Coordination Compounds : CHEM 197
What are we going to learn?
• Transition metal ion, complex ion, ligands
• Nomenclature of coordination compounds
• Coordination number and geometry
• Crystal field theory and splitting:
• Color of coordination compounds
• Magnetic properties of coordination compounds
Transition elements in periodic table:
Our interest: First row transition metals: Sc to Zn
M Config. M Config. M2+ Config. M Config.
Sc [Ar]4s23d1 Fe [Ar]4s23d6 Sc2+ [Ar]3d1 Fe [Ar]3d6
Ti [Ar]4s23d2 Co [Ar]4s23d7 Ti2+ [Ar]3d2 Co [Ar]3d7
V [Ar]4s23d3 Ni [Ar]4s23d8 V2+ [Ar]3d3 Ni [Ar]3d8
Cr [Ar]4s23d4 Cu [Ar]4s23d9 Cr2+ [Ar]3d4 Cu [Ar]3d9
Mn2+ [Ar]3d5 Zn [Ar]3d10
Mn [Ar]4s23d5 Zn [Ar]4s23d10
Note: (i) 4s gets filled in before 3d for neutral atom.
(ii) In ion forming, 4s gets ionized first before 3d
(iii) Cr and Cu are special (reason: half/full-filled d-orbitals)
Properties of Transition elements:
• high density (very hard)
• high melting point
• high boiling point
• high electrical conductivity
• low ionization energies
• Variable positive oxidation states (+2, +3 (most common),
+4,+5,+6 exist, highest oxidation states with O2, F2, Cl2)
• Form coordination compounds of varying colors.
Consist of two parts
Complex ion Counter ion
Consists of two parts
A central atom/ion (transition One/more ligand (s)
metal atom/cation) Two types
Bond formation momodentate polydentate
Coordinate covalent bonds: ligands donate electron-pair (Lewis base)
and central atom/ion accepts electron pair (Lewis acid)
1. [Cr(NH3)5H2O](NO3)3 : complex ion=[Cr(NH3)5H2O]3+
counter ions: NO3 (three of them)
central ion : Cr3+
ligands: five NH3 , one H2O
2. [Co(en)2Cl2]Cl : complex ion: [Co(en)2Cl2]+
counter ion : Cl-
central ion: Co3+
ligands: en (H2N – CH2 – CH2 – NH2), two Cl-
3. [Fe(CO)5]: complex ion: nothing, central atom: Fe, ligand: 5 CO
Monodentate: Ligand forming only one bond with the central ion.
This means ligand has only one tooth (dent)! This
uses only one electron pair to form bonds, others
are not pointing in the right direction. (Cl-, NH3, H2O,
Bidentate: Ligands forming two bonds with the central ion. They use
two of their electron pairs to form two bonds.
Tetradentate: Ligands forming four bonds with a central ion.
Example: Porphyrin in Hemoglobin.
are called chelating
(i) Cation is named first, then anion.
(ii) In a complex ion, ligands are named first, in alphabetical order.
Then the central atom/ion is named.
(iii) Anionic ligands end with the letter ‘o’. Neutral ligands get the
name of the molecule. (H2O, CO, NH3 get special names, aquo,
(iv) di-, tri-, tetra, penta-, hexa- for # of same ligands.
(v) Oxidation # of the central ion (I), (II), (III), …..
(vi) Anionic complex ion ends with –ate.
1. [Cr(NH3)4Cl2] Cl
Central ion Complex ion, a cation
2. K3[Fe(CN)6] Potassiumhexacyanoferrate(III)
Coordination number (CN) and structure:
CN= # of donor atoms surrounding the central atom/ion.
2 linear L–M–L
4 Tetrahedral or
1. [Co(NH3 )4(NO2)Cl]+
CN=6,octahedral, the coplex has ligands: 4 NH3, 1 NO2- , 1 Cl-
Oxidation # of central ion = 3+
2. tri-ethylenediamine cobalt(III) [Co(en)3]3+
en=H2N – CH2 – CH2 – NH2
Crystal Field Theory (CFT)
1. Characteristics of coordination complexes:
CFT is the first theory that explains two of the most
important characteristics of coordination compounds.
(i) Why many coordination complexes are colored?
(ii) Why many coordination complexes are paramagnetic?
2. Types of electrostatic interactions:
Attraction: central ion’s + charge Repulsion: d-orbital electrons and
ligands’ electrons ligands’ electrons
(This is the binding force.)
