/molecule/ ion that are linked through coordination bond.
  2. The coordination compounds can have positive or negative charge which depends upon the charge on the ligand and metal ion.
  3. The charged coordination complex is known as complex ion.
  4. For example; in Cu (NH3)4SO4, the complex ion is Cu(NH3)42+ and SO42- is counter ion.
  5. Here in the complex ion; there are four ammonia molecule act as ligand and donate the lone pair of electrons of nitrogen atoms of each ammonia molecule.
  6. The metal ion Cu2+ ion have vacant d-orbitals that can accept electrons from the ligands to form coordination covalent bond.
  7. The complex ions can be positively or negatively charged such as [Cu(NH3)4]2+, [Cr(H2O)6]3+, [Fe(CN)6]3-, [Pt(NH3)4]2+, [PtCl4]2-, Cr(H3O)63+ or CoCl63–.
  8. The neutral complexes do not contain any charge such as Pt(NH3)2Cl2.
  9. The charge on complex ion depends upon the charge on the charge on ligand and metal ion.
  10. Ligands can have positive, negative or neutral in nature. Of the total charge on ligand gets balance by the charge of metal ion, the complex compound will be neutral in nature.
On the contrary, the imbalance between charge of metal ion and total charge on ligands create charge on complex to form complex ion. Some common examples of negatively charged ligands are Cl, OH, CN, Br, and SCN. The number of coordination bonds between ligands and metal ion is called as coordination number.

The coordination linkage between ligands and metal atom/ion represents in a square bracket and called as coordination sphere or coordination entity for example; [Cu(NH3)4](NO3)2, [Pt(NH3)4][PtCl4] etc.

Complex Ion Examples

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There are several complex compounds which are composed of certain metal ions with ligands through coordination bonds. The complex ions have certain charge on them can be either positive or negative.

For example; the aqueous solution of [Cr(H2O)6]Cl3 crystal results the formation of one mole of Cr(H2O)63+ ion with three moles of chloride ions. The presence of complex ions can detect with the conductivity observations or precipitation reactions.

For example; [Pt(NH3)4]Cl2 shows 3.0 A V–1 dm2mol–1 and forms two moles of AgCl precipitated with AgNO3 solution. It proves that the complex ion is [Pt(NH3)4]2+ ion that is bonded with two chloride ions.

On the other hand; the contrary the [Pt(NH3)3Cl]Cl contains one complex ion [Pt(NH3)3Cl]+ charge and one chloride ion as counter ion. Another example of complex ion is Pt(NH3)2Cl2 shows almost zero conductivity and form two moles of ions; one complex ion with one counter ion.

Complex Ion Formation

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Any complex is composed of two parts; a central atom and some electron donor ligands. Usually transition metals involve in the formation of complex ions due to the presence of vacant orbitals in it. They are linked through the coordinate covalent bond through the electron transfer from ligand to metal atom/ion.

The formation constant Kf, is used to describe the formation of a complex ion from its constituents. The formation constant is also called as stability constant or association constant for the formation of complex ion. Let’s take an example of complex ion; MaLb which is composed of an atoms of M and b of ligands.

The formation constant for this complex can be written as given below;

Kform = [MaLb] / [M]a[L]b

Let’s take an example of the formation of a complex ion of silver metal ion with cyano group to form the dicyanoargentate (I) ion ([Ag (CN)2]):

Kform= [Ag(CN)2]][Ag+][CN]2

The high magnitude of formation constant represents the favorable product or complex ion and reaction moves toward the forward direction. The value of formation constant helps in the prediction of stability of complexes. The reverse value of formation constant is called as instability constant or dissociation constant denoted as Kd.

Complex Ion Naming

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Complex ion contains two components; metal atom/ion and ligand. Both the components are bonded with coordinate covalent bond. The complex ions can carry positive or negative charge on complex entity. The naming of complex ions also follows some rules for the nomenclature like organic and inorganic compounds.

