Chemqueries: 2018

Friday, 28 December 2018

Factors Affecting the magnitude of ∆

There are several factors that affect the magnitude of splitting (0)of d-orbitals by the surrounding ligands.

1. Oxidation State of the Metal Cation:

The higher the oxidation state of the metal cation, the greater will be the magnitude of ∆. The higher the oxidation state of the metal causes the ligand to approach more closely to it and, therefore the ligand causes more splitting of metal d-orbitals.
     For example               ∆0 for [Co(H2O)6]2+ = 9200 cm-1
             and                     ∆0 for [Co(H2O)6]3+ = 20760 cm-1

2. Same Oxidation State of Metal Cation but the number of d-electrons is                   Different:

In general, for a given series of transition elements(say 3d-series), in complexes having the metal cation with the same oxidation state but the different number of electrons in the d-orbitals, the magnitude of  decreases with increase in the number of d-electrons. It is due to the fact that the higher number of d-electrons prevents the ligands to come closer to the metal cation.
                                 For example        0 for [Co(H2O)6]2+ = 9200 cm-1(3d7)
                                        and                ∆0 for [Ni(H2O)6]2+ = 8500 cm-1(3d8)

3. Principal Quantum Number(n) of the d-orbital of the Metal Cation:

In case of complexes having the metal cation with the same oxidation states and the same number of d-electrons, the magnitude of for analogous complexes within a given group increases about 30% to 50% from 3d to 4d and by about the same amount from 4d to 5d. It is because:
  (1) On moving from 3d to 4d and 4d to 5d, the size of the d-orbital increases and electron density              decreases in them. Therefore the ligands can approach the metal cation with larger d-orbital                more closely.
  (2) There is less steric hindrance around a larger metal cation.
        For example      ∆0 for [Co(NH3)6]2+ = 2300 cm-1 
                                  ∆0 for [Rh( NH3)6]2+= 34100 cm-1 
                                  ∆0 for [Ir( NH3)6]2+= 41200 cm-1 

 4. Nature of Ligands:

The ligands are classified as a weak and strong ligand. The ligand which causes a small degree of splitting of d-orbitals are called weak ligands and the ligands which cause a large splitting are called strong ligands. The common ligands have been arranged in order of their increasing crystal field splitting power to cause splitting of d-orbitals from a study of their effects on the spectra of transition metal ions.
(weak end)O22−< I < Br < S2− < SCN (S–bonded) < Cl− < N3 < F< NCO < OH < C2O42− < H2O < NCS (N–bonded) < CH3CN < gly (glycine) < py (pyridine) < NH3 < en (ethylenediamine) < bipy (2,2'-bipyridine) < phen (1,10-phenanthroline) < NO2 < PPh3 < CN < CO < CH2(strong end)
 This order s usually called as Spectrochemical series
The order of the field strength of common ligands is independent of the nature of the metal cation and the geometry of the complex.

5. Number of Ligands:

The magnitude of crystal field splitting(∆)increases with the increase of the number of ligands.For example  > ∆t
 Though the number of ligands in a square planar complex is smaller than that of octahedral complexes, the magnitude of  ∆sp is greater than ∆0. It is because of the fact that square planar complexes are formed by much strong ligands with dmetal cation of 3d-series transition metal cation and 4d and 5d series d8metal cation with either weak or strong ligand. The very strong ligands and 4d or 5d series transition metal cations are responsible for higher crystal field splitting. Also in square planar complexes of dmetal cation, the dZ2 orbital with two electrons is stabilized and the vacant dX2-y2 orbital is destabilized.

