Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organicand inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by acompound positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is todetermine the chemical functional groups in the sample. Functional groups are identified basedon vibrational modes of the groups such a stretching, bending etc. Different vibrational modesabsorb characteristic frequencies of IR radiation. An infrared spectrophotometer is aninstrument that passes infrared light through a molecule and produces a spectrum that containsa plot of the amount of light transmitted on the vertical axis against the wavelength of infraredradiation on the horizontal axis. Absorption of radiation lowers the percentage transmittancevalue.

Infrared Spectroscopy‐ A spectro‐analytical tool in chemistry 

AJELIAS L2‐S5

Infrared Spectroscopy‐ Spectra of Metal Carbonyls  

MnOC

OC

CO

CO

OC

Mn

OC

CO

CO

CO

OC

FeOC

OC

OC

Fe

OC

CO

CO

CO

CO

OC

AJELIAS L2‐S6

The range in which the band appears decides bridging or terminal .

The number of bands is only related to the symmetry of the molecule

bridging

terminal

terminal

M

C

OM M

C

O

terminal bridging μ 2

M

M

M

C

O

bridging μ 32120-1850 cm-1

νCO

1850-1700 cm-1 1730-1620 cm-1

CrOC

OC CO

CO

CO

CO Fe

Fe

Fe

OCFeOC

CO

CO

Cp

Cp

Cp

Cp

1620 cm‐12018, 1826 cm‐12000 cm‐1

Terminal versus bridging carbonyls

AJELIAS L2‐S7

Variation in νCO (cm–1) of the first row transition metal carbonyls

free CO2143

Ni(CO)42057Co(CO)4

-

1890Co2(CO)82044(av, ter)

[Fe(CO)4]2-

1815Fe(CO)52030

[Mn(CO)4]3-

1600,1790Mn(CO)6 +2098

Mn2(CO)102013 (av)

[Cr(CO)4]4-

1462,1657Cr(CO)62000V(CO)6

¯

1860V(CO)61976

Ti(CO)62-

1747

As the electron density on a metal centre increases, more π‐back‐bonding to the CO ligand(s) takes place. This weakens the C–O bond further as more electron density is pumped into the empty π* anti‐bonding carbonyl orbital.  This increases the M–C bond order and reduces the C‐O bond order. That is, the resonance structure M=C=O becomes more dominant.

M C O M C Oν CO Higher ν CO Lower

AJELIAS L2‐S8

Factors which affect νCO stretching frequencies

More back bonding

1.Charge on the metal2. Effect of other ligands

Other spectator ligands: Phosphines 

PR3 νCO, (cm–1) χ(cm–1)Δ νCO wrt P(t-Bu)3

PR3 νCO, (cm–1) χ(cm–1)Δ νCO wrt P(t-Bu)3

P(t-Bu)3 2056.1 0.0 PPh2(C6F5) 2074.8 18.7

PCy3 2056.4 0.3 P(OEt)3 2076.3 20.2

P(i-Pr)3 2059.2 3.1 P(p-C6H4-CF3)3 2076.6 20.5

PEt3 2061.7 5.6 P(OMe)3 2079.5 23.4

P(NMe2)3 2061.9 5.8 PH3 2083.2 27.1

PMe3 2064.1 8.0 P(OPh)3 2085.3 29.2

PBz3 2066.4 10.3 P(C6F5)3 2090.9 34.8

P(o-Tol)3 2066.6 10.5 PCl3 2097.0 40.9

PPh3 2068.9 12.8 PF3 2110.8 54.7

PPh2H 2073.3 17.2 P(CF3)3 2115.0 58.9

PR3

NiOC CO

CO

Lowest CO stretching frequencyMost donating phosphine

best σ donor

Highest CO stretching frequencyLeast donating phosphine

best π acceptor

AJELIAS L2‐S9

Effect of different co-ligands on νCO (cm-1) of Mo(CO)3L3

Complex (fac isomers)

ν CO cm–1

Mo(CO)3(PF3)3 2090, 2055Mo(CO)3(PCl3)3 2040, 1991Mo(CO)3[P(OMe)3]3 1977, 1888

Mo(CO)3(PPh3)3 1934, 1835Mo(CO)3(NCCH3)3 1915, 1783Mo(CO)3(dien)* 1898, 1758Mo(CO)3(Py)3 1888, 1746

With each negative charge added to the metal centre, the CO stretching frequency decreases by approximately 100 cm–1.