Like ionic compounds, the positive coordination entity comes first followed by negative entity of the molecule. In the name of complex ion, the name of ligand will come first followed by the name of metal ion. If there is more than one ligand in the complex ion, write the name of ligand has to written in alphabetical order.
  1. In the naming of anionic ligand, the name of ligand ended with –ide suffix like -chloro becomes chloride.
  2. Those ligands whose names are ended with ‘ite’ like nirite are replaced with -ato and become –ito.
  3. Some of the neutral ligands are named differently like water as ‘aqua’, ammonia as ‘ammine’, carbon monoxide as ‘carbonyl’, and the N2 and O2 are called ‘dinitrogen’ and ‘dioxygen’.
  4. Some common anionic ligands are O2- (oxo), OH- (Hydroxo), CN- (cyano), C2O42- (oxalate), CO32- (carbonato), CH3COO- (acetate) etc.
  5. Neutral ligands are NO (nitrosyl), CO (carbonyl), C5H5N (pyridine) and H2NCH2CH2NH2 (ethylenediamine).
  6. The number of ligands can represent by Greek prefix; mono, di, tri etc. complex ion, e.g. di-, tri- and tetra- etc.
  7. The number of polydentate ligands is shown by prefixes bis-, tris-, tetrakis-, pentakis- etc.
  8. The cationic complex ion is ended with the same as the metal name while the name of anionic complex ion is ended with –ate as the suffix of metal name.
  9. The name of metal is followed by the oxidation state of metal in Roman numeral in parentheses.
  10. Some of the complex ions are well known with their common names such as Fe(CN)63- and Fe(CN)64- are named as ferricyanide and ferrocyanide respectively.

Complex Ion Nomenclature

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Let’s discuss the nomenclature of complex ions. The name of [Cr(NH3)3(H2O)3]3+ is triamminetriaquachromium(III) ion as there are six ligands; three aqua and three ammine ligands with chromium (III) ion. The entire complex ion has 3+ positive charges therefore the name of complex ion is ended with chromium ion. Both ligands ammine and aqua are arranged in alphabetical manner. Another example of complex ion is [Pt(NH3)5Cl]3+ is named as pentaamminechloroplatinum(IV) ion.

The cation has 3+ positive charges with five ammine groups and one chloro group. Both ligands must arrange in alphabetical manner, therefore ammine comes first followed by chloro group. The positive charge on the complex is due to -1charge on chloro group and platinum (IV) ion that makes 3+ charge on the complex ion. In [Pt(H2NCH2CH2NH2)2Cl2]2+ ion, there are three ligands, two ethylenediammine and two chloro ligands.

Since ethylenediammine ligand is a poly-dentate ligand, therefore it is written as ‘bis(ethylenediamine)’ and two chloro group. The name of complex ion will be dichlorobis(ethylenediamine)platinum(IV) ion. The name of [Co(H2NCH2CH2NH2)3]3+ can be written tris(ethylenediamine)cobalt(III) ion. Let’s take an example of anionic complex ion. The name of [Fe(CN)6]4- is hexacyanoferrate(II) ion since it is composed of six cyano ligands with 4- charge on complex entity.

The name of metal is ended with –ate suffix sue to the presence of negative charge on it. The name of [NiCl4]2- is tetrachloronickelate(II) ion as there are four chloro ligands bonded with Nickel (II) ion. Since there is 2- charge on the complex ion, the name of nickel is ended with –ate suffix followed by oxidation number of metal ion in parenthesis.

Some other examples of complex ions with their names are as follow;

S.No Complex ion Name of complex ion
1 [Pt(NH3)2Cl2]2+ diamminedichloroplatinum(II) ion
2 [Ni(C2O4)2(H2O)2]2- diaquabis(oxalato)nickelate(II) ion
3 [Ag(NH3)2]+ diamminesilver(I) ion
4 [Fe(NH3)6]3+ hexaammineiron(III) ion
5 [CuCl4]2- tetrachlorocuprate(II) ion
6 [FeCl(CN)5]2- monochloropentacyanoferrate(III) ion
7 [CoF6]3- hexafluorocobaltate(III) ion
8 [CoBr(NH3)5]2- pentaamminebromocobalt(III) ion
9 [Fe(NH3)6]3+ hexaammineiron(III) ion
10 [Cr(CN)6]3- hexacyanochromate (III) ion
11 [Co(SO4)(NH3)5]+ pentaamminesulfatocobalt(III) ion
12 [Fe(OH)(H2O)5]2+ pentaaquahydroxoiron(III) ion
13 [Ag(CN)2]- dicyanoargentate(I) ion