3. Repulsion and splitting of d-orbitals
Crystal field theory focuses on splitting of the d-orbitals of the
central atom/ion. The splitting is the result of electrostatic
repulsion between the electrons in the ligands and the electron
in the d-orbitals of the central transition metal ion.
What is splitting?
Splitting means removal of degeneracy of d-orbitals. In free ion,
5 d-orbitals have same energy (this is called degeneracy).
In presence of the ligands, the five d-orbitals split into different groups
of varying energy. This grouping of d-orbitals is called splitting.
What does splitting depend on?
Splitting depends on two things
Geometry or coordination # (CN) Types of ligands
of the central ion
Octahedral splitting Tetrahedral splitting Square planar
(CN=6) (CN=4) Splitting (CN=4)
Octahedral splitting: (CN=6): The following diagram shows
the orientation of five d-orbitals and the positions of 6 ligands
for an octahedral geometry.
What do we see from the above diagram?
1. Six green circles represent six lone-pairs of six ligands.
2. Ligands are pointed directly at the x-, y- and z-axis.
3. dx2-y2 and dz2 are pointed directly at the x-, y- and z-axis.
4. dxy, dxz and dyz are NOT pointed directly at the x-, y- and
What do these observations mean?
• dx2-y2 and dz2 orbitals’ electrons will experience larger
repulsion than others. So, the energies of dx2-y2 and dz2 will
be higher than that of dxy, dxz, dyz. This is splitting.
Octahedral splitting energy diagram:
Free ion in octahedral
= energy gap, called crystal field splitting.
x+y=, 2y – 3x = 0 CFSE=(n1*3/5-n2*2/5)
x=2/5 and y=3/5 n1=# of el in eg ; n2=# of el in t2g
Magnitude of Splitting ( )
1. Metal: Larger metal larger
Higher Oxidation State larger
2. Ligand: Spectrochemical series
Spectrochemical series: Increasing
I-<Br-<Cl-< F- < H2O < NH3 < en < CN- < CO
(Weak field Ligands) (High field ligands)
(small ) (large )
High-spin complexes Low-spin complexes
A. Compare splitting in [FeF6]3- and [Fe(CN)6]3-
F- is a weak-field ligand, CN- is strong field ligand.
Central ion is Fe3+ , electronic configuration: d5
Strong paramagnetic CFSE=-2.0
CFSE=0 Weak paramagnetic
B. Compare splitting in [CoF6]3- and [Co(CN)6]3-
F- is a weak-field ligand, CN- is strong field ligand.
Central ion is Co3+ , electronic configuration: d6
How do we measure ?
• is the amount of energy needed to excite an electron from
the lower energy orbital to a higher energy orbital.
c=speed of light
l=wavelength of absorbed light
• So, the wavelength of the light that the compound absorbs
tells us how large/small is.
Color of coordination complex ions:
• When white light (which has seven colors) falls on a compound, it
absorbs light of particular frequency corresponding to D. Every
frequency corresponds to a definite color.
• Every absorbed color has its complimentary color.
• Color that we see is the complimentary color of the absorbed light.
Wave length (A) Absorbed color Complimentary
6800 red green
6100 Orange blue
5800 Yellow Indigo
4300 Blue yellow
Colors of some Co3+ complexes:
Complex Ion Wavelength of Color of Light Color of Complex
light absorbed Absorbed
[CoF6] 3+ 700 (nm) Red Green
[Co(C2O4)3] 3+ 600, 420 Yellow, violet Dark green
[Co(H2O)6] 3+ 600, 400 Yellow, violet Blue-green
[Co(NH3)6] 3+ 475, 340 Blue, violet Yellow-orange
[Co(en)3] 3+ 470, 340 Blue, ultraviolet Yellow-orange
[Co(CN)6] 3+ 310 Ultraviolet Pale Yellow
Tetrahedral splitting, CN=4:
• t, the tetrahedral splitting is smaller than octahedral splitting
because of the interactions with 4 ligands, and the interaction
is not direct. t = (4/9) o
• So, the tetrahedral complex ions are usually high-spin complexes.
[Ni(CN)4]2- : CN=4, Oxidation # = 2+, e-configuration: d8
• unpaired electron (s) = paramagnetic
• no unpaired electron = diamagnetic
• the more # of unpaired electrons, the more paramagnetic the
• Measure of paramagnetism = Magnetic moment
• Magnetic moment = [n(n+2)]1/2 B
[CoF6]3- has magnetic moment = (4(4+2))1/2 = (24)1/2 =4.9 B
[FeF6]3- has magnetic moment = (5(5+2))1/2 = (35)1/2 =5.9 B