Tuesday, 25 December 2018

Splitting of d-orbital in octahedral complex

Crystal field splitting in Octahedral complex:


  •  In a free metal cation all the five d-orbitals are degenerate(i.e.these have the same energy.In octahedral complex say [ML6]n+the metal cation is placed at the center of the octahedron and the six ligands are at the six corners. These six corners are directed along the cartesian coordinates i.e. along the x, y, and z-axis. When all the ligands are at an infinite distance from the metal cation, the five d-orbital of the metal cation will not be affected by the ligand electrostatic field and will remain degenerate. When the ligands move towards the metal cation, there are two electrostatic forces i.e. 
                (1)The attraction between metal cation and ligand
                (2)Electrostatic repulsion between d-electrons of the metal cation and lone pairs of ligands
  • Greater the repulsion between metal cation and ligands, ligands will be more closer to the metal cation and hence more will be the repulsion between the metal d-electrons and the lone pair of electrons on the ligand. When the ligands are closer to the metal cation an electrostatic force of repulsion also exists among the ligands.These two repulsion cause to adopt the octahedral geometry that locates the ligand at the internuclear distance from the metal cation and as far apart from one another as possible.
  • The force of repulsion between metal d-electron and the ligand electrons cause to increase in potential energy of metal d-electrons. Remember that greater the force of repulsion higher will be the potential energy. If all the six ligands approaching the metal cation surrounds it spherically symmetric i.e. all the six ligands are at equal distance from each of the d-orbitals.
  • The energy of each d-orbital will raised by the same amount and all the five d-orbital will remain degenerate. This is a hypothetical situation and has the average energy of a set of d-orbitals.In an actual octahedral complex, a spherically symmetric field is never obtained. Therefore all the five d-orbitals are not affected by the same extent.
  • Since the two d-orbitals( dx2-y2 and  dz2 ) points directly towards the ligands and the three d- orbitals(  dxy ,dyz and dzx ) point in between the path of the approaching ligand. Therefore the  dx2-y2 and  dz2 orbitals will be more strongly repelled than the  dxy ,dyz and dzx orbitals. Therefore the energy of the dx2-y2 and  dz2 orbitals will be raised and that of the other three orbitals which lie far away from the ligand will be decreased relative to the hypothetical energy state.
               
  • The five d-orbital which were degenerate in a free metal cation is now split into two sets of d-orbitals of different energies, a higher energy level with two orbitals(dx2-y2 and  dz2)having the same energy and a lower level with three equal energy orbitals(dxy,dyz, and dzx). The set of   dx2-y2 and dz2orbitals are referred to as eset which is doubly degenerate and the set of dxy,dyz, and dzx is referred to as t2g set which is triply degenerate.
  • Since the distance between metal cation and the ligands has remained the same, the net potential energy(or average energy) of the system must remain the same as that of the spherical field before splitting. This state of average energy is called the barycentre.
  • The separation of five d-orbitals of metal cation into two sets of different energies is called crystal field splitting. The energy difference between two sets of orbitals which arise from an octahedral field is measured in terms of the parameter ∆0 or 10Dq where o in ∆0 stands for octahedral. 
  • Since the energy of barycentre remains constant, the total energy decrease of the t2g set must be equal to the total energy increase of the eg set. Therefore since there are two eg orbitals, they must increase by 0.6∆0 or 6Dq and the three t2g orbitals must decrease by 0.4∆0 or 4Dq per electron. The decreased energy t2g of orbitals stabilizes the complex by 0.4∆0 and the increase in energy of eorbitals destabilizes the complex by 0.6∆0.




Thursday, 20 December 2018

Crystal Field Theory

Crystal Field Theory:

