VIBRATIONAL SPECTRA OF CARBONYL COMPOUNDS

IR spectra have been usually used in the study of metal carbonyls. Since C=O stretching frequencies give very strong sharp bands, well separated from other vibrational modes of any other ligands that may also be present. The co molecule has a stretching frequency of 2143 cm⁻³ Terminal co groups in neutral metal Carbonyl molecule are formed in the range 2125 - 1850 cm⁻¹. Showing the reduction in CO bond order. Moreover when changes are made that should increase the extent of M-C back bonding. Co frequencies are shifted to even lower values.

The most important use of IR spectra of carbonyl compounds is in structural diagnosis. Where by bonding and terminal co groups can be recognised. For bridging co groups, the range is from 1850 - 1750 cm⁻¹.

                    Fe₂ (co)₉ has strong bonds in both the terminal and the bridging regions. It could be infra red, the structure must contains both type of co groups. X - ray study conforms the above result.

                      For OS₃ (co)₁₂ several structures consitance with the general rules of valence can be written. Some of these would have bridging co groups while the actual one doesn't. The IR spectrum alone shows that no structure with bridging co groups in acceptable. Since there is no absorption bond below 2100 cm⁻¹.

TREND IN CARBONYL COMPOUNDS ABOUT STRETCHING FREQUENCIES :-

[ Mn (co)₆]⁺ − 2090 cm⁻¹

[ Cr (co)₆] − 2000 cm⁻¹

[ V (co)₆ ]⁻ − 1860 cm⁻¹

Order in the decrease in the back bonding.

[V(co)₆]⁻ > [Cr (Co)₆] > [Mn(Co)₆]⁺

[V(Co)₆]⁻, where more negative charge must be taken from the metal atom, a bond is found at ≈ 1860 cm⁻¹ corresponding to the found at 2000 cm⁻¹ in [Or(co)₆] conversely a change that would tend to inhibit the shift of the e⁻s from the metal to corbon monoxide π orbital, such as placing a positive charge on the metal should cause the co frequencies to rise.

In [V(co)₆]⁻ because of the negative charge on the metal, a partial additional metal - carbon back bonding is possible which decreases bond order in co and so co absorption frequency decreases appreciably.

Example :2

The co stretching frequency for

Ni (co)₄ ,      [Co(co]⁻       [Fe(co)₄]²⁻

2060cm⁻¹   1890cm⁻¹     1790 cm⁻¹

This can be explained on the basis of M-C π bonding (back bonding). As the extent of bonding increases e⁻s are depleted on M, and therefore co bond a greater e⁻  in the carbonylate anions, the metal has a greater e⁻ density to be released for π bonding enhancing the M to C π bonding and consequently decreasing the co bond strength.

METAL CARBONYL HYDRIDES :-

Properties :-

1) Metal carbonyl hydrides are unstable

2) which is easily oxidised by air

3) It's acidic, it seems incorrect to formulate compound with dissociate to give H⁺ as a complex containing hydride H⁻. But the explanation lies in the stabilities of carbonylates anion. Which results open acids dissociation. The hydrogen present in the metal carbonyl hydride is added to olefins.

H CO(CO)₄ RCH = CH₂ →RCH₃CH₂CO(CO)₄

[ π - C₅H₅ Mo (CO)₅ H] + CCl₄ →[π-C₅H₅ Mo (Co)₃ cl ] + CHCl₃.

X- ray studies of H₂[RO₃ (CO)₁₂]²⁻ indicates that the Re atom forming as isolated triangle. Each Re atom carry four terminal CO group. Two Re-H-Re bridges in the Re₃ plane.