Complex Ion Effect

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During some of the reaction, the complex ions form during the reaction that determines the solubility of the compounds. Let’s discuss one of the examples of one of complex ion formation in the solution. The saturated solution of silver bromide also contains additional un-dissolved solid silver bromide.

The reaction of this solution with high concentrations of ammonia results the formation complex ion; diammineargentate(I) ion [Ag(NH3)2]+. The formation of complex ion in the solution affects the solubility of the solution. A complex ion is formed with the combination of anion or ligand or Lewis base with a metal cation which acts as Lewis acid and accepts electrons.

During the reaction of the formation of complex ion, the formation of product depends upon the amount of complex ion product. As the concentration of complex ion increases, the equilibrium shifts towards the original molecule; reactant molecule. The equilibrium constant for the reaction of silver chloride and ammonia for the formation of ammine complex can be written as below.

Kc = Kform x Ksp
Where

Kform = Formation constant of the complex ion (1.7 x 107)
Ksp = Solubility product of complex ion formed during reaction (1.77 x 10-10)

Complex Ion Solubility

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The presence of any common ion in the solution, affects the solubility of the solution. The solubility of the solution denoted with the help of solubility product; Ksp which is limited to soluble solutes.

The solubility of complex ions can determine with the help of ionic product of it. If the ionic product is less than the solubility product, the precipitation cannot occur.
While the high value of ionic product compare to solubility product results the formation of precipitation of the insoluble salt.

The equal value of ionic product and solubility product results the formation of a saturated solution. Let’s take an example of the reaction of silver bromide solution with excess of ammonia to form complex ion in the solution. The solubility product for silver bromide is 5.0 x 10-13. In the saturated solution of silver bromide, some silver bromide remains un-dissolved in the solution that react by the addition of excess of ammonia solution in the reaction medium to form a complex ion, Ag(NH3)2+. The reaction of the formation of this complex ion in aqueous solution can be written as below;

Ag+(aq) + 2 NH3(aq) $\Leftrightarrow$ Ag(NH3)2+(aq)

The addition of ammonia in the saturated solution of silver bromide upsets the equilibrium of the solution and pull silver ions from the equilibrium to form complex ion.

This pulling of silver ions towards the formation of complex ions, initiates the decomposition of silver bromide in the solution of increase the concentration of silver ions in the solution that increases the solubility of silver bromide salt due to the presence of complex ion in the solution.The original equilibrium was in between silver ions and bromide ions.

AgBr(s) $\Leftrightarrow$ Ag+(aq) + Br-(aq)
Ksp = [Ag+] [Br-]

That disturbed due to the addition of ammonia in the solution to set a new equilibrium.

Ag+(aq) + 2 NH3(aq) $\Leftrightarrow$ Ag(NH3)2+(aq)
Kform= [Ag(NH3)2+]/ [Ag+] [NH3]2

Therefore the overall equilibrium will be;

AgBr(s) + 2 NH3(aq) $\Leftrightarrow$ Ag(NH3)2+(aq) + Br-(aq)
Kc = [Ag(NH3)2+] [Br-] / [NH3]2

Therefore; Kc = Kform x Ksp

Complex Ion Equilibria

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In a chemical reaction between two reactant molecules, results the formation of product molecules. The reaction will continue up to that extant until the concentration of reactant and product remain constant. This point is called as equilibrium point.

At the equilibrium point, the rate of forward reaction equals to the rate of backward reaction for a reversible reaction. The chemical equilibrium of a reaction can affect by external forces such as change in concentration of reactant or product, temperature, pressure etc. The value of equilibrium constant (Kc) depends upon the concentration of reactant and product.