          Valence bond theory is useful to visualize the bonding in complexes, but it fails to explain the nature of ligands, colour and electronic spectra, effect of temperature on magnetic moment and magnetic susceptibilities, why some complexes are high spin and others are low spin, stability of complexes.
To explain these properties Bethe and van Vleck proposed the crystal field theory.This theory was originally applied to ionic crystals and is therefore called crystal field theory.
This theory is based on the following assumptions:
  • Ionic ligands such as Cl-,OH-,CN- are regarded as negative point charges(or simply point charges) and the neutral ligands such as  H20,NH3,Py are regarded as dipole(or simply dipoles) because these ligands are dipolar.If the ligand is neutral molecule like the negative end of the dipole is directed towards the metal ion.
  • Metal-ligand bond is not covalent i.e. there is no overlapping of orbitals.Instead of bonding in complexes is purely electrostatic in nature.In complexes two types of  electrostatic forces come into account,
             (1)One is the attraction between metal cation and the negatively charged ligand or the                                negative end of the polar ligand(i.e. dipole)
             (2)The second type of electrostatic interaction is the electrostatic repulsion between the                            lone pairs of electrons on the ligands and the electrons in the d-orbital of the metal cation                    and the repulsion between nuclei of metal cation and the ligands but to a small extent.
  • Another repulsion also come into account that occurs among the ligands.
  • the five d-orbital in a free metal ion are degenerate(i.e.have same energy).When a complex is formed, the electrostatic field of ligands destroy the degeneracy of these d-orbitals i.e. these orbitals now have same energies.
  • The orbital lying in the direction of the lidands are raised in energy more than those lying away from the ligands because of the repulsion between the d-electrons and the ligands.
  • In order to understand CFT, it is necessary to know the geometry and orientation of the five d-orbitals.      

Monday, 17 December 2018

Hybridisation in outer orbital octahedral complex

Outer orbital octahedral complex:

        Let's take an example   [Fe(H2O)5(NO)]2+
  • In this complex ion oxidation state of Fe is +1, because NO exists in +1 oxidation state in a complex of Fe and it is valence shell electronic configuration is 3d6, 4s1
  • Magnetic moment measurement indicates that its experimental magnetic moment is 3.89 B.M. which corresponds to three unpaired electrons in the d-orbital of complexion.
  • The single NOstrong ligand has little tendency to pair up only two unpaired electrons. Since  H2O is a weak ligand, therefore, it has no tendency to pair up electrons and none of the five 3d orbitals is vacant. 
  • Therefore the  4s, 4p and two of the five 4d orbitals (i.e.4dx2-y2  and  4dz)combine to give six sp3d2 hybrid orbitals.
  • These hybrid orbital form bonds with six ligands by accepting six pairs of electrons, one pair from each of the six ligands.


  • Here as outer d-orbital is involved in hybridization it gives outer orbital octahedral geometry.

Note:

  • The octahedral complex of  d1, d2, and d3 metal cation are always inner orbital octahedral complexes whether the ligands are strong or weak.
  • The octahedral complex of d8, d9, and d10 metal cation are always outer orbital complexes either the ligands are strong or weak.
  • The complexes of d4, d5,d6, and d7 metal cation are outer orbital complex if the ligands are weak.

Inner orbital octahedral complex

Let's discuss inner orbital complexing taking an example, [Co(CN)6]3-  ion
  • In this complex oxidation state of cobalt is +3
             Valence shell electronic configuration is
  • Magnetic moment measurement indicates that  [Co(CN)6]3-C ion is diamagnetic(given). So that all the d-electrons arranged in such a way, that no unpaired electron is left in the d-orbital. 
  • In other words as CN-  ion is a strong ligand, it causes the pairing of 3d-electrons.
     
  •  Here two vacant 3d-orbital combine with the vacant 4s and 4p orbital to form six  d2sp3
     hybrid orbital. 
  • Then six hybrid orbital overlap with six filled orbital of  CN- ligand, one on each of the ligand and thus six coordinate covalent bonds are formed which gives d2sp3  hybridization.
  • As inner d-orbitals are involved in hybridization, it gives inner orbital octahedral geometry.

