Infrared in Organometallic Infrared in Organometallic compoundscompounds

IR-basicsIR-basicsIndex

IntroductionIntroduction

IR is one of the first technique inorganic chemists used (since 1940)

Molecular VibrationMolecular Vibration

Newton’s law of motion is used classically to calculate force constant

r

reFFFF The basic picture : atoms (mass) are

connected with bonding electrons. Re is the equilibrium distance and FF: force to restore equilibrium

F(x) = -kx where X is displacement from equilibrium

Analyzing inorganic molecules by IRAnalyzing inorganic molecules by IR

• With IR, we might be able to determine the number of atoms in a With IR, we might be able to determine the number of atoms in a groupgroup

Distinguish MXMX22 and MXMX33 groups

• We might distinguish monodentate from bidentate sulfateWe might distinguish monodentate from bidentate sulfate

• We might distinguish terminal from bridging We might distinguish terminal from bridging COCO ligands ligands

• We can use variation in We can use variation in COCO stretching frequency in metal carbonyl stretching frequency in metal carbonyl to make deduction about electronic nature of the other ligandsto make deduction about electronic nature of the other ligands

Bond Stretching FrequenciesBond Stretching Frequencies

1.1. Stretching Frequency is lower for heavier atomsStretching Frequency is lower for heavier atoms

2.2. Stretching Frequency is lower for weaker bondsStretching Frequency is lower for weaker bonds

3.3. Stretching Frequency vary over a narrow range Stretching Frequency vary over a narrow range for a set of related compoundsfor a set of related compounds

Vibration frequency of a bond depends on the Vibration frequency of a bond depends on the mass of mass of bonded atomsbonded atoms and on the and on the force constantforce constant of the bond of the bond

General Principle:General Principle:

Bond Stretching Frequencies: Bond Stretching Frequencies: HydrogenHydrogen

Hydrogen: Hydrogen: all bond stretch occur in the range: all bond stretch occur in the range:

40004000 to to 1700 1700 cmcm-1-1 (for (for H-FH-F down to down to H-PbH-Pb))

Going down any main group in periodic table Going down any main group in periodic table increase the massincrease the massAnd And decrease the bond strength decrease the bond strength => Lowering stretching Frequency=> Lowering stretching Frequency

From Left to right along a row: From Left to right along a row: the effect of increasing the massthe effect of increasing the mass is is outweighedoutweighed by the by the increase in Bond strenght increase in Bond strenght

=> Frequency increase=> Frequency increase

Bond Stretching Frequencies: Bond Stretching Frequencies: HydrogenHydrogen

Increase (cmIncrease (cm-1-1))

Decrease (cmDecrease (cm-1-1))

Bond Stretching Frequencies: Bond Stretching Frequencies: other nucleiother nuclei

Stretching of bonds not involving Hydrogen are lower (below 1000 cm-1)

Except for multiple bond with higher force constantExcept for multiple bond with higher force constant

Or for single bond involving nuclei in the first row (C-F, B-O)Or for single bond involving nuclei in the first row (C-F, B-O)

Bond Stretching Frequencies: Bond Stretching Frequencies: CarbonylCarbonyl

• Terminal COTerminal CO correlate with electron-righness electron-righness of the metalof the metal

• BackbondingBackbonding from the d-orbital of the metalfrom the d-orbital of the metal to the * antibonding orbital* antibonding orbital weaken CO bondweaken CO bond => lower stretching frequencylower stretching frequency (from free CO)

Important group of frequencies is due to Carbonyl ligandCarbonyl ligand in Metal complex

Bond Stretching Frequencies: Bond Stretching Frequencies: CarbonylCarbonyl

CoCo (COCO)(NONO)(PPClClXXPh3-X)2

Table illustrating how the electronegative Chlorineelectronegative Chlorine on on Phosphorus ligand Phosphorus ligand decrease the electron density on Cobaltdecrease the electron density on Cobalt (central atom) (central atom) Decreasing d -> p* backbonding Decreasing d -> p* backbonding raising CO raising CO and and NONO

Patterns of group Frequencies: Patterns of group Frequencies: CarbonylCarbonyl

Clearly defined group frequencies like COCO are very important in determining how many of the grouphow many of the group occur in each molecule and symmetry relationship between themsymmetry relationship between them

• There is 1 stretching mode for each bond in a molecule in principle we can count the number ofcount the number of COCO frequencies (caution as some vib. Might not be active in IR)

• Symmetry relating equivalent groups 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 COCO ligands, the number of bands will be less than the number of ligands: some are degenerated,

Patterns of group Frequencies: Patterns of group Frequencies: CarbonylCarbonyl

M

L

CO

CO

OC Has only 2 CO bands: Has only 2 CO bands: provided that the provided that the ligand preserves M(CO)ligand preserves M(CO)33 3 fold symmetry 3 fold symmetry

M

L

L

CO

OC M

OC

OC

cis-octahedral complexThe 22 CO CO transtrans to each other can be treated togetherThe 22 CO CO ciscis to each other can be treated together

There are Sym. And Asym stretch for both groupsThere are Sym. And Asym stretch for both groups=> Therefore there are => Therefore there are 4 CO4 CO stretch expected stretch expected

