1.
Coordination Complexes:
Bonding & Magnetism
Dr. Anjali Devi J S
Assistant Professor (Contract Faculty), Mahatma Gandhi University, Kerala

2.
Bonding in coordination compounds
• Werner’s theory- primary secondary valence- Alfred Werner
• Valence Bond Theory (Linus Pauling in 1930s)
• Crystal field theory (Hans Bethe in 1929)
• Ligand Field Theory
• Molecular orbital theory

3.
1. Valence Bond Theory
Assumptions
1. Formation of a complex involves reaction between Lewis bases
(ligands) and Lewis acid (central metal atom or metal ion) with the
formation of coordinate covalent (or dative) bonds between them .
2. The model utilizes hybridization of metal s, p, and d, valence
orbitals to account for the structure and magnetic properties of
complexes.
Fe NH3

4.
Coordination Complex geometry
• Complex geometry can be linked to orbital hybridization.
Coordination
number
Geometry Hybrid orbitals
2 Linear sp
4 Tetrahedral sp3
4 Square planar dsp2
6 Octahedral d2sp3 or sp3d2

5.
Demonstration:
Structure of Ni(II) complexes –[Ni(Cl)4]2-

6.
Structure of Ni(II) complexes –[Ni(CN)4]2-

7.
Valence Bond Theory-Magnetism
And
Diamagnetic
And
Paramagnetic

8.
Strong & Weak field Ligands

9.
Describe the bonding in
(a) Ni(NH3)6]2+,
(b) Pd(NH3)6]2+ and
(c) Pt(NH3)6]2+
with valence bond theory.
Question

10.
2. Crystal field theory
• Ligand lone pair is modelled as a point negative charge (or as the
partial charge of an electric dipole) that repels electrons in the d
orbitals of central metal ion.
• The resulting splitting of the d orbitals into groups with different
energies , and uses that splitting to rationalize and correlate the
optical spectra, thermodynamic stability, and magnetic properties of
complexes.
Purely electrostatic
interaction

11.
d orbitals

12.
Crystal field splitting

13.
Crystal Field Splitting

14.
Crystal field theory
• In the presence of an octahedral crystal field, d orbitals are split into a
lower energy triply degenerate set (t2g) and a higher energy doubly
degenerate set (eg) separated by an energy Δo; the ligand field
splitting parameter increases along a spectrochemical series of
ligands and varies under the identity and charge of the metal atom.

15.
Crystal field theory

16.
Crystal Field Theory (CFT)

17.
• The ligand field strength depends on ligand (spectrochemical series)
• The ligand field strength depends on identity of central metal atom.
• The values of Δo increases with increase in oxidation state (compare
Co spexcies and Fe species).
• And Δo increases down the group (see Co, Rh and Ir)
Mn2+ < Ni2+<Co2+ <Fe2+<V2+<Fe3+> Co3+ <Mo3+< Rh3+ <Ru3+<Pd4+<
Ir3+ <Pt4+
Factors affecting crystal field splitting
parameter, Δo

18.
Crystal field stabilization energy (CFSE)
In the d1 case: t2g
1
It has an energy of -0.4 Δo relative to the barycenter of the d orbital.
For d2 :t2g
2
The electron obey Hund’s rule and occupy different degenerate t2g
orbitals, which has an energy of -0.4 Δo relative to the barycenter of
the d orbital.
System Configuratio
n
CFSE
d1 t2g
1 0.4 Δo
d2 t2g
2 0.8 Δo
d3 t2g
3 1.2 Δo

19.
In the d4 case:
(1) For Δo< pairing energy(P) { weak field or high spin condition}
t2g
3eg
1
CFSE= (3X+0.4 Δo) –(1X+).6 Δo )=0.6 Δo ]
relative to the barycenter of the d orbital.
(2) For Δo> pairing energy(P) { strong field or low spin condition}
t2g
4eg
0
Crystal field stabilization energy (CFSE)

20.
• Determine the CFSE for the following octahedral ion:
(a) d3
(b) High spin d5
(c) Low spin d6
(d) d9
Question
(a)1.2 Δo
(b)0
Answer
(c)2.4 Δo-2P
(d) 0.6Δo