Test your understanding:

    Explain hybridisation in, 
     [Co(NO2)6]4-   (one unpaired electron) 
     [Mn(CN)6]3-    (two unpaired electron)
     [Cr(CN)6]3-C   (paramagnetic corresponding to three unpaired electron)

Friday, 14 December 2018

How to determine hybridization in coordination complex

To understand hybridization  let's take an example,  [Co(NH3)6]3+
Here it is clear that the coordination number of this complex is 6. So the complex must adopt octahedral geometry. The only thing we have to predict is whether it's hybridization is  sp3d2 ord2sp3

Steps to predict hybridization:

1. In the first step, we have to calculate the oxidation state of the metal ion. So the oxidation state of cobalt is +3.
2. Then predict whether the ligand is strong or weak and then according to this arrange electrons in the d-orbital.
3. If the ligand is strong, then pairing occurs from the initial condition(low spin complex) and if the ligand is weak then first all the d-orbital is singly filled and then pairing occur(High spin complex)
4. In the given example  NH is a strong ligand so that it will form a low spin complex.
    
5. From the above picture, we can see that  6 vacant orbitals of metal ion combine with 6   NHligands to give d2sp3  hybridization.
6. As the d-orbital present in the inner side, it is an inner orbital octahedral complex.
    For more details follow this link
           Hybridization in a coordination compound
           High spin and low spin complex

Wednesday, 12 December 2018

Hybridization in coordination compound

Hybridization in coordination compound:

       The main application of Valence Bond Theory is to predict hybridization in a coordination compound.
  • Hybridization is the concept of mixing of atomic orbital into a new hybrid orbital.
  • For example, if one s-orbital combine with one p-orbital it will form a new sp hybrid orbital.
          Like that, sp2hybrid orbital = one s-orbital + two p-orbital
                           sp3 hybrid orbital = one s-orbital + three p-orbital
                           sp2d hybrid orbital = one s-orbital + two p-orbital + three p-orbital
                           sp3d2 hybrid orbital =  one s-orbital + three p-orbital + two d-orbital
  • The coordination number of the metal atom or ion decide the hybridization and geometry of the complex.
          For example, if the coordination number is 6, then 6 hybrid orbitals combine among themselves and give  sp3dhybridization.
  • If the coordination number is 6 two possibilities is there i.e.
                    1. Inner orbital Octahedral complex( d2sp3hybridization.)
                    2. Outer orbital Octahedral complex( sp3d2hybridization.)
               But, both the hybridization leads to Octahedral geometry.
  • If the coordination number is 4 there is the possibilities of two types of geometry i.e.
                    1. Tetrahedral complex ( sphybridization)
                    2. Square planar complex( dsp2hybridization)

  • Outer orbital complex: 
          Here d-orbital of outermost shell or nth shell participate in bonding. So the hybridization will be sp3d(d-orbital present in outer side)
  • Inner orbital complex:
          Here d-orbital of inner shell or penultimate shell or (n-1)th shell participate in bonding. So the hybridization will be d2sp3 (d-orbital present in inner side)

Tuesday, 11 December 2018

High spin and Low spin complex

High spin and low spin complex are two possible classifications of spin states that occur in a coordination compound.
  • Before going to this topic we must have an idea about strong ligand and weak ligand. To know which ligand is strong and which ligand is weak, we must go through spectrochemical series i.e.
  • The spectrochemical series : (weak end)O22−< I < Br < S2− < SCN (S–bonded) < Cl− < N3 < F< NCO < OH < C2O42− < H2O < NCS (N–bonded) < CH3CN < gly (glycine) < py (pyridine) < NH3 < en (ethylenediamine) < bipy (2,2'-bipyridine) < phen (1,10-phenanthroline) < NO2 < PPh3 < CN < CO < CH2(strong end)
  • The ligands which are present on the left of the series are consider to be strong ligahds and those which are present on the right of the series are consider to be weak ligand 

High spin complex:

  • It is also called spin free complex.
  • The complex having a maximum number of unpaired electrons are called high-spin or spin-free complex.
  • In the high spin complex, first all the d-orbital are singly filled and then pairing occour .
  • Strong ligand i.e. ligands which are on the left of the spectrochemical series are always form high spin or spin free complex.