ML L

CO

CO

M

OC

OC

trans-octahedral complexThe 44 CO CO are all related by symmetry

=> There is only one active vibration in IR=> There is only one active vibration in IR

Group Frequencies: Group Frequencies: Type of BindingType of BindingMany ligands hae different modes of binding to other atomsMany ligands hae different modes of binding to other atoms

TerminalTerminal

M CO

BridgingBridging

M

CO

MM

CO

M

M

Triple BridgeTriple Bridge

2130 – 1700 cm2130 – 1700 cm-1-1

1900 – 1780 cm1900 – 1780 cm-1-1

1900 – 1780 cm1900 – 1780 cm-1-1

We can therefore state: We can therefore state: COCO above 1900 =>above 1900 => terminal COterminal CO

Below 1900Below 1900 : Can be due to bridging COCan be due to bridging CO or terminal COterminal CO with with unusual reduction of CO strenghtunusual reduction of CO strenght (d -> * back bonding)

Group Frequencies: Group Frequencies: Type of BindingType of Binding

For exampleFor example: Ru: Ru33(CO)(CO)1212 : : CO 2060, 2030, 2010 cmCO 2060, 2030, 2010 cm-1-1 onlyonly

=> Ru(CO)=> Ru(CO)44 units units held together by Ru-Ru bondsheld together by Ru-Ru bonds

Another exampleAnother example: Fe: Fe33(CO)(CO)1212 : : CO 2040, 2020, 1997, CO 2040, 2020, 1997, 18401840 cm cm-1-1

Iron complex has Iron complex has bridging CObridging CO as well as as well as terminal COterminal CO

Halogens: Halogens: Type of BindingType 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 / absenceFrom the above observations, determine the presence / absenceOf bridging in a new compound Of bridging in a new compound

Polyatomic ligands: Polyatomic ligands: Type of BindingType of BindingPolyatomic ligands can attach at different donor site. Polyatomic ligands can attach at different donor site.

Monothioacetate ligandMonothioacetate ligand

Case 1Case 1 Through Oxygen onlyThrough Oxygen onlyM

O

S

CH3

Case 2Case 2 Through Sulfur (1-2 metal)Through Sulfur (1-2 metal)

M

S

O

CH3

M

M

S

O

CH3

Case 3Case 3 Through both Sulfur and OxygenThrough both Sulfur and OxygenM

S

O

CH3

Case 4Case 4 Through Sulfur one M and Through Sulfur one M and Oxygen through other MOxygen through other M

M

S

O

CH3

M

Polyatomic ligands: Polyatomic ligands: Type of BindingType of Binding

Case 2Case 2

M

S

O

CH3

M

M

S

O

CH3

The only one that have The only one that have C=O : 1600 cmC=O : 1600 cm-1-1

Case 1Case 1

M

O

S

CH3

The The C=S : Is less characteristic: C=S : Is less characteristic: weaker and in more crowded regionweaker and in more crowded regionBand near 950 cmBand near 950 cm-1-1 indicate case 1 indicate case 1

Case 3Case 3 Case 4Case 4andandInvolve chelating and bridging Involve chelating and bridging ligands: yield slightly reduced ligands: yield slightly reduced frequency of both stretch:frequency of both stretch:

~ 1500 cm~ 1500 cm-1-1 for CO for CO~ 900 cm~ 900 cm-1 -1 for CSfor CS

Isotopic substitution: Isotopic substitution: to interpret vibrational spectrato 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-HM-H stretching decrease by several hundred cm-1 by replacing M-DM-D

• This substitution can be used to remove This substitution can be used to remove M-HM-H bands where bands where they may hide other bandsthey may hide other bands

Example: Co(Example: Co(COCO))44HH : : 4 IR bands ~ 2000 cm4 IR bands ~ 2000 cm-1-1

Only the lowest one shift when D replace HOnly the lowest one shift when D replace H

Example: Co(Example: Co(COCO))44HH : : 4 IR bands ~ 2000 cm4 IR bands ~ 2000 cm-1-1

Only the lowest one shift when D replace HOnly the lowest one shift when D replace H

Isotopic substitution: Isotopic substitution: to interpret vibrational spectrato interpret vibrational spectra

Isotopic substitution: Isotopic substitution: to interpret vibrational spectrato interpret vibrational spectra

For other nuclei than Hydrogen, the mass changes are small: For other nuclei than Hydrogen, the mass changes are small: only few cmonly few cm-1-1 change change

Naturally occuring isotope mixture and isotopically enriched Naturally occuring isotope mixture and isotopically enriched mixture can give useful information mixture can give useful information For For ClCl, the , the relative shift relative shift is less than is less than 0.50.5 ( (m/m), which is m/m), which is resolvable if the bands is narrowresolvable if the bands is narrow

Isotopic substitution: Isotopic substitution: to interpret vibrational spectrato interpret vibrational spectra

Isotopic substitution is most useful to identify metal-ligand Isotopic substitution is most useful to identify metal-ligand vibrationsvibrationsK[OsOK[OsO33N]N] Band just above 1000 cmBand just above 1000 cm-1 -1 shift when shift when enrichingenriching

with with 1515NN