Coordination Compounds
Double salt : A compound formed by combination of two or more simple compounds, which is stable in solid state only is called double salt. In solution it breaks into component ions.
e.g., K2SO4·Al2(SO4)3·24H2O; Potash alum
FeSO4·(NH4)2SO4·6H2O; Mohr’s salt
KCl·MgCl2·6H2O; Carnallite
Complex compound : A compound formed by combination of two or more simple compounds which retain its identity both in solid and solution states is called complex compound.
e.g., K4[Fe(CN)6], Potassium ferrocyanide
[Cu(NH3)4]SO4, Cupramine sulphate
Some important terms pertaining
coordination compound :
Coordination entity : The central metal atom or ion and ligand taken together is called coordination entity. It may be positive, negative or neutral.
e.g., [Cu(NH3)4]2+, [Fe(CN)6]4–, [Ni(CO)4]
Central atom : The atom or ion with which definite number of ligands are attached in a definite geometry is called central atom/ion. Any atom/ion which has high positive charge density or vacant orbitals of suitable energy may be central atom or ion, e.g., transition metals, lanthanoids. It is Lewis acid (electron acceptor).
Ligands : Molecules or ions which are bound to the central atom/ion in the coordination entity are called ligands. A molecule or ion which has high negative charge or dipole or lone pair of electrons may be ligands. It is Lewis base (electron donor).
Classification of ligands :
(i) On the basis of charge – Negative ligands (CN–, F–, Cl–, NO2 – , NO3–, OH–, O2–) , Positive ligands (NO2
+, NO+, N2H5) , Neutral ligands (H2O, NH3, CO, NH2OH, CH3NH2)
(ii) On the basis of number of donor sites – Monodentate : Only one donor site e.g., H2O, NH3, Bidentate : Two donor sites e.g., Oxalato, Ethylenediamine , Polydentate : More than two donor sites e.g., EDTA (Hexadentate)
(iii) On the basis of bonding – Chelating ligands : A bidentate or polydentate ligand which forms more than one coordinate bonds in such a way that a ring is formed.( Ethylenediamine), Ambidentate ligands : Monodentate ligand which
contains more than one coordinating atom (or donor atom).
Coordination number (C.N.) : The total number of coordinate bonds through which the central metal atom or ion is attached with ligands is known as coordination number.
Coordination sphere : The central atom and the ligands which are directly attached are collectively known as coordination sphere. It is non-ionisable and written enclosed in square brackets. The ionisable groups are written outside the brackets.
Homoleptic complexes : Complexes in which a metal is bound to only one kind of ligands are called homoleptic complexes.
Heteroleptic complexes : Complexes in which the central atom is bound to different type of ligands are called heteroleptic complexes.
Nomenclature of coordination compounds :
Rules for writing the formula of coordination compounds :
– Formula of the cation whether simple or complex must be written first, followed by anion.
– The coordination sphere is written in square brackets.
– Within the coordination sphere the sequence of symbols is, first the metal atom followed by anionic ligand then neutral ligand finally cationic ligand.
Ligands of same type are arranged alphabetically.
– Polyatomic ligands are enclosed in parentheses.
– The number of cations or anions to be written in the formula is calculated on the basis that total positive charge must be equal to the total negative charge, as the complex as a whole is electrically neutral.
Rules for naming coordination compounds :
– The cation is named first then the anion.
– In naming coordination sphere, ligands are named first in alphabetical order followed by metal atom and then oxidation state of metal by a roman numeral in parentheses.
– The complex part is written as one word.
– When the coordination sphere is anionic, name of central metal ends in –ate.
Naming of ligands :
– Name of anionic ligands end in –o.e.g., Cl– : Chlorido
– Neutral ligands (with a few exceptions) retain their names e.g., NH3 : Ammine
– Name of cationic ligands end in – ium. e.g., NO2 + : Nitronium
– Certain ligands are represented by abbreviations in parentheses instead of their complex structural formulae. e.g., ethylenediamine(en).
– Ambidentate ligands are named by using different names of ligands or by placing the symbol of donor atom.
e.g., —SCN– (Thiocyanato-S or Thiocyanato), —NCS– (Thiocyanato-N or Isothiocyanato), —ONO– (Nitrito-O or Nitrito), —NO2 – (Nitrito-N or Nitro)
– The prefixes di-, tri-, tetra-, pentaand, hexa– are used to indicate the number of each ligand. If the ligand
name includes such a prefix, the ligand name should be placed in parentheses and preceded by bis-(2), tris-(3), tetrakis-(4), pentakis-(5) and hexakis-(6).
Isomerism : Two or more substances having the same molecular formula but different structural or spatial arrangement are called isomers and phenomenon is called isomerism.
Structural isomerism
Ionisation isomerism :- Compounds, which give different ions in solution due to the exchange of ions in coordination sphere and counter ions are called ionisation isomers.
[CoBr(NH3)5]SO4, [CoSO4(NH3)5]Br
Hydrate isomerism :- Isomers, which differ in the number of water molecules attached to the metal atom or ion as ligands are called hydrate isomers.
[Cr(H2O)6]Cl3, [Cr(H2O)5Cl]Cl2·H2O
Linkage isomerism:- When an ambidentate ligand attached to metal through different atoms then the compounds are said to be linkage isomers.
[Co(NO2)(NH3)5]Cl’ [Co(ONO)(NH3)5]Cl
Coordination isomerism:- When both positive and negative ions of a salt are complex ions and two isomers differ in the distribution of ligands in two complex ions then the isomers are called coordination isomers.
