Isomerism in Coordination Compounds-
Coordination compounds exhibit the following types of isomerism:
1.Structural Isomerism-In this isomerism, isomers have different bonding pattern. Different types of structural isomers are
(i)Linkage isomerism– This type of isomerism is shown by the coordination compounds having ambidentate ligands. e.g.,[Co(NH3)5(NO2)]Cl and [Co(NH3)5(ONO)]Cl or
pentaammine nitrito- N Cobalt (III) chloride and pentaammine nitrito-O’Cobalt (III) chloride.
(ii)Coordination isomerism– This type of isomerism arises from the interchange of ligands between cationic and anionic complexes of different metal ions present in a
complex, e.g.,[Cr(NH3)6) [CO(CN)6] and [CO(NH3)6] [Cr (CN)6]
(iii) Ionisation isomerism– This isomerism arise due to exchange of ionisable anion with anionic ligand.
(iv) Solvate isomerism-This is also known as hydrate isomerism. In this isomerism, water is taken as solvent. It has different number of water molecules in the coordination
sphere and outside it. e.g..[Co(H2O)6]CI3, [Co(H2O) 4C12]Cl·2H2O, [Co(H2O)3Cl3].3H2O
2. Stereoisomerism– Stereoisomers have the same chemical formula and chemical bonds but they have different spatial arrangement. These are of two types :
(i)Geometrical isomerism-Geometrical isomers are of two types i.e., cis and trans isomers. This isomensm is common in complexes with coordination number 4 and 6.
Geometrical isomerism in complexes with coordination number 4
(i)Tetrahedral complexes do not show geometrical isomerism.
(ii) Square planar complexes of formula [MX2L2] (X and L are unidentate) show geometrical isomerism. The two X ligands may be arranged adjacent to each other in a cis isomer, or opposite to each other in a trans isomer,
(iii) Square planar complex of the type [MABXL] (where A, B, X, L, are unidentate ligands) shows three isomers, two cis and one trans.e.g., [Pt(NH3) (Br)(Cl)(Py)].
Geometrical isomerism in complexes with coordination number 6.
Octahedral complexes of formula [MX2L4], in which the two X ligands may be oriented cis or trans to each other, e.g., [Co(NH3)4Cl2)]+. Octahedral complexes of formula [MX2A2], where X are unidentate ligands and A are bidentate ligand.form cis and
trans isomers, e.g., [CoC12(en)2]’.
In octahedral complexes of formula [MA3X3], if three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face. it is known as facial (fae)
isomer, when the positions are around the meridian of the octahedron, it is known as meridional (mer) isomer. e.g., [Co(NH3)3(NO2)3]
(ii) Optical isomerism– These are the complexes which have chiral structures. It arises when mirror images cannot be superimposed on one another. These mirror images are
called enantiomers. The two forms are called dextro (d) and laevo (l) forms.
Tetrahedral complexes with formula [M(AB)2] show optical isomers and octahedral complexes (cis form) exhibit optical isomerism.
Bonding in Coordination Compounds-
Werner’s Theory– Metals exhibit two types of valencies in the formation of complexes.These are primary valencies and secondary valencies.
1. Primary valencies correspond to oxidation number (ON) of the metal and are satisfied by anions. These are ionisable and non-directional.
2. Secondary valencies correspond to coordination number (CN) of the metal atom and are satisfied by ligands. These are non-ionisable and directional. Hence, geometry is
decided by these valencies.
Valence Bond Theory(VBT)-
This theory was proposed by L.Pauling in 1930 s. According to this theory, when a complex is formed, the metal ion/atom provides empty orbitals to the surrounding
ligands. Coordination number shows the number of such empty orbitals, i.e., number of empty orbitals is equal to the coordination number. These empty orbitals hybridised
before participation in bonding and the nature of hybridisation depends on the nature of metal and on the nature of approaching ligand.
Inner orbital complexes or outer orbital complexes.
When outer d-orbital are used in bonding, the complexes are called outer orbital complexes. They are formed due to weak field ligands or high spin ligands and hybridisation is sp3d2. They have octahedral shape.
When d- orbitals of (n – 1) shell are used, these are known as inner orbital complex, they are formed due to strong field ligands or low spin ligands and hybridisation is d2sp3.
They are also octahedral shape.
1. 6 – ligands(unidentate), octahedral entity.