The better the σ donating capability of the other ligands on the metal, moreelectron density given to the metal, more back bonding (electrons in theantibonding orbital of CO) and lower the CO stretching frequency.

MoL

L CO

CO

CO

L

Effect of a ligands trans to CO

AJELIAS L2‐S10

More back bonding =More lowering of the C=O bond order = More lower ν CO stretching frequency

Synthesis of Metal Carbonyls

Direct carbonylation

Reductive carbonylation

AJELIAS L2‐S11

W(CO)6 + PPh3hν W(CO)5(PPh3) + CO

Fe(CO)5 + hν Fe(CO)3 + 2CO

Reactions  of Metal Carbonyls

Photochemical substitution

Co2(CO)8 + 2Na 2 Na[Co(CO)4]

Fe(CO)5 + Na/Hg Na 2Fe(CO)4

Mn2(CO)10 + 2Na 2 Na[Mn(CO)5]

Reduction : Carbonyl anions

V(CO)6 + Na Na[V(CO)6]

Oxidation : Iodocarbonyls

Mn2(CO)10 + I2 2 Mn(CO)5I

Fe(CO)5 + I2 Fe(CO)4I2

AJELIAS L2‐S12

In the presence of  UV radiation a monodentateligand displaces only one CO unit

Reactions  of Metal Carbonyls

WOC

OC CO

CO

CO

OC

RLi

etherW

OC

OC CO

CO

C

OC

[Me3O]BF4

R OLi

WOC

OC CO

CO

C

OC

R OCH3

Fischer Carbene

Nucleophilic addition to CO

Carbenes are catalysts for olefin metathesis

AJELIAS L2‐S13

MnOC

OC CO

CO

Me

OC

COhigh pressure Mn

OC

OC CO

CO

C

OC

OMe

Migratory insertion of CO

Mn2(CO)10 + 2 Na 2 NaMn(CO)5

NaMn(CO)5 + CH3I CH3Mn(CO)5

CH3Mn(CO)5 + CO CH3C(O)Mn(CO)5 ( migratory insertion)CH3C(O)Mn(CO)5 + PPh3 hv

CH3C(O)Mn(CO)4PPh3

Or at step 3 direct reaction with acyl chloride instead of MeI. Step 1 otherreducing agents e.g. AlEt3 can also be used.

2 Mn(OAc)2 + 4 Na + 10 CO Mn2(CO)10 + 4 NaOAchigh temphigh pressure

Give a scheme for the synthesis of Mn(CO)4(PPh3)[C(O)CH3] starting from Manganese acetate, Mn(OAc)2.

Problem solving ‐ synthesis

Metal‐ Sandwich compounds

AJELIAS L2‐S14

Hapticity of sandwich compounds varies from 1‐8

Why metal – sandwich compounds are important? 

1. Transition metal/ metal ion embedded inside an organic matrix: Makes a metal ion solubleeven in hydrocarbon solvents. E.g. Ferrocene is soluble in hexane while Fe2+ as such is not.Outcome: a hydrocarbon soluble additive/catalyst

2. Coordination to an electropositive metal often changes the reactivity and electronic properties of the π system bound to it  (benzene vs ferrocene)

3. A stericially protected metal site where a wide range of catalytic applications are possible  on the. e.g alkene polymerization

4. Metal sandwich compounds are excellent substrates to make planar chiral compounds. Applications as chiral catalysts in asymmetric catalysis 

Fe

Y

X

Fe

Y

X

Planr chirality:Non‐ super‐imposable 

mirror images

AJELIAS L2‐S15

Cyclopentadienyl (Cp−)

• Cyclopentadienyl (Cp−)  the most important of all the polyenyl ligands• It gets firmly bound to the metal  

• generally inert to nucleophilic reagents. 

