Question

Draw and label crystal field splitting diagrams (including orbital labels) for the following complexes. Calculate he ligand field stabilization energy as a factor of Delta for each complex.

a. [Co(OH2)6]2+

b. [RH(NH3)6]2+

c. [NiCl4]2-

Answer 1

First of all calculate the oxidation state of central atom. Assume the oxidation state to be x for central metal and add the charge of all other atoms present in complex. This sum is equal to the charge present present on the complex. If no charge is present then sum is equal to zero.

In first complex central metal atom is cobalt. 6 H₂O ligands are attached to the complex. So, it is an octahedral complex. In octahedral complex, when ligands approach metal atom the five degenerate d orbitals split into two sets namely t_2g and e_{g .} t_2g is a triply degenerate orbital and is of low energy whereas e_g is doubly degenerate orbital and is of high energy. t_2g orbital is lowered by 0.4 delta O and eg orbital is raised by 0.6 delta O. The splitting of d orbitals into two sets of different energy is called crystal field splitting. The parameter used to determine crystal field splitting is deta O or Dq.

Calculating oxidation state of central atom

In a first complex

H₂O is a neutral ligand . Let us consider the oxidation state of cobalt is x.

x +6×0 = +2

x +0 = +2

x = 2 - 0 = +2

So, Co is in +2 oxidation state.

b) second complex is also octahedral because 6 ammonia ligands are attached to central metal atom rhodium. NH₃ Is a neutral ligand .

Oxidation state of central atom

x +6× 0 =+2

x + 0 = +2

x =2-0 = +2

Thus, oxidation state of Rh is +2

(c) third complex is tetrahedral because 4 ligands are attached to nickel.  

Oxidation state of nickel

x + 4× (-1) = -2

x - 4 = - 2

x = - 2 +4

x = +2

Thus, oxidation state of nickel is +2

In tetrahedral complex splitting of d orbitals is opposite to that of octahedral . e_g orbital is of low energy and t_2g orbital is of high energy. Thus eg orbital lies belows t₂g orbitals. The energy of eg orbital is lowered by o. 6 delta t and energy of t_2g is raised by 0.4 delta t

Next step is to look whether the ligand attached to the metal is strong field or weak field. This can be identified with the help of spectrochemical series. According to the series, water ammonia and chloride are weak field ligands. ligands whose orbitals interact weakly with the metal orbitals are called weak field ligands and splitting between t_2g ​​​​​​and e_g orbitals  is small. Moreover, number of unpaired electrons are more in complexes containing weak field ligand. Such complexes are called high spin complexes. In case of second complex Rh belongs to second transition series i. e. 4d series. Complexes of second and third transition series form low spin complexes irrespective of the type of ligand attached to central atom. In case of high spin complex first fill the electron singly in low energy orbital and then in high energy orbital .if electrons are still left, then electrons can be paired. In low spin complex after filling the electron singly in low energy orbital, pairing of electron should be done in low energy. If still electrons are left then fill the high energy orbitals. For example in second complex Rh forms low spin complex, first 6 electrons are filled in t_2g and then the electron left is filled in e_g whereas first complex is high spin. In this case also 7 electrons are to be filled in t_2g and e_g​​​​​​. However, here first electrons are filled singly i.e. 3 in t_2g and 2 in e_g  ​​​​​​. Still 2 electrons are left so now pairing of electrons will take place.

Splitting in tetrahedral complexes is less than octahedral because number of ligands are less in tetrahedral.

Answers : a) crystal field splitting energy ( CFSE) IN DELTA O

CFSE= - 0.80 delta O

b) CFSE = - 1.80 DELTA O +P

C) CFSE = - 0.8delta t