Low spin complex:

  • It is also called spin paired complex.
  • The complex having a minimum number of unpaired electron i.e. more number of paired electrons are called low spin or spin paired complex.
  • In a low spin octahedral complex pairing of d electrons take place from the initial condition.
  • Weak ligand i.e. ligands which are present on the right of the spectrochemical series always form low spin or spin paired complex.  

Sunday, 9 December 2018

State function and Path function

State Function:

  • In thermodynamics, a state function, state quantity, or a function of the state, is a property of a system that depends only on the initial and final state the system, not on the way in which the system acquired that state.
  • A state function describes the equilibrium state of a system and thus also describes the types of system.
  • The cyclic integral involving a state function is always zero.
  • All the thermodynamics property satisfy the requirements of state function.
  • U = q + w         change in thermodynamic energy
    S = qrev/T            entropy
    H = U = PV         enthalpy
    G = H – TS          Gibb’s free energy
    A = U – TS          Helmholtz free energy
  • Internal energy, enthalpy, entropy are the example of state functions.

Path Function:

  • Path function depends on the path taken to reach that specific value, not on the initial and final state of that value.
  • Path function needs multiple integral and limits of the integration in order to integrate.
  • It is based on how the state of a system was established.
  • Work, heat, arc length are the example of path function.

Saturday, 8 December 2018

Thermodynamic process

Thermodynamic process:

        It is the path or operation by which a system changes from one state to another state.

1. Isothermal process:

        The temperature of the system remains constant during each step.
        T = constant => dT = 0

     Details:

  • For such change system should be contained in a perfectly conducting container.
  • Perfect isothermal change is impossible but when a change is carried out very slowly approximate isothermal change occurs.
  • It follows Boyle's law.
  • Work done in the isothermal process is graphically given by the area under the P-V curve.
  • ∆H = nCp∆T  and  ∆H = nCv∆T
              In isothermal process ∆T = 0
                                             => ∆H = 0 and ∆E = 0
  • Specific change at constant T is infinitely great in the isothermal process.

2. Adiabatic process:

        There is no heat exchange between the system and surroundings.
             q = constant
        => dq = 0
        Perfectly adiabatic change is impossible but when a process is carried out very rapidly fairly
        approximate adiabatic change occurs.

3. Isobaric process:

              It is the system in which pressure of the system remains constant during each step.
              P = constant , dp = 0

4. Polytropic process:

         In this process heat capacity of the body remains constant.
            Cp = constant  and  Cv = constant

       => dCp = 0  and  dC = 0

5. Quasistatic process:

         The process in which the deviation from thermodynamic equilibrium is infinitesimal and all the states through which the system passes can be considered as equilibrium states.

6. Isochoric process :

         When there is no change in the volume of the system during various operations the change is said to be isochoric.
       V = constant  => dv = 0
    For example, the combustion of a substance in a bomb calorimeter is an isochoric process.

7. Cyclic process:

           The process which brings back a system to its original state after a series of changes is called acyclic process.
   As the E and H depend only on their states, E and H are constant.
   So, dE = 0 and  dH = 0


Friday, 7 December 2018

Reversiable and Irreversiable process

Reversible Process:

                   A thermodynamically reversible process is one in which all the changes occurring in any part of the process are exactly reversed when it is carried out in opposite direction.

Characteristic of the reversible process:

  • It has to be carried out in infinitesimal amount and hence require infinite time. Therefore changes in this process are very slow.
  • At all time during the process, the driving force for the change is opposed by a restraining force infinitesimally smaller than the driving force.
  • It can be reversed by an infinitesimal increase in opposing force.
  • The work produced in the reversible process is maximum.

Irreversible process:

                The process which occurs suddenly or spontaneously without the restriction of occurring in the successive stage of infinitesimal quantities.

Characteristic of the irreversible process:

  • In an irreversible process work done in the forward direction and in the backward direction are not equal.
  • If the initial and final stages are specified, the internal energy change would always be the same, whether the process has been affected by reversibly or irreversibly.
  • In an irreversible process, since the work done(dw) in two opposite direction are unequal, the heat transfer(dq)would also be unequal.
  • All the natural processes are irreversible.
                   e.g.=>  (1) flow of heat from high temperature to low temperature
                               (2) expansion of gas from high pressure to low pressure 

Thursday, 6 December 2018

VALENCE BOND THEORY

Why Valence bond theory is introduced?

