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UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 1
UNIT IV
THEORIES OF BONDING IN COMPLEXES AND CRYSTAL FIELD THEORY
VALENCE BOND (VB) THEORY:
Salient features of the VBT theory: The central metal atom (or) ion has the required number
of vacant orbitals for accommodating the electrons donated by the ligands. The number of
vacant orbitals is equal to the coordination number of the metal ion for a particular complex.
Vacant orbital s, p, d, f orbital. This vacant orbital goes hybridization to form same no. of
hybrid orbitals. Each ligand has at least one orbital containing lone pair of electrons. Vacant
hybrid orbital filled with ligand to form coordination bond. Coordinate bond is stronger if the
overlapping between the orbitals is greater.
The vacant orbitals of the metal atom (or) ion undergo suitable hybridisation to yield a set of
equivalent hybrid orbitals of definite geometry. Geometrical shape depends upon the
hybridization of the metal orbital. If (n-1) d orbitals are used for hybridization, the complexes
is called inner complexes and ‘nd’ called outer complexes.
Structure of complex compounds based on VBT 1. Structure of nickel tetracarbonyl
[Ni(CO)4] 2. Formation of [NiCl4]2-
3. Structure of [Ni(CN)4]2-
4. Structure of [CoF6]3-
5.
Structure of [Co(NH3)6]3+
GEOMETRY OF COMPLEX COMPOUND:
UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 2
OCTAHETRAL COMPLEX ION
SQUARE PLANAR COMPLEX ION
UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 3
TETRAHEDRAL COMPLEX ION ION
Drawbacks of the valence bond theory: a) Does not explain the colour of coordination
compound. b) cannot explain magnetic behavior based on geometry. c) does not explain why
some called inner and outer complex same metal ion in the same oxidation state. d) Fails to
predict the exact geometry of the complexes with the coordination number four. e) Doesnot
distinguish weak field and strong field of ligand. f) It cannot predict exactly the tetrahedral
and square planar structure of 4-cordinate.
CRYSTAL FIELD THEORY
Silent features of CFT: Central metal atom or ion is surrounded by various ligands which are
either negative charge or neutral molecule. Electrostatic interaction between ML. Eg. Fe- and
Co3+.
Central metal atom have 5 degenerated orbital dxy, dyz, dxz, d(x2-y2), dz2. The ligands
approach the metal ion, due to repulsion forces, the degeneracy of d- orbitals is destroyed and
they split into two groups of different energy level t2g and eg orbital. This effect is called
crystal field splitting(∆0 or10Dq). Due to repulsion, the orbitals along the axes of ligands
acquire higher energy while those lying in between the ligands acquire less energy. • It doent
show the overlapping. • From the Crystal field stability energy the stability of the complexes
can be known.
UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 4
Definition: Crystal field splitting- Splitting of 5 degenerated d-orbital. Crystal filed
stabilization energy (CFSE) - Change in energy achieved by filling up electron in orbital in
complex metal atom. High spin complex-(spin free) Greater no. of unpaired electrons and
hence higher value of resultant spin and magnetic moment is called. • Low spin complex •
Pairing energy- The energy required to pair 2 electron against the electron electron repulsion
in the same orbital of a metal atom.
Application of CFT to tetrahedral complexes • In the tetrahedral complex, [MX4]n, the metal
atom or ion is placed at centre of the regular tetrahedron and the 4 ligands, are placed at four
corners of the tetrahedron. • Ligand approach the central Metal atom in between 3 cordinate
x, y, z.
In case of strong field ligands, the electrons prefer to pair up in eg orbital giving low spin
complexes while in case of weak field ligands, the electrons prefer to enter higher energy t2g
orbitals giving more unpaird electrons and hence form high spin complexes.
Application of Crystal field theory to octahedral complexes [MX6]n the metal atom or ion is
placed at the centre of regular octahedron while 6 ligands Occupy the positions at 6 vertices
of the octahedron. • Two orbital dx2-y2 and dz2 are Axial greater repulsion and dxy, dyz, dxz less
repulsion.
UNIT –IV INORGANIC CHEMISTRY BY Dr. A. NILAMADANTHAI, DEPT. OF CHEMISTRY GASCWKM Page 5
Five d orbital lose degenracy and split into two point group. The group t2g lower
energy while eg group have higher energy. Experimental calculation show that the energy of
t2g orbital is lowered by 0.4∆0 or 4Dq and energy of eg orbital is increased by 0.6∆0 or 6Dq.
Thus energy difference between t2g and eg orbitals is ∆0 or 10Dq which is crystal field
splitting energy. CFSE increases with the increasing strength of ligands and oxidation state of
central metal ion.
Limitation of crystal field theory: Does not explain the s and p orbital. Does not explain bi-
bonding. Cannot explain partly covalent nature of the metal ligand bond.
Spectrochemical series: The decreasing order of field strength of some of the ligands is, CO >
CN > NH3 > EDTA > H2O > OH- > F- > S2- > Cl- This series depends on the power of
splitting the d orbitals and is called spectrochemical series.