1. Infrared Photodissociation Spectroscopy of Silicon Carbonyl Cations
Antonio Brathwaite and Michael Duncan
maduncan.myweb.uga.edu/
Department of Chemistry, University of Georgia
Athens, GA 30602

2. Transition Metal-Carbonyls
Stable transition metal-carbonyls have been studied for over a century
Cr(CO)6
Fe(CO)5
Ni(CO)4
Stability determined by 18 electron rule
We studied cations for comparison
Mn(CO)6+
Co(CO)5+
Ni(CO)4+
Free molecular C-O vibration cm-1
C-O stretching frequency shifts systematically depending on the metal atom, charge and electronic structure
There are limited studies on main group element-carbonyl systems
We use infrared photodissociation spectroscopy to study silicon carbonyls by probing the carbonyl stretching region

3. Classical Metal Carbonyl Bonding
Dewar-Chatt-Duncanson complexation model can be used to explain CO shifts
σ donation occurs along the CO axis into empty metal d orbitals
Results in a blue-shifted CO frequency
Filled d orbitals back donate into the antibonding * orbital on CO
This causes a red-shift in the CO stretching frequency
Combined effect produces a red shifted CO frequency

4. Experiment

5. Photodissociation by elimination of excess ligands
Mass selected ion
with excess CO
h (tunable IR)
Fragment ion after
CO elimination
External CO
Weakly bound
(CO-CO dimer, D0 ~100 cm-1)

6. Rare Gas Tagging Si+(CO)2Ar Si+(CO)2 h (tunable IR)
Bonds are sometimes too strong (D0 > 23 kcal/mol) to break with infrared light (e.g., C-O stretch 2143 cm-1 = 5.7 kcal/mol)
We attach a weakly bound “tag” atom to enhance fragmentation.
They are eliminated when light is absorbed providing indirect evidence of absorption
Computations on tagged and untagged ions are done
Si+(CO)2Ar
Si+(CO)2
h (tunable IR)
Mass selected ion,
argon tagged
Fragment ion after argon elimination

7. Ions vs. Neutrals Molecule IR frequency Fe(CO)5 2013, 2034 cm-1
 backbonding is the most important interaction in neutral metal-carbonyls
Cations have less electron density to disperse as the charge on the metal atom will contract the valence electrons.
Cations have reduced  backbonding and less red shifted frequencies than their isoelectronic neutrals.
How does Si(CO)2+ compare to its isoelectronic neutral Al(CO)2.
Molecule
IR frequency
Fe(CO)5
2013, 2034 cm-1
Co(CO)5+
2140, 2150 cm-1
Cr(CO)6
2003 cm-1
Mn(CO)6+
2114 cm-1
Compare relative shifts of transition metal neutrals and ions vs main group carbonyl
J. Phys. Chem. A 2009, 113, J. Am. Soc. Mass Spectrum. 2010, 21, 5.

8. Asymmetric Carbonyl Coordination
A. J. Lupinetti, S. Fau, G. Frenking, S. H. Strauss, "Theoretical Analysis of the Bonding between CO and Positively Charged Atoms," J. Phys. Chem. 101 (1997) 9551
A. J. Lupinetti, S. Fau, G. Frenking, S. H. Strauss, "Theoretical Analysis of the Bonding between CO and Positively Charged Atoms," J. Phys. Chem. 101 (1997) 9551
Asymmetric Carbonyl Coordination
Asymmetric ligand clustering has been observed for Mg+ and Al+
Initial ion-ligand interactions cause polarization of the occupied 3 s orbital.
Subsequent ligands tend to bind on the same side as the first
Si+ has similar 3s orbital occupation with and an additional electron in the 3p orbital
What kind of bonding is present in Si(CO)n + complexes?
Another interesting comparison will be the coordination of these systems
G. Gregoire, N. R. Brinkman, D. van Heijnsbergen, H. F. Shaefer, M. A. Duncan, J. Phys. Chem. A 2003, 107, 218
Walters, R. S. Jaeger, T. D. Gregoire, N. R. Brinkman, H. F. Shaefer, M. A. Duncan, J. Phys. Chem. A 2003, 107, 7396

9. Mass Spectrum of Si(CO)n+

10. Infrared photofragmentation mass spectra
All complexes larger than n = 2, fragment by sequential ligand termination ending at n= 2
Weakly bound ligands are easily eliminated by IR photons
These results are consistent with a coordination of two

11. Infrared photodissociation Spectra of Si(CO)n+ Ar clusters

12. Infrared photodissociation Spectra of Si(CO)n+ clusters
Spectra detected by elimination of CO
The bands at 2123 cm-1 represent the asymmetric stretch and the bands at cm-1 represent the symmetric stretch
The blue-shifted band at approximately cm-1 observed.
This band is attributed to the weakly bound “external” CO ligands

13. Structures of neutrals and ions
Experiment and theory compared to Al(CO)n measured by Douberly and co- workers
Though greater in magnitude, the frequencies observed here are qualitatively consistent with transition metal-carbonyl trends
Molecule
IR frequency
Asymmetric
Symmetric
Si(CO)2+
2123 cm-1
2154 cm-1
Al(CO)2
1920 cm-1
1960 cm-1
Note basis set used.
Liang, T. Flynn, S. D. Morrison, A. M. Douberly, G. J. Phys. Chem. A 2009, 113, 4701

14. Conclusions Si(CO)2+ is the fully coordinated silicon carbonyl cation
Si(CO)2+ has a V-shaped structure analogous to that of isoelectronic Al(CO)2
Si(CO)n+ and Al(CO)n have the same qualitative trend as transition metal- carbonyl isoelectronic analogues

15. Future Work Early transition metal-carbonyls Metal cluster-carbonyls
Test the limits of metal carbonyl coordination
Metal cluster-carbonyls
Different binding sites have characteristic frequencies
Metal oxide-carbonyls
Pure metal vs. metal oxides