Tuesday, 11 December 2018

High spin and Low spin complex

High spin and low spin complex are two possible classification of spin states that occour in coordination compound.
  • Before going to this topic we must have 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 maximum number of unpaired electrons are called highspin or spinfree complex.
  • In 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 minimum number of unpaired electron i.e. more number of paired electrons are called low spin or spin paired complex.
  • In 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 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 need multiple integral and limits of the integration in order to integrate.
  • It is based on how 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:

        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 isothermal process is graphically given by area under 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 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 a 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 depends only on their states, E and H are conatant.
   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 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 then the driving force.
  • It can be reversed by infinitesimal increase in opposing force.
  • The work produced in reversible process is maximum.

Irreversible process:

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

Characteristic of irreversible process:

  • In an irreversible process work done in the forward direction and in the backward direction are not equal.
  • If initial and final stages be specified, the internal energy change would always be same, whether the process has been affected 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 valance 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 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 ligand have the tendency to pair of the electrons where as 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 magnetic moment and magnetic suscesptibility.
  • It could not explain the deviation of experimental magnetic moment from calculated by 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 it's 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 mirror image of each other but are non-superimposable when 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 isomer 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 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 ligand coordinated to the metal atom or cation but with different spatial(spatial arrangement means arrangement in space)arrangements are called stereoisomers.
  • Stereo isomerism 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 caton is different are called geometrical isomers and the phenomenon is called geometrical isomerism.
  • Geometrical isomers cannot be interconverted without breaking metal-ligand(M-L) bond.
  • Geometrical isomerism is shown by that compound which can be converted into its cis and trans form.
  • 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 ligand occupy opposite position to each other is called trans -isomer.
  • Cis and trans isomer are different compound 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 coordination compound ihis thpe of isomerism is found mainly in heteroleptic complex because because here multiple geometrical arrangement of ligand around the central mteal atom is 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 isomer are those in which three donor atom of the same ligand occupy 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]  ?