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Infrared Spectroscopy- A Spectro-analytical tool in chemistry Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a compound positioned in the path of an I...

Infrared Spectroscopy- A Spectro-analytical tool in chemistry Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a compound positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample. Functional groups are identified based on vibrational modes of the groups such a stretching, bending etc. Different vibrational modes absorb characteristic frequencies of IR radiation. An infrared spectrophotometer is an instrument that passes infrared light through a molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis. Absorption of radiation lowers the percentage transmittance value. Infrared Spectroscopy- Spectra of Metal Carbonyls CO OC CO OC OC Mn Mn CO OC CO CO OC terminal The range in which the band appears decides bridging or terminal. OC O C O C CO bridging The number of bands is OC Fe Fe CO only related to the OC C CO symmetry of the O terminal molecule  Terminal versus bridging carbonyls O M O C C C M M M M O M terminal bridging  2 bridging  3 CO 2120-1850 cm-1 1850-1700 cm-1 1730-1620 cm-1 Cp Fe CO CO OC CO OC Fe Cp Cr OC Fe Cp OC CO CO Fe Cp CO 2000 cm-1 2018, 1826 cm-1 1620 cm-1 Factors which affect CO stretching frequencies Variation in CO (cm–1) of the first-row transition metal carbonyls 1. Charge on the metal free CO 2143 Ni(CO)4 2057 As the electron density on a metal centre increases, more -backbonding Co(CO)4- Co2(CO)8 1890 2044 (av, ter) to the CO ligand(s) takes place. This [Fe(CO)4]2- Fe(CO)5 weakens the C–O bond further as more 1815 2030 electron density is pumped into the [Mn(CO)4]3- Mn(CO)6 + Mn2(CO)10 empty * anti-bonding carbonyl 1600,1790 2098 2013 (av) orbital. It increases the M–C bond [Cr(CO)4]4- Cr(CO)6 order and reduces the C-O bond order. 1462,1657 2000 The resonance structure M=C=O V(CO)6¯ V(CO)6 becomes more dominant. 1860 1976 Ti(CO)62- 1747 M C O M C O CO Higher CO Lower More back bonding  Other spectator ligands: Phosphines PR3 CO, (cm–1) (cm–1) PR3 CO, (cm–1) (cm–1) 2. Effect of other ligands  CO wrt  CO wrt P(t-Bu)3 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 Lowest CO stretching frequency: Most donating PMe3 2064.1 8.0 P(OPh)3 2085.3 29.2 phosphine (best −donor) PBz3 2066.4 10.3 P(C6F5)3 2090.9 34.8 Highest CO stretching P(o-Tol)3 2066.6 10.5 PCl3 2097.0 40.9 frequency: Least donating PPh3 2068.9 12.8 PF3 2110.8 54.7 phosphine (best -acceptor) PPh2H 2073.3 17.2 P(CF3)3 2115.0 58.9  Effect of different Co-ligands Effect of different co-ligands on CO (cm-1) of Mo(CO)3L3 Complex CO cm–1 (fac isomers) Mo(CO)3(PF3)3 2090, 2055 CO Mo(CO)3(PCl3)3 2040, 1991 Mo(CO)3[P(OMe)3]3 1977, 1888 More back bonding = L CO Mo Mo(CO)3(PPh3)3 1934, 1835 More lowering of the C=O L CO Mo(CO)3(NCCH3)3 1915, 1783 bond order = Less CO L Mo(CO)3(dien)* 1898, 1758 stretching frequency Mo(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  donating capability of other ligands on the metal, more electron density given to the metal, more back bonding (electrons in the antibonding orbital of CO) and lower the CO stretching frequency.  Unique reactions in organometallic chemistry Oxidative Addition Reductive Elimination Migratory Insertion  - Hydrogen Elimination  Oxidative addition When addition of ligands is accompanied by oxidation of the metal, it is called an oxidative addition reaction H H2 oxidative OX state of metal increases by 2 units Ph3P PPh3 addition H PPh3 Rh Rh Coordination number increases by 2 units Ph3P Cl Ph3P Cl PPh3 2 new anionic ligands are added to the metal Rh+1 Rh+3 Requirements for oxidative addition Availability of nonbonded electron density on the metal. Two vacant coordination sites on the reacting complex (LnM), that is, the complex must be coordinatively unsaturated. A metal with stable oxidation states separated by two units; the higher oxidation state must be energetically accessible and stable. Examples of Oxidative addition: Cis or trans ? Homonuclear systems (H2, Cl2, O2, C2H2) Cis Heteronuclear systems (MeI) Cis or trans  Reductive elimination The reverse of the Oxidative Addition reaction. CH3 Ph2 Ph2 P CH3 P CH3 reductive elimination + H3C CH3 Pt Pt 165 °C, days P CH3 P CH3 Ph2 Ph2 CH3 Pt4+ Pt2+ Oxidation state of metal decreases by 2 units Coordination number decreases by 2 units 2 cis oriented anionic ligands form a stable  bond and leave the metal Factors which facilitate reductive elimination A high formal positive charge on the metal The presence of bulky groups on the metal, and An electronically stable organic product Cis orientation of the groups taking part in reductive elimination is a MUST  Migratory Insertion X L +L M Y [M-Y-X] M Y X dn dn No change in the formal oxidation state of the metal A vacant coordination site is generated during a migratory insertion (which gets occupied by the incoming ligand) The groups undergoing migratory insertion must be cis to one another CH3 Ph3P O OC OC CO C Mn + PPh3 Mn CH3 OC CO OC CO OC OC These reactions are enthalpy driven and although the reaction is entropy prohibited the large enthalpy term dominates  Migratory Insertion X X M A B M A 1, 1 - migratory insertion B X X A B 1, 2 - migratory insertion M M A B CO O CH2CH2R 1, 1-migratory Ph P CCH2CH2R Ph3P insertion 3 Rh Rh OC PPh3 OC PPh3 H R 1, 2-migratory CH2CH2R Ph3P Ph3P insertion Rh Rh Ph3P OC PPh3 CO