• used as a stabilising ligand for many complexes. 

MM

M

η5η3η1

mostcommon

Leastcommon

(η5‐Cp)(η3‐Cp)W(CO)2

•Neutral cyclopentadiene (C5H6) is a weak acid with a pKa of around 15 

•Deprotonated with strong base or  alkali metals to generate the anionic Cp−

AJELIAS L2‐S16

Synthesis of Cp (C5H5‐) based sandwich compounds

FeCl2 + 2 C5H6 + 2 Et2NH Cp2Fe + 2 [Et2NH2]Cl

RuCl3(H2O)n + 3C5H6 + 3/2 Zn Cp2Ru + C5H8 + 3/2 Zn2+

2 C5H6 + 2 KOH + Tl2SO4H2O 2 CpTl + K2SO4 + H2O

CpTl + Mn(CO)5Cl CpMn(CO)3 + TlCl + 2 CO

(poisonous)

H H

H H

180°C

H H

Na2 NaCp

MCl2 + 2 NaCp Cp2M [ M = V, Cr, Mn, Fe, Co]Solvent: THF, DME, Liquid NH3 etc

+ H2cracking

dicyclopentadiene

AJELIAS L2‐S17

CpTl based chemistry is not practiced nowadays due to toxicity

Ferrocene: synthesisFe

Fe + 2 (R3NH)Cl FeCl2 + 2 R3N + H2

FeCl2 + 2 C5H6 + 2 R3N Cp2Fe + 2(R3NH)Cl

Lab Synthesis

FeCl2 + 2 NaCp Cp2Fe

AJELIAS L2‐S18

Reactions of Ferrocene

Ferrocene undergoes electrophilic substitution reactions. Many of its reactions are faster than similar reactions of benzene

Necessary requirement: The electrophile should not be oxidizing in nature

Fe FeI2I3

The oxidized Cp2Fe+, ferrocenium cation, will repel the electrophile away. Therefore direct nitration, halogenation and similar reactions cannot be carried out on ferrocene. 

Acetylation

3.3 x 106 times faster than benzene

FeFe

H3C(O)C

Fe

C(O)CH3

C(O)CH3

90 %90 %

Ac2O/ H3PO460 min, 50 °C

CH3C(O)ClAlCl3(1:2:2)

FeC(O)CH3

C(O)CH3

traces

FeCp2 + HBF4.OEt2p- benzoquinone

Et2O[FeCp2][BF4]

FeCp2 + NH4PF6 H2O/Acetone[FeCp2][PF6]

FeCl3

AJELIAS L2‐S19

FeFe Fe

HgClHg(OAc)2

Hg(OAc)

LiCl

Br, I derivatives

Br2/I2

Chloromercuration (hazardous)

109 times faster than benzene

FeFeHCHO/R2NH

H2C

H3PO4

NR2

Mannich reaction

Does not happen with benzene; only with bromobenzene

Lithiation reaction

FeFe

Li

Fe

Lit-BuLi n-BuLi

TMEDALi

N

N

(3:2 adduct)

Does not happen with benzene; only with phenols/anilines

AJELIAS L2‐S20

dppf

[1]ferrocenophane

Lithiation and 1,1’‐di‐lithiation – access to range of new derivatives

AJELIAS L2‐S21

FeFe

Fe

HOOC

Fe

I

Cl3Si

(BuO) 3B

SiCl4

1/8 S8

Fe

SLi

LiCO2/H+

I2

H+

Fe

(HO)2B

NaCN

Fe

CN

AJELIAS L2‐S22

SiFeMe

MeFe

SiMe Me

n

130 °C

M. Wt: 3.4 X 105

Polymers with ferrocene in the backbone

Cr3 CrCl3 + 2 Al + AlCl3 + 6 C6H6 3AlCl4 Na2S2O4

KOH

Cr

Bisbenzene chromium:  Prepared by Fischer and Hafner

Problem solving ‐ synthesis

Starting  fro m ferrocene show  minimum  number of steps for  preparing 1,1’‐ ferrocene dicarboxylic acid