Organometallic infra red notes

1. Infrared in Organometallic
compounds
IR-basics
Index

2. Introduction
IR is one of the first technique inorganic chemists used (since 1940)
Molecular Vibration
Newton’s law of motion is used classically to calculate force constant
r
re
F
F The basic picture : atoms (mass) are
connected with bonding electrons. Re is
the equilibrium distance and F: force to
restore equilibrium
F(x) = -kx where X is displacement from equilibrium

3. Analyzing inorganic molecules by IR
• With IR, we might be able to determine the number of atoms in a
group
Distinguish MX2 and MX3 groups
• We might distinguish monodentate from bidentate sulfate
• We might distinguish terminal from bridging CO ligands
• We can use variation in CO stretching frequency in metal carbonyl
to make deduction about electronic nature of the other ligands

4. Bond Stretching Frequencies
1. Stretching Frequency is lower for heavier atoms
2. Stretching Frequency is lower for weaker bonds
3. Stretching Frequency vary over a narrow range
for a set of related compounds
Vibration frequency of a bond depends on the mass of
bonded atoms and on the force constant of the bond
General Principle:

5. Bond Stretching Frequencies: Hydrogen
Hydrogen: all bond stretch occur in the range:
4000 to 1700 cm-1 (for H-F down to H-Pb)
Going down any main group in periodic table increase the mass
And decrease the bond strength
=> Lowering stretching Frequency
From Left to right along a row: the effect of increasing the mass is
outweighed by the increase in Bond strenght
=> Frequency increase

6. Bond Stretching Frequencies: Hydrogen
Increase (cm-1)
Decrease (cm-1)

7. Bond Stretching Frequencies: other nuclei
Stretching of bonds not involving Hydrogen are lower (below 1000 cm-1)
Except for multiple bond with higher force constant
Or for single bond involving nuclei in the first row (C-F, B-O)

8. Bond Stretching Frequencies: Carbonyl
• Terminal CO correlate with electron-righness
of the metal
• Backbonding from the d-orbital of the metal to
the p* antibonding orbital weaken CO bond
=> lower stretching frequency (from free CO)
Important group of frequencies is due to Carbonyl ligand in Metal
complex

9. Bond Stretching Frequencies: Carbonyl
Co (CO)(NO)(PClXPh3-X)2
Table illustrating how the electronegative Chlorine on Phosphorus
ligand decrease the electron density on Cobalt (central atom)
Decreasing d -> p* backbonding  raising CO and NO

10. Patterns of group Frequencies: Carbonyl
Clearly defined group frequencies like CO are very important in
determining how many of the group occur in each molecule and
symmetry relationship between them
• There is 1 stretching mode for each bond in a molecule
in principle we can count the number of CO frequencies
(caution as some vib. Might not be active in IR)
• Symmetry relating equivalent groups govern the activity of various
stretching mode in IR and Raman
• If there is a rotation axis relating three or more CO ligands, the
number of bands will be less than the number of ligands: some are
degenerated,

11. Patterns of group Frequencies: Carbonyl
M
L
CO
CO
OC Has only 2 CO bands: provided that the
ligand preserves M(CO)3 3 fold symmetry
M
L
L
CO
OC M
OC
OC
cis-octahedral complex
The 2 CO trans to each other can be treated together
The 2 CO cis to each other can be treated together
There are Sym. And Asym stretch for both groups
=> Therefore there are 4 CO stretch expected
M
L L
CO
CO
M
OC
OC
trans-octahedral complex
The 4 CO are all related by symmetry
=> There is only one active vibration in IR

12. Group Frequencies: Type of Binding
Many ligands hae different modes of binding to other atoms
Terminal
M CO
Bridging
M
CO
M
M
CO
M
M
Triple Bridge
2130 – 1700 cm-1
1900 – 1780 cm-1
1900 – 1780 cm-1
We can therefore state: CO above 1900 => terminal CO
Below 1900 : Can be due to bridging CO or terminal CO with
unusual reduction of CO strenght (d -> p* back bonding)

13. Group Frequencies: Type of Binding
For example: Ru3(CO)12 : CO 2060, 2030, 2010 cm-1 only
=> Ru(CO)4 units held together by Ru-Ru bonds
Another example: Fe3(CO)12 : CO 2040, 2020, 1997, 1840 cm-1
Iron complex has bridging CO as well as terminal CO

14. Halogens: Type of Binding
Halogens may also act as bridging/terminal ligands (e.g. Al2Cl6)
• Study compounds of known structure that has terminal M-X
• Study compounds that has bridging ligands
From the above observations, determine the presence / absence
Of bridging in a new compound

15. Polyatomic ligands: Type of Binding
Polyatomic ligands can attach at different donor site.
Monothioacetate ligand
Case 1 Through Oxygen only
M
O
S
CH3
Case 2 Through Sulfur (1-2 metal)
M
S
O
CH3
M
M
S
O
CH3
Case 3 Through both Sulfur and Oxygen
M
S
O
CH3
Case 4 Through Sulfur one M and
Oxygen through other M
M
S
O
CH3
M

16. Polyatomic ligands: Type of Binding
Case 2
M
S
O
CH3
M
M
S
O
CH3
The only one that have
C=O : 1600 cm-1
Case 1
M
O
S
CH3
The C=S : Is less characteristic:
weaker and in more crowded region
Band near 950 cm-1 indicate case 1
Case 3 Case 4
and
Involve chelating and bridging
ligands: yield slightly reduced
frequency of both stretch:
~ 1500 cm-1 for CO
~ 900 cm-1 for CS

17. Isotopic substitution:
to interpret vibrational spectra
• Vibrational frequency depend on Masses of moving atoms
• Deuterium substitution produce large mass increase => large
frequency decrease by up to 0.717 (1/√ 2)
• M-H stretching decrease by several hundred cm-1 by replacing
M-D
• This substitution can be used to remove M-H bands where
they may hide other bands
Example: Co(CO)4H : 4 IR bands ~ 2000 cm-1
Only the lowest one shift when D replace H

18. Example: Co(CO)4H : 4 IR bands ~ 2000 cm-1
Only the lowest one shift when D replace H
Isotopic substitution:
to interpret vibrational spectra

19. Isotopic substitution:
to interpret vibrational spectra
For other nuclei than Hydrogen, the mass changes are small:
only few cm-1 change
Naturally occuring isotope mixture and isotopically enriched
mixture can give useful information
For Cl, the relative shift Dn/n is less than 0.5 (Dm/m), which is
resolvable if the bands is narrow

20. Isotopic substitution:
to interpret vibrational spectra
Isotopic substitution is most useful to identify metal-ligand
vibrations
K[OsO3N] Band just above 1000 cm-1 shift when enriching
with 15N