B.Sc.
(H) Chemistry
Semester - IV
Core Course - VIII (CC-VIII)
Inorganic Chemistry - III
I. Coordination Chemistry
16. Coordination Numbers and Structures
Dr. Rajeev Ranjan
University Department of Chemistry
Dr. Shyama Prasad Mukherjee University, Ranchi
Coordination Chemistry: 20 Lectures
Werner’s theory, valence bond theory (inner and outer orbital complexes), electroneutrality
principle and back bonding. Crystal field theory, measurement of 10 Dq (Δo), CFSE in weak
and strong fields, pairing energies, factors affecting the magnitude of 10 Dq (Δo, Δt).
Octahedral vs. tetrahedral coordination, tetragonal distortions from octahedral geometry
Jahn-Teller theorem, square planar geometry. Qualitative aspect of Ligand field and MO
Theory.
IUPAC nomenclature of coordination compounds, isomerism in coordination compounds.
Stereochemistry of complexes with 4 and 6 coordination numbers. Chelate effect, polynuclear
complexes, Labile and inert complexes.
Coverage:
1. Coordination Numbers and Structures
2
Dr. Rajeev Ranjan
Coordination Numbers and Structures
• Factors considered when determining structures.
The number of bonds. Bond formation is exothermic; the
more the better.
VSEPR arguments
Occupancy of d orbitals.
Steric interference by large ligands.
Crystal packing effect.
It may be difficult to predict shapes.
3
Dr. Rajeev Ranjan
Coordination Numbers and Structures
Low Coordination Numbers (C.N.)
• C.N. 1 is rare except in ion pairs in the gas phase.
• C.N. 2 is also rare.
[Ag(NH3)2]+, Ag is d10 (how?)
VSEPR predicts a linear structure.
Large ligands help force a linear or near-linear arrangment.
• [Mn(N[SiMePh2]2)2].
• C.N. 3 is more likely with d10 ions.
Trigonal-planar structure is the most common.
[Cu(SPPh3)3]+, adopts a low C.N. due to ligand crowding.
4
Dr. Rajeev Ranjan
Coordination Numbers and Structures
Coordination Numbers 4
• Tetrahedral and square planar complexes are the most
common.
Small ions and/or large ligands prevent high coordination
numbers (Mn(VII) or Cr(VI)).
• Many d0 or d10 complexes have tetrahedral structures (only
consider bonds).
MnO4- and [Ni(CO)4]
Jahn-Teller distortion
5
Dr. Rajeev Ranjan
Coordination Numbers and Structures
Coordination Numbers 4
• Square-planar geometry
d8 ions (Ni(II), Pd(II), and Pt(III))
• [Pt(NH3)2Cl2]
The energy difference between square-planar and
tetrahedral structures can be quite small.
• Can depend on both the ligand and counter-ion.
6
Dr. Rajeev Ranjan
Coordination Numbers and Structures
Coordination Number 5
• Common structures are trigonal bipyramid and square
pyramid.
The energy difference between the two is small. In many
measurements, the five ligands appear identical due to
fluxional behavior.
How would you modify the experiment to differentiate
between the two structures?
• Five-coordinate compounds are known for the full range of
transition metals.
7
Dr. Rajeev Ranjan
Coordination Numbers and Structures
Coordination Number 6
• This is the most common C.N. with the most common
structure being octahedral.
If the d electrons are ignored, this is the predicted shape.
• [Co(en)3]3+
• This C.N. exists for all transition metals (d0 to d10).
8
Dr. Rajeev Ranjan
Coordination Numbers and Structures
Distortions of Complexes Containing C.N. 6
• Elongation and compression :
Produces a trigonal antiprism structure when the angle
between the top and bottom triangular faces is 60.
Trigonal prism structures are produced when the faces
are eclipsed.
• Most trigonal prismatic complexes have three
bidentate ligands (Figure 9-30).
• interactions may stabilize some of these structures.
The Jahn-Teller effect is useful in predicting observed
distortions.
9
Dr. Rajeev Ranjan
Coordination Numbers and Structures
Higher Coordination Numbers
• C.N. 7 is not common
• C.N. 8
There are many 8-coordinate complexes for large
transition elements.
• Square antiprism and dodecahedron
• C.N.’s up to 16 have been observed.
Thank You
10