21.
Crystal field stabilization energy of high spin
octahedral complexes
dn Example N (high spin
complexes)
CFSE/ Δo
d0 Sc3+ 0 0
d1 Ti3+ 1 0.4
d2 V3+ 2 0.8
d3 Cr3+ 3 1.2
d4 Cr2+ 4 0.6
d5 Mn2+, Fe3+ 5 0
d6 Fe2+ 6 0.4

22.
Crystal field stabilization energy of low spin
octahedral complexes
dn Example N (high spin
complexes)
CFSE/ Δo
d4 Cr2+ 2 1.6-P
d5 Fe3+, Mn2+ 1 2.0 -2P
d6 Fe2+ 0 2.4-2P
d7 Co2+ 1 1.8-P

23.
Six negative charges arranged octahedrally
around a central metal ion- Visual

24.
Tetrahedral arrangement of four negative
charges around a cation - Visual
∆𝑡 =
4
9
∆𝑜

25.
Energy level diagram showing splitting of a set of d
orbitals by octahedral and tetrahedral crystal field.

26.
Tetragonally distorted Octahedral complex
Octahedral array of
ligands becomes
progressively distorted
by the withdrawal of
two trans ligands,
especially those lying on
the z axis.
For a square pyramidal (spy) set of ligands, the
splitting diagram has to be qualitatively similar to
tat of square set.

27.
Trigonal bi pyramidal complex
• The tbp has D3h symmetry.
• Taking 3-fold axis as z axis,
• dz2,
• dxy, dx2-y2
• dxz, dyz

28.
Magnetic Properties

29.
Bohr magnetons
• The magnetic moments of atoms, ions, and molecules are expressed
in units called Bohr magnetons (B.M.)
• 1 𝐵. 𝑀. =
𝑒ℎ
4𝜋𝑚𝑐

30.
Magnetic moment of electron
• The magnetic moment 𝜇𝑠 of a singe electron is given by the equation,
𝜇𝑠 (𝑖𝑛 𝐵. 𝑀. )= g 𝑠(𝑠 + 1)

31.
Question
• For a free electron, g has the value 2.00023 which may be taken as
2.00 for most purpose. Find spin magnetic moment of one electron.
• 𝜇𝑠 (𝑖𝑛 𝐵. 𝑀. )= g 𝑠(𝑠 + 1)
• Answer: 𝜇𝑠 (𝑖𝑛 𝐵. 𝑀. )= 2
1
2
(
1
2
+ 1) =
• 3 = 1.73

32.
Magnetic moment of Metal ions-Special Case
• MnII , FeIIIand GdIII (the ions whose ground states are S states) :
There is no orbital angular momentum even in the free ion. There
cannot be any orbital contribution to the magnetic moment. The
observed magnetic moments agrees well with spin only values.

33.
The transition metal ion with in their ground state D, or F being most
common, do possess orbital angular momentum.
𝜇𝑆+𝐿 = g 4𝑆 𝑆 + 1 + 𝐿(𝐿 + 1)
Magnetic moment of First Series Transition
Metal ions

34.
The observed values of 𝜇 frequently exceeds
𝜇S but seldom are as high as 𝜇S+L
• Because, the metal ions on its compounds restricts orbital motion of
the electrons so tha the orbital angular momentum are wholly or
partially quenched.

35.
Temperature independent paramagnetism
(TIP)
• In many systems that contain unpaired electrons, as well as in a few, eg,
CrO4
2-, that do not , weak paramagnetism that is independent of
temperature can arise by a coupling of the ground state of the system with
excited state of high energy under the influence of the magnetic field.
• This TIP resembles diamagnetism in that it is not due to any magnetic
dipole existing in the molecule but is induced when the substance is placed
in the magnetic field
• It also resembles diamagnetism in its order of magnitude 0-500 x 10-6 cgs
units per mole

36.
High spin Low spin crossovers
• Spin crossover , sometimes referred to as spin transition or spin
equilibrium behavior, is a phenomenon that occurs in some metal
complexes wherein spin state of the complex changes due to external
stimuli such as variation of temperature, pressure, light irradiation or
influence of magnetic field.

37.
Spin Crossover
High
Spin
Low
Spin

38.
Spin crossovers
• This phenomenon is commonly observed with some first row
transition metal complexes with a d4-d7 electron configuration in
octahedral ligand geometry.

39.
∆= 𝑃
High spin and
low spin states
have same
energy
High spin and
low spin states
can coexist in
equilibrium
Spin state
equilibrium
Spin crossovers

40.
Thank You