             After the failure of VSEPR Theory scientists had to developed valence bond theory. VSEPR theory mainly fails to explain the nature of the simple molecule and geometry of the complex
molecule. Hence a new theory is developed which helps to explain the shape of atomic orbitals, electronic configuration of elements, overlapping and hybridization of atomic orbital, which is known as valence bond theory. The metal atom or ion must have vacant s, p, and d-orbital for the accommodation of electron donated by the ligand.

Postulates of Valence Bond Theory:

  • The suitable number of vacant orbital of comparable energies of metal undergoes hybridization. The orbital after hybridization have the same energy and these orbitals are called hybrid orbitals.
  • The hybrid orbitals overlap with the ligand to form coordinate bonds. Based on the pattern of overlapping, there are two types of covalent bonds i.e. sigma bond and a pi bond. The covalent bond formed by sidewise overlapping of atomic orbitals is known as pi bond whereas the bond formed by overlapping of atomic orbital along the inter nucleus axis is known as a sigma bond.

                                      Valence Bond Theory

  •  Ligands are classified into two categories i.e. weak ligand and strong ligand.
  • Strong ligands have the tendency to pair of the electrons whereas weak ligands have no such tendencies.

Limitations of Valence Bond Theory;

  • It could not explain the nature of the ligand whether the ligand is strong or weak.
  • It could not explain the pairing of electrons in the presence of strong ligand.
  • It could not explain colour and electronic spectra of complexes.
  • It could not explain the effect of temperature on the magnetic moment and magnetic susceptibility.
  • It could not explain the deviation of experimental magnetic moment from calculated by the spin only formula.
  • It could not explain the kinetics and mechanism of reaction in complexes.

Thursday, 22 November 2018

Optical isomerism in coordination compound

What is optical isomerism?

Optical isomers is also called enantiomers and these are the pair of molecules or ions that are non- superimposable mirror images of each other.
                                                   Image result for optical isomerism

What does the term superimposable mean?

The term superimposable means that if one structure is laid over the other of the same molecule the position of all the atoms should be matched and these two are called non-superimposable mirror image of each other.
  • For example, if a pipette is placed in front of a mirror, the image reflected on the mirror is identical to the pipette itself. So, in this case, we say that pipette and its mirror image are superimposable with each other.
  • If the left hand is placed in front of a mirror, the image reflected on the mirror will look like the right hand. Thus we can say that left and right hand are the mirror image of each other but are non-superimposable when the left hand is placed over the right hand keeping the palms down, they do not match. This superimposable property of left and right hand is called handedness.
    The optical isomers have handedness and are said to be chiral.

What are chiral molecules?

  • The molecules which are optically active and rotate the plane polarized light towards left or right are known as chiral molecules.
  • If the plane polarized light is rotated to the right, the isomer is said to be dextrorotatory(d or +) and if it is rotated to the left, the isomer is said to be levorotatory(l or -).The d- and l- isomer of a chiral substance are called enantiomers.
  • An equimolar mixture of d- and l- isomer, called a racemic mixture.
  • The essential condition for a substance to be chiral(or optically active) is the substance must have non-superimposable mirror image and it don't have any plane of symmetry.

Optical isomerism in the square planar complex:

Why square planar compound does not  show optical isomerism?

  •  Square planar complex rarely show optical  isomerism whether all the four ligands are different are same because they have  all the four ligands and the metal cation in the same plane and hence have a plane of symmetry. 
  • How ever there are exceptionally some complexes which exhibit optical isomerism i.e. (isobutylenediamine) (meso-diphenylethylenediamine) palladium (II) or palladium(II) complex.