Let’s take an example of complex ion equilibrium reaction. The reaction of Fe3+ ions with ligand as thiocyanato ion (SCN-) forms a deep red color complex ion thiocyanatoiron (III) ion ([FeSCN]2+). At the initial stage of the reaction, the concentration of both reactants decreases with the same rate as each one mole of Fe3+ ion reacts with one mole of SCN- ion. The equilibrium constant expression for the given complex ion reaction can be written as given below;

Kc= [FeSCN]2+/ [Fe3+] [SCN-]

At a certain temperature, the value of equilibrium will remain constant; therefore the mixture of both reactants will react until to reach to the equilibrium stage.
With all the value of concentration of reactants, the value of equilibrium constant will remain same.

We can determine the value of Kc with the help of spectrophotometric experiment to measure the red color appearance of the product which is absorbed at red color wavelength (447 nm).

The magnitude of absorbance (A) is directly related to the concentration or molarity (M) of the solution; A = k M where k is a constant value. According to the Beer-Lambert law the amount of light being absorbed to the by a complex ion at a certain concentration depends upon the path length of the light and absorbance.
Therefore;
A = $\varepsilon$bc
Where;
A = Absorbance
$\varepsilon$ = Absorptivity coefficient
b = Path length
c = Concentration

The calibration curve between the absorbance and concentration can used to determine the concentration of complex ion formed in the reaction mixture.
Overall in an aqueous solution of the reaction mixture, the complex ion exists in equilibrium with its constituents; metal ion and ligands. Another example of complex ion equilibrium is the formation of blue color complex of Cu (NH3)42+ ion from the reaction of copper ions with excess of ammonia. Here copper (II) ion acts as central metal ion and four ammonia molecules act as donor molecules or ligands. The reaction between central metal ion and ligands can be written as given below;

Cu2+(aq) + 4 NH3(aq) $\Leftrightarrow$ Cu(NH3)42+(aq)

The formation constant for the given complex will be

Kform = [Cu(NH3)42+]/ [Cu2+] [NH3]4

As we know the formation constant of the complex ion is also known as stability constant for it. Therefore as the magnitude of the stability constant increases, the stability of the complex increases which shows that the metal ions and ligands tend to bind and form complex ion in the reaction mixture that moves the reaction in the forward direction.

The reverse of stability constant is known as instability constant that increases with increasing the rate of backward reaction.

Complex Ion Reactions

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Complex ions are generally involves in the precipitations reactions and are used for the qualitative analysis. As we know that complex ions are mainly formed with transition metals due to the presence of incomplete valence shell, therefore addition of donor molecules such as ammonia, water, hydrogen sulphide etc. results the formation of transition complex ions in the solution that may precipitate during reaction and easy to identify due to their solubility.

  1. If the metal cation can form stable complex ions with ammonia or hydroxide ions; ammonia solution of sodium hydroxide can be used for the precipitation.
  2. Therefore, they are termed as complexing agents that initiate the formation of complex ions in the solution in the form of precipitate.
  3. The dissociation constant and solubility product are used to determine the equilibrium constant form the reaction.
  4. Here the dissociation constant denotes the extent of the cation to be converted into complex ion and solubility product denotes the fraction of metal ion that remains in the solution after the formation of complex ion.
  5. The reaction of the solution of Nickel (II) ion with excess of ammonia results the formation of [Ni (NH3)4] 2+ complex ion. 
  6. Some of the complex ions with ammonia are Ag(NH3)2+, Cd(NH3)42+, Cu(NH3)42+, Ni(NH3)62+, and Zn(NH3)42+.
  7. Similarly the complex ions formed due to the addition of sodium hydroxide in the solution are Al(OH)4-, Pb(OH)3-, Sb(OH)4-, Sn(OH)62-, and Zn(OH)42-.
  8. Usually strong inorganic acids require dissolving these precipitates such as addition of hydrochloric acid to the solution of tetra-amine-copper (II) sulfate dissociates the complex to copper (II) ions and ammonium ions in the solution.
  9. On the contrary, the addition of sodium hydroxide to the saturated solution of aluminum hydroxide forms [Al (OH)6] 3− complex ion.