Infrared in Organometallic compounds Index IR-basics

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 The basic picture : atoms (mass) are connected with bonding electrons. Re is the equilibrium distance and F: force to restore equilibrium F F re F(x) = -kx where X is displacement from equilibrium

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

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

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

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

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)

Bond Stretching Frequencies: Carbonyl Important group of frequencies is due to Carbonyl ligand in Metal complex 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)

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

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,

Patterns of group Frequencies: Carbonyl Has only 2 CO bands: provided that the ligand preserves M(CO)3 3 fold symmetry 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 trans-octahedral complex The 4 CO are all related by symmetry => There is only one active vibration in IR

Group Frequencies: Type of Binding Many ligands hae different modes of binding to other atoms Terminal Bridging 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)

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

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

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

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

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

Isotopic substitution: to interpret vibrational spectra 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 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

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

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