1. Analysis of η 2 vs. η 5 Bonding of the Pentazolato Ligand to Manganese Monika Wiedmann April 25, 2005

2. Introduction  Experimentally, the pentazolate anion is unknown.

3. Pentazolate vs. Cyclopentadienyl Ion

4. The Pentazolato Ligand on Mn  Is it η 5 ?  Or is it η 2 ?

5. Calculations  Calculations were done on Mn(N 5 )(CO) 3 with the pentazolato ligand in both the η 5 and η 2 orientations.  Geometry optimization and frequency calculations were performed at the RB3LYP 6-31+G(d) level.  Single point energy calculations were then run at the RB3LYP 6-311+G(3df) level.  NBO analysis was done to visualize the interactions between Mn and its ligands.

6. The Pentazolato Ligand on Mn  Is it η 5 ?  Or is it η 2 ? 12 kcal/mol lower in energy

7. Natural Bond Orbital Analysis  Regular quantum mechanical calculations result in delocalized molecular orbitals.  NBO takes these and transforms them into localized orbitals familiar to chemists (s, p, d, σ, σ*, π, π*).  Interactions between specific orbitals can be visualized.

8. η 2 Bonding of Pentazolate to Mn

9. Occupied 3d Orbitals on Mn

10. Unoccupied 3d and 4s Orbitals on Mn

11. Nitrogen → Mn Electron Donation  Filled N-N