[Co(NH3)6][Cr(CN)6], [Co(CN)6][Cr(NH3)6]
Valence bond theory : It was developed by Pauling.
– A suitable number of vacant orbitals must be present in the central metal atom or ion for the formation of coordinate bonds with the ligands.
– Central metal ion can use appropriate number of s, p or d-orbitals for hybridisation depending upon the total number of ligands.
– The outer orbital (high spin) or inner orbital (low spin) complexes are formed depending upon whether outer d-orbitals or inner d-orbitals are used.
– Low spin complexes are generally diamagnetic and high spin complexes are paramagnetic.
– Paramagnetism ∝ No. of unpaired electrons.
– Magnetic moment = 
B.M.
where n = number of unpaired electrons.
Crystal field theory : It assumes the ligands to be point charges and there is electrostatic force of attraction between ligands and metal atom or ion. When ligands approach the central metal ion, then the five degenerate orbitals do not possess equal energy any more and results in splitting, which depends upon nature of ligand field strength.
– Greater the ease with which the ligand can approach the metal ion, the greater will be the crystal field splitting caused by it.
– Crystal field splitting in octahedral coordination complexes is shown as :
If Δo < P (where ‘P’ is energy required for forced pairing of electrons) then the electrons will remain unpaired and a high spin complex is formed.
– If Δo > P, then pairing of electrons takes place and a low spin complex is formed.
– Crystal field splitting in tetrahedral complexes is shown as :
Colour of coordination compounds : The magnitude of CFSE (Δo) for most of the complexes is of the same order as the energy of a photon of visible light. Hence, whenever d–d transition takes place, it imparts colour to the complex. The colour of the complex is the colour complementary to the wavelength absorbed.
Applications of coordination compounds :
Coordination compounds are of great importance in biological system. e.g., chlorophyll, haemoglobin, vitamin B12, etc. are coordinate compounds of Mg, Fe and Co respectively.
Coordination compounds are used for qualitative and quantitative analysis, extraction of metals, electroplating, photography and as dyes.
cis-Platin is used in cancer treatment, EDTA is often used for treatment of lead poisoning.
Coordination compounds are also used as catalyst.
What is the difference between a complex and a double salt?
Double salts dissociate into ions completely when dissolved in water but the complex ion does not dissociate.
The coordination number of ‘Co’ in the complex [Co(en)3]3+ is
(a) 3 (b) 6
(c) 4 (d) 5
Write the coordination number and oxidation state of platinum in the complex [Pt(en)2Cl2].
Coordination number and oxidation state of Pt in the complex [Pt(en)2Cl2] are 6 and +2 because en is a bidentate and neutral ligand.
Which of the following is more stable complex and why?
[Co(NH3)6]3+ and [Co(en)3]3+
[Co(en)3]3+ is more stable complex than [Co(NH3)6]3+ due to chelate effect as it forms rings.
Explain the following terms giving a suitable example in each case :
(i) Ambidenate ligand
(ii) Denticity of a ligand
(i) Ambidenate ligand : A unidentate ligand which can coordinate to central metal atom through two different atoms is called ambidentate ligand. For example, NO2– ion can coordinate either through nitrogen or through oxygen to the central metal atom/ion.
Denticity : The number of coordinating groups present in a ligand is called the denticity of ligand. For example, bidentate ligand ethane-1, 2-diamine has two donor nitrogen atoms which can link to central metal atom.
Write the formula of the following coordination compound :
Iron (III) hexacyanidoferrate(II)
Ans Fe4[Fe(CN)6]3
Write the IUPAC name of the following complex : [Cr(NH3)3Cl3].
Ans triamminetrichloridochromium(III)
Write the IUPAC name of the following complex : [Co(NH3)5(CO3)]Cl.
Ans Pentaamminecarbonatocobalt(III) chloride
Write the IUPAC name of the complex [Cr(NH3)4Cl2]Cl.
Ans tetraamminedichloridochromium(III) chloride.
Write down the formula of : Tetrammineaquachloridocobalt(III)chloride.
Ans : [Co(NH3)4(H2O)(Cl)]Cl2
Give the formulae of the following compounds :
(a) Potassium tetrahydroxidozincate(II)
(b) Hexaammineplatinum(IV) chloride
Ans (a) K2[Zn(OH)4]
Potassium tetrahydroxidozincate(II)
(b) [Pt(NH3)6]Cl4
Hexaammineplatinum(IV) chloride
What type of isomerism is exhibited by the complex [Co(NH3)5Cl]SO4 ?
Ionisation isomerism : [Co(NH3)5Cl]SO4 and [Co(NH3)5SO4]Cl
What type of isomerism is shown by the complex [Co(NH3)6][Cr(CN)6]?
Ans Coordination isomerism
What type of isomerism is shown by the complex [Co(en)3]Cl3?
The complex, [Co(en)3]Cl3 shows optical isomerism.
What type of isomerism is shown by the complex [Co(NH3)5(SCN)]2+?
The complex [Co(NH3)5(SCN)]2+ shows linkage isomerism as SCN– is an ambidentate ligand.
Give two examples of coordination compounds used in industries.
Na2EDTA is used for estimation of hardness of water.
Wilkinson’s catalyst is used as catalyst for hydrogenation.
Give names of two complexes which are used in medicines.
cis-Platin is used in the treatment of cancer. EDTA is used in lead poisoning.