(i) Inner orbital complex- [Co(NH3)6]3+ All electrons are paired, therefore complex will be diamagnetic in nature.
(ii) Outer orbital complex, [CoF6]3- Complex has unpaired electrons, therefore, it will be paramagnetic in nature.
2. 4-ligands(unidentate) tetrahedral entity
(i)Inner orbital complex, [Ni(CN)4]2- All electrons are paired so complex will be diamagnetic in nature.
(ii) Outer orbital complex,[CoCI4]– Since, complex has unpaired electrons. so it will be paramagnetic in nature.
Limitations of VBT-
This theory could not explain the quantisation of the magnetic data, existence of inner orbital and outer orbital complex, change of magnetic moment with temperature and
colour of complexes.
Crystal Field Theory (CFT)-
This theory was proposed by H.Bethe and van Vleck. Orgel. in 1952, applied this theory to coordination compounds. In this theory, ligands are treated as point charges in case of
anions and dipoles in case of neutral molecules.
The five d-orbitals are classified as
(i) Three d-orbitals i.e., dxy, dyz and dzx are oriented in between the coordinate axes and are called t2g – orbitals.
(ii) The other two d-orbitals, i.e., dx2– y2 and dz2 oriented along the x – y % axes are called eg – orbitals.
Due to approach of ligands, the five degenerate d-orbitals split. Splitting of d-orbitals depends on the nature of the crystal field.[The energy difference between t2g and eg
level is designated by Δ and is called crystal field splitting energy.] By using spectroscopic data for a number of coordination compounds, having the same metal ions but
different ligand, the crystal field splitting for each ligand has been calculated. A series in which ligand are arranged in order of increasing magnitude of crystal field splitting, is
called spectrochemical series.
Limitations of CFT
1. It does not consider the formation of 7t bonding in complexes.
2. It is also unable to account satisfactorily for the relative strengths of ligands e.g., it does not explain why H2O is stronger ligand than OH–
3. It gives no account of the partly covalent nature of metal-metal bonds.
Ligand Field or Molecular Orbital Theory-
This theory was put forward by Hund and Mulliken. According to this theory, all the atomic orbitals of the atom participating in molecule formation get mixed to give rise an
equivalent number of new orbitals, called the molecular orbitals. The electrons are now under the influence of all the nuclei.
Stability of Coordination Compounds
The stability of complex in solution refers to the degree of association between the two species involved in the state of equilibrium. It is expressed as stability constant (K).The
factors on which stability of the complex depends :
(i)Charge on the central metal atom. As the magnitude of charge on metal atom increases, stability of the complex increases.
(ii)Nature of metal ion. The stability order is 3d < 4d <5d series.
(iii)Basic nature of ligands. Strong field ligands form stable complex.
The instability constant or the dissociation constant of
compounds is defined as the reciprocal of the formation or
Importance and Applications of Coordination Compounds –
1. They are used in many qualitative and quantitative analysis.
2. Hardness of water is estimated by simple titration with Na2 EDTA.
3. Purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds.
4. They have great importance in biological systems.
5. They are used as catalyst for many industrial processes.
6. In medicinal chemistry, there is a growing interest of chelating therapy.
Organometallic Compounds- They contain one or more metal-carbon bond in their molecules. They are of the following types:
1.Sigma (σ) bonded compounds- Metal-carbon bond is sigma bond, e.g.,(C2H5)4 Pb, Zn(C2H5)2 R – Mg – X, etc.
2.Pi(π) bonded compounds- In which molecules/ions containing π bonds act as a ligand. e.g., Ferrocene, Dibenzene chromium and Zeise’s salt. Zeise’s salts is K [PtCI3(η2 – C2H4)] In which ethylene acts as a ligand which do not have a lone pair oi electron.In ferrocene, Fe (η5 – C5H5)2 represents the number of carbon atoms with which metal ion is directly attached.
3.σ and π bonded compounds- Metal carbonyls are their examples. Metal-carbon bond of metal carbonyls have both σ and π – bond character. They have CO molecule as
Wilkinson’s catalyst (Rh(PPh3)3CI] is used as homogeneous catalyst in the hydrogenation of alkenes. Zeigler-Natta catalyst [Ti CI4 + (C2H5>3Al] acts as heterogeneous catalyst in the polymerisation of ethylene.