Learning Objectives:

1. Synthesize and identify metal carbonyl complexes using Raman spectroscopy

2. Understand how the effects of pi back-bonding can be seen on a Raman plot 

 

Background:

A carbon-oxygen bond has various properties depending in its surroundings. For example, in carbon monoxide, although oxygen is a more electronegative atom than carbon, the oxygen end is partially positive.  In carbonyl ligands, this remains the case. When a carbonyl group bonds to a transition metal, it can undergo pi backbonding. Pi backbonding is a partial transfer of electrons from a metal's d-orbitals to the carbonyl’s pi antibonding orbitals. This process strengthens the metal-carbon bond while weakening the carbon-oxygen bond. This results in a detectable shift in both the metal-carbon and carbon-oxygen peaks in Raman and infrared spectroscopy based on the magnitude of this change in bond strength.

 

It generally is expected that elements of the same group have similar properties. This is not the case for first-row transition metals. This is not because there is something unexpected occurring in these metals, but rather the effect of the f-shell on third-row and fourth-row transition metals. The f-shell does not effectively shield the outer orbitals from the nucleus, resulting in a smaller than expected atomic radius. This keeps the radius similar to those found in the second row of transition metals. and thus the properties from these metals differs from their first-row counterparts. For example, many first row metal carbonyls such as Cr(CO)6 and Ni(CO)4 have been found not to significantly pi backbond. 

 

Coordination complexes with transition metal centers can be described by the potential spin configurations of the metal’s d-orbital electrons. According to Hund’s rule, electron's spins should align, partially filling each orbital prior to fully filling any. This condition is a high-spin state. Low spin states can occur when d orbitals split in energy. Eg orbitals are naturally higher in energy than T2g orbitals. This energy difference can be effected by the charge of the metal ion, the period of the metal ion, and the field strength due to the attached ligands according to the spectrochemical series. If this energy difference becomes significantly large, the lower T1g orbitals are filled completely prior to the filling of the higher Eg orbitals. Carbonyl ligands are at the high end of the spectrochemical series and thus likely form low-spin complexes.

See Figure A.

 

References:

(1) Blanco, F.; Alkorta, I.; Solimannejad, M.; Elguero, J. J. Phys. Chem. A 2009, 113 (13), 3237–3244.

(2) Johnson, J. B.; Klemperer, W. G. J. Am. Chem. Soc. 1977, 99 (22), 7132–7137.
(3) Bursten, B. E.; Freier, D. G.; Fenske, R. F. Inorg. Chem. 1980, 19 (6), 1810–1811.
(4) Ehlers, A. W.; Dapprich, S.; Vyboishchikov, S. F.; Frenking, G. Organometallics 1996, 15 (1), 105–117.

(5) Vincent, A. Molecular symmetry and group theory: a programmed introduction to chemical applications; John Wiley & Sons, 2013.
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