Tuesday, 20 November 2018

Stereoisomerism

  • The isomer in which same type and number of ligands coordinated to the metal atom or cation but with different spatial(spatial arrangement means an arrangement in space)arrangements are called stereoisomers.
  • Stereoisomerism is classified into two types i.e.
    1. Geometrical isomerism
    2. Optical isomerism

Geometrical isomerism:

  • Stereoisomers in which relative position or orientation of the ligand is different i.e. donor atom around the central metal cation is different are called geometrical isomers and the phenomenon is called geometrical isomerism.
  • Geometrical isomers cannot be interconverted without breaking the metal-ligand(M-L) bond.
  • Geometrical isomerism is shown by that compound which can be converted into its cis and transform.
  • The isomer in which two particular ligands occupy the adjacent position of each other is called cis- isomer and the isomer in which two adjacent ligands occupy an opposite position to each other is called trans -isomer.
  • Cis and trans isomer are different compounds with different properties like melting point, dipole moment, solubility, colours, and chemical properties.
  • Geometrical isomerism is most common in complexes having coordination number 4 and 6 but the complexes having coordination number 2 and 3 do not exhibit geometrical isomerism.
  • cis/trans-2-butene









  • But in the coordination compound, this type of isomerism is found mainly in the heteroleptic complex because here multiple geometrical arrangements of ligand around the central metal atom are possible.
  • Square planar complexes are coordination compounds with coordination number 4 having [MX2L2] type formula, where X and L are unidentate ligands. The two ligands X could either be adjacent to each other in a cis isomer or opposite to each other to form a trans isomer.
    Square planar complexes with MABXL type formula show three isomers-two cis and one trans.
  • Tetrahedral geometry does not display these isomers. However, octahedral complexes do show cis and trans isomerism. In complexes with formula [MX2L4] type, we can have the X ligands in the arrangement of cis or trans to each other.
  • We also observe this type of isomerism when bidentate ligands L–L [e.g., NH2 CH2 CH2 NH2 (en)] are present in complexes with [MX2(L–L)2] type formula. There is another type of geometrical isomerism that we find in octahedral coordination entities with [Ma3b3] type formula. An example is [Co(NH3)3(NO2)3].

  • Facial and Meridional isomer:

    • Facial isomers are those in which three donor atom of the same ligand occupies an adjacent position at the corner of an octahedral face. They have the ligand in the cis arrangement.
    • And we get meridional isomer when the position of the ligands are around the meridian of the octahedron. Here the ligands are in the trans arrangement .

    Session Quiz:
    1. The number of possible isomer for the octahedral complex ion [Co(en)Cl2Br2]- is _______ .
    2.What is the number of isomer exist for [Mo(C5H5N)3(CO)3]  ?




     

Sunday, 18 November 2018

Structural Isomerism

Before discussing about this topic please go through the link  and get idea about isomerism .https://chemqueries.blogspot.com/2018/11/isomerism-in-coordination-compound.html

Classification of  Structural isomerism:

1.Ionisation isomerism:

  • In these isomer there is exchange of ligand between coordination sphere and ionization sphere.
  • Here the chemical formula is same, only the ligands present inside and outside the coordination sphere are exchanged.
  • These isomers give different ions when dissolved in water.
  • For example
    [Co(NH3)5Br]SO4   and [Co(NH3)5SO4  ]Br  and show ionization isomerism. Here we noticed that in both the compound only sulphate ion and bromide ion are among themselves.
  • Some other examples are  ,
    [Co(NH3)5Cl]SO4   and   [Co(NH3)5SO4]Cl 
    [Co(NH3)5NO3]SO4   and   [Co(NH3)5SO4]NO3 
    [Co(en)2Cl(NO2)]SCN   and   [Co(en)2Cl(SCN)]NO2 

2.Hydrate isomerism:

  • Hydrate means the word water, so in this type of isomerism only the water molecule is exchanged between coordination sphere and ionization sphere.
  • When we dissolve these isomers in aqueous solution, they give different colour.
  • For example,
    [Cr(H2O)6]Cl3 (violet)    and    [Cr(H2O)5Cl]Cl2.H2O (pale, green)
  •  Here we all notice one thing that, number of water and chlorine molecule is same    in both the  compound. The only difference is that in 1st compound all the water molecule is present inside the coordination sphere but in second one five is present inside and one is present outside the  coordination sphere.

3.Linkage isomerism: 

  • Linkage isomerism is shown by ambidentate ligand(ligand that have two different donor atom).
  • Linkage isomerism arises when an ambidentate ligand can coordinate to a metal cation through either of the two donor atoms.
  • Lets take an ambidentate ligand
    NO2-  ion, it has two donor atom N-atom and  O-atom, it can coordinate to the metal cation either through nitrogen atom or through oxygen atom.
  • The linkage isomers containing NO2- as ligand are
               [Co(NH3)5(NO2)]2+     ( N-atom coordinate to Co3+ )
       and  [Co(NH3)5(ONO)]2+    ( O-atom coordinate to  Co3+ )

4.Coordination isomerism:

  • The compound in which both cation and anion are complex ions and there nay be exchange of ligands between these two complex ions are known as coordination isomer.
  • In the pairs of these isomers,the central metal cation in the two coordination sphere may be same or different.
  • Some examples are 
    [Co(NH3)6][Cr(CN)6]    and      [Cr(NH3)6][Co(CN)6]
    [Cu(NH3)4][PtCl4]     and     [Pt(NH3)4][CuCl4] 

5. Coordination position isomerism:

  • This type of isomerism is formed by the bridging complex and there is a exchange of ligand between two metal cations.
  • For example,
2098_Coordination Position Isomerism.png
  •  Here we can see that only ligands are exchange in between the coordination so here and ionization sphere.

6. Ligand isomerism: 

  • If the ligand itself exist in two or more isomeric form, then the complexes containing such type of ligand show ligand isomerism.
  • For exampleCH3 – CH – CH2         and       CH3 – CH – CH2              
                                     |         |                            |                    |
                                   NH2   NH2                        NH2                 NH2
                                1,2-diaminopropane        1,3-diaminopropane

     Here we see that only the ligands are exchanged their position in order to show ligand isomerism.

7. Polymerization isomerism:


  • Polymers are not real isomers.
  • These type of isomers have same emperical formula but different molecular formula.
  • All these isomers have same ratio of metal atoms and ligands in them.
  • Coordintion polymers of Pt2 ion:
    Complex compound
    Number of  Pt2+
    Number of  NH3
    Number of  Cl-
    Ratio
    [Pt(NH3)2Cl2
    1
    2
    2
     1:2:2                 
    [Pt(NH3)4][PtCl4]
    2
    4
    4
    [Pt(NH3)4][Pt(NH3)Cl3]2
    3
    6
    6
    [Pt(NH3)3Cl]2[PtCl4]
    3
    6
    6

Friday, 16 November 2018

Isomerism in coordination compound

  •  Compound having same chemical formula but different arrangement in their constituent atoms are called isomers.
  • Due to the complicated formula of many coordination compound, their is a possibility of different types of bond and number of shapes, so different types of isomerism may occur. 
  • Since their atoms are arranged differently, therefore, isomers have different physical and chemical properties(colour, melting point, boiling point and solubility) with different reactivity also.
  • Coordination compound exhibit the same types of isomer as organic compound.
  • Isomers are mainly classified into two types i.e.
               1. Structural isomerism
               2. Stereo isomerism
  • Structural isomerism is further classified into seven types i.e.
               1. Ionisation isomerism
               2. Hydrate isomerism
               3. Linkage isomerism
               4. Coordination isomerism
               5. Coordination position isomerism
               6. Ligand isomerism
               7. Polymerization isomerism
  • And stereo isomerism is classified into two types i.e.
               1. Geometrical isomerism
                2. Optical isomerism
      I will update in detail about this topic in my next article.