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Coordination Complex

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A molecule is formed by the chemical bonding between atoms. During the formation of any molecule, the constituent atoms tend to complete their octet configuration either sharing or complete transfer of electrons with other atoms. 

The equal sharing of valance electrons between bonding atoms results the formation of covalent bond between atoms. On the contrary, the complete transfer of electrons between bonding atoms leads to the formation of cations and anions which are bonded through electrostatic force of attraction known as ionic bond.

Sometimes the bonding atoms involve in unequal sharing of electrons, i.e. One of the bonding atom act as a donor and another atom accepts those electrons, acts as acceptor. Such type of bonding between donor and acceptor is known as coordination bonds. 

There are many such compounds in which the bonding atoms involve in coordination linkage which is also known as dative bond. The most common example of coordination linkage is the bonding between ammonia molecule and boron trifluoride. As nitrogen atom in ammonia contains one lain pair or electrons which can donate to boron; an electron deficient atom with six electrons.
 Ammonia Boron Trifluoride

Coordination Complex Definition

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"The transition metal or d-block elements also involve in coordination bonding with electron deficient molecule or ions or atoms. Such adducts are known as coordination compounds or coordination complexes."
For example, in FeCl4- ion and CrCl3 .6NH3, metal ion acts as acceptor and chloride ion or ammonia molecule is known as donor part to form coordination bond. In other words; metal atom accepts electrons from donor atom or molecule in its vacant orbitals to form the coordination linkage.

Here the donor atom or molecule is known as ligand and the complex is known as Lewis acid-base complexes as metal atom/ion acts as Lewis acid (electron acceptor) and ligand acts as Lewis base (electron donor). If each ligand is bonded through one coordination bond with the central metal atom, then the total number of coordination bonds between metal and ligands is called as coordination number.

Generally transition metals are involved in the formation of coordination complexes due to availability of vacant d-orbitals. But many p-block elements also involve in the formation of such complexes such as aluminum and tin forms AlF63-, SnCl42- with halide ions. Similarly lead forms PbI42- ions with iodide ligand. Remember, the coordination complexes can carry either negative or positive charge on them and known as complex ions while a neutral molecule is considered as the coordinate compound.
  1. The chemical and physical nature of coordination compounds depends upon their bonding constitutes.
  2. All the bonded ligands with central metal atoms.ions are written in a square bracket and this representation is known as coordination sphere or coordination entity.
  3. The study of coordination complexes started from the study of cobalt complexes by Alfred Werner who later developed the Werner's Theory of Coordination Complexes.
  4. He purposed that there are three different cobalt (III) complexes which can be isolated by dissolving the CoCl2 in aqueous ammonia and further oxidized by air to convert in +3 oxidation state.
  5. All these complexes show different properties due to the different type of bonding and structural arrangement of ligands around central metal ion.
  6. Some of these complexes do not react with HCl even at high temperature also.
  7. If we added excess of silver nitrate to the solution of these complexes, some of them do not form a white precipitate of silver chloride.
  8. Werner also measured the conductivity of these complexes with the help of their solution and found that all of them show different values of conductivities.
On the basis of his observations, Werner purposed that in coordination complexes, ligands are bonded to central metal atom/ion through two types of bonding or we can say that the central metal atom/ion can show two types of valencies; primary and secondary valency.
  1. The primary valence can satisfy by negative ions to satisfy the charge on the metal ion while secondary valance shows the number of molecules/atom or ions which are bonded through coordination bonds to metal ion/atom.
  2. The primary valance can involve in chemical reactions while secondary valance is responsible for the geometry of the molecule.

Coordination Complex Examples

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Let’s discuss some of the examples of coordination complexes with their ligands. Ligand can be any ion/ molecule/atom which have an extra pair or lone pair of electrons with it and tends to donate them to an electron deficient entity too form coordination bond.

On the basis of the number of coordination bonds from ligands, they can be classified as mono-dentate, bi-dentate, poly-dentate. A monodentate ligand can form only one coordinate bond with metals atom/ion while a bi-dentate ligand can form two coordinate bonds with the same metal atom/ion either through the same bonding atom or another one.

Some typical monodentate ligands are halides, oxides, hydroxy, water, ammonia, the oxygen molecule, carbon monoxide, carbon dioxide, a cyano group and thiocyano. Ethylenediamine is an example of bidentate ligands which can form two coordinate bond through the lone pairs of electrons locate on the amino groups of the molecule. Bi and polydentate ligands tend to form a cage like structure with metals which is known as chelate, and the process is called as chelation.

The formation of chelate through bonding stabilise a coordination complex. Mainly five and six membered rings are much more stable compare to others. The number of coordination bonds between metal and ligand is called as coordination numbers. Usually coordination compounds with 2,4, and 6 coordination number. Let's discuss some examples of complexes with their coordination numbers.

S.No Complex Ligands Coordination number
1 AgCl2- Cl- 2
2 Ag(NH3)2+ NH3 2
3 Pb(OAc)2 -OAc- 2
4 Cu(NH3)2+ NH3 2
5 Ag(S2O3)23- S2O32- 2
6 Cu(NH3)42+ NH3 4
7 HgI42- -I- 4
8 Zn(CN)42- -CN- 4
9 Na2Co(SCN)4 -SCN- 4
10 [Ni(CN)4]2- -CN- 4
11 Co(SCN)42- -SCN- 4
12 Fe(H2O)62+ H2O 6
13 Ni(NH3)62+ NH3 6
14 Fe(CN)64- -CN- 6
15 Co(NH3)63+ NH3 6
16 Fe(H2O)63+ H2O 6
17 Ni(NH3)6(NO3)2 NH3 6
18 K2PtCl6 Cl- 6
19 [Ni(NH3)6]2+ NH3 6

Remember that the charge on the complex ion is not localized any one of the atom/ion, but delocalized over the molecule. According to G.N. Lewis, the bonding between metal and ligand is like Lewis acid-base reaction in which metal atom/ion acts as Lewis acid and ligand acts as Lewis base.

For example, in the complex of Co3+ ion and ammonia, the bond formation can be shown as below.

Coordination Complex Ion


Here Co3+ ion contains [Ar] 3d64s04p0 electronic configuration and has vacant 4s and 4p-orbitals with incomplete 3d-sub shell which can accept electrons.

Similarly each ammonia molecule contains a lone pair of electrons over nitrogen atom which can donate its pair of electrons to metal ion and acts as Lewis base to form coordination bond.

Solved Example

Question: Identify the oxidation state of central atom, ligands, and coordination number in the given complexes.
  1. [Fe(CN)6]4
  2. [Co(H2O)4SO4]-
  3. [Pt(en)2]2+
  4. [Cu(NH2CH2CH2NH2)3]2+

Solution:
 

Complex compound/ion Oxidation state of metal atom/ion Ligands Coordination number
[Fe(CN)6]4- 2+ CN- 6
[Co(H2O)4SO4]- 1+ H2O, SO42- 6
[Pt(en)2]2+ 2+ en 4
[Cu(NH2CH2CH2NH2)3]2+ 2+ NH2CH2CH2NH2 6


 

Coordination Complex Nomenclature

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The naming of coordination complexes also follows IUPAC rules.
  1. In the writing of the formula of coordination compounds, the name will be written as [Metal Anions Neutrals]±Charge, the name of coordination compound can be written as Prefix Ligands Metal (Oxidation State).
  2. Remember the coordination complexes can have positive charge like [Co(NH3)6]3+ or negative charge like [CoCl4(NH3)2]- or can be neutral like [CoCl3(NH3)3].
  3. The negative or positive charge on complex ion can be neutralized by counter ion like in K4 [Fe(CN)6]; potassium ‘K’ acts counter ion for [Fe (CN)6]-4.
We can write the name of coordination complexes with help of the formula. In the naming of coordination complex; first we have to write name of the ligand. If there is more than one linage in the complex compound, the name of ligands will follow alphabetical order followed by the name of metal atom/ ion with the oxidation state in Roman numeral in parentheses such as (II, IV).

The naming of ligands also follows some crucial rules like the name of anionic ligand is ended with the replacement of –idde with –o suffix such as chloride $\rightarrow$ chloro. If the name of anions ended with "-ite" and "-ate", than ot will replace with "-ito" and "-ato" suffix such as Nitrite will change to Nitrito and Nitrate will change to Nitrato.

Some other anionic ligands with their names are as follows.

S.No Ligands formula Ligand name
1 OH- Hydroxo
2 SO42- Sulphato
3 S2O32-- Thiosulphato
4 CN- Cyano
5 NO2- Nitro
6 ONO- Nitrito
7
O2- Oxo
8 SCN- Thiocyanato
9 NC- Isocyano
10 NCS- Isothiocyanato


Usually neutral ligands are written with their normal names such as methylamine (CH3NH2) and pyridine (C5H5N). Some of neutral ligands have certain names used only in the naming of coordination complexes such as ammine (NH3), aqua (H2O), carbonyl (CO), and nitrosyl (NO).

In the name of coordination complexes, the number of ligands are written with the prefix; mono, bi, tri etc. which are always written before the name of ligands such as the name of [FeCl(CO)2(NH3)3]2+ can be written as triaminechlorodicarbonyliron(III).
Similarly the name of [CrCl2(H2O)4]+ could be written as tetraaquadichlorochromium(III)ion. While the name of [CoCl2(en)2]+would be dichlorobis(ethylenediamine)cobalt(III) ion.

Solved Examples

Question 1: Write the name of given coordination complexes.
  1. [Cr(H2O)3(CO)3]Cl3
  2. [Pt(H2O)5Cl]Cl3
  3. [Pt(H2NCH2CH2NH2)2Br2]I2
  4. [Cr(H2NCH2CH2NH2)3]2(SO4)3
  5. Na4[Fe(CN)6]
  6. K2[CuCl4]
  7. Pt(H2O)2Cl4
  8. (NH4)2[Ni(C2O4)2(NH3)2]
  9. [Ag(H2O)2][Ag(CN)2]

Solution:
 
  1. triaquatricarbonylchromium(III)chloride
  2. pentaaquachloroplatinum(IV)chloride
  3. dibromobis(ethylenediamine)platinum(IV)iodide
  4. tris(ethylenediamine)chrominum(III)sulphate
  5. sodiumhexacyanoferrate(II)
  6. potassium tetrachlorocuprate(II)
  7. diaquatetrachloroplatinum(IV)
  8. ammonium diammniebis(oxalato)nickelate(II)
  9. diaquasilver(I)dicyanoargentate(I)

 

Question 2: Sodium chloride is an ionic compound while sodium hexacyanoferrate(II) is a coordination compound. Why?
Solution:
 
An ionic bond is formed due to complete transfer of electrons form one of the bonding atom to another and results the formation of cation & anion. These two oppositely charged entities are bonded through an electrostatic force of attraction which is known as ionic bond and such compounds are called, as ionic compounds like sodium chloride. On the contrary, some of the compounds form due to unequal sharing of electrons between bonded atom which results the formation of coordination bond and such compounds are called as coordination compounds or coordination complexes like sodium hexacyanoferrate(II).
 

Coordination Complex Naming

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Before writing the name or formula of coordination compounds, we must know the oxidation state of central metal atom which is written in parenthesis in the name of compounds. We can calculate the oxidation state of by removing all the chargeable ligands from the total charge on the complex compound or ion. In the name of an anionic complex, the name of the metal is added with suffix –ate such as cooper will written as cuprate (II).

Similarly tin (Stannate), silver (Argentate), gold (Aurate), lead (plumbate) and iron (Ferrate). The chargeable entity is recognized as the coordination ion or complex ion while a neutral molecule is considered as coordination compound. For example; [Cr(OH)4]-is a complex ion or coordination ion with -1 charge and named tetrahydroxochromate(III) ion.

Similarly [CuCl4]2- is named as tetrachlorocuprate(II) ion as there are four chloride ions which acts as ligands (-chloro) with copper metal in 2+ oxidation state.
Since there is 2- charge on the complex, therefore the name of metal is ended with cuprate (II) ion.

On the contrary, [Pt(NH3)4)][Pt(Cl)4] is a neutral compound in which both cationic and anionic entities are coordination entities. There are four ammine groups in cationic part of molecule with platinum (II) and anion contains four chloro group with platinum in 2+ oxidation state.

Therefore the name of compound will be tetraammineplatinum(II) tetrachloroplatinate(II). Remember; if both cation and anion are coordination entity, like other ionic compound, the name of cation will come first followed by the name of anion. In [CoCl(NO2)(NH3)4]+ complex ion, there are three different types of ligands; nitro, chloro and another is amine.

  1. The name of ligands will come in alphabetical order with the name of metal ion. Therefore the name of complex will be tetraaminechloronitrito-N-cobalt(III) ion.
  2. Nitro is an ambidentate ligand which can form coordination bond with metal through nitrogen or oxygen atom.
  3. If it is bonded through nitrogen atom, the name of ligand will be –Nitro, but the bonding with oxygen atom makes the name –Nitrito. Therefore the name of [CoCl(ONO)(NH3)4]+complex ion will be tetraaminechloronitrito-O-cobalt(III).
  4. In the formula writing for coordination complexes, first the symbol of metal atom /ion is written followed by the symbols of ligands in alphabetical order according to the first letter in their chemical formula.
  5. The number of each ligand in the complex is shown as subscript on the chemical formula of it.
  6. For example the formula of tetrafluorochromium(VI) chloride will be [CrF4]Cl2 as there are four fluoro ligand (F4) with one chromium metal ion in 6+ oxidation state.
  7. The chloride at the end of name represents the counter ion whose number will be determined by the charge on complex entity.
  8. Similarly the formula of aminetetraaquachromium(II) ion can be written as [Cr(H2O)4(NH3)]+2. Here in this complex ion, both ligands; aqua and amine are neutral in nature therefore we have to follow alphabetical order during writing the formula of the compound.
  9. The formula of aminesulfatochromium(II) will be [Cr(SO4)(NH3)]. SO4.
  10. Here sulphate ion acts as counter ion to balance the charge of coordination ion, but amine and one of the sulphato group act as ligand and written inside the square bracket.

Solved Example

Question: Write the formula of given coordination compounds or ions.
  1. triamminetrichlorocobalt(III)
  2. triamminetrinitro-cobalt(III)
  3. diamminedioxalatoferrate(III) ion
  4. silver(I) tetrachloromercurate(II)

Solution:
 
  1. [CoCl3(NH3)3]
  2. [Co(NO2)3(NH3)3]
  3. [Fe(ox)2(NH3)2]-
  4. Ag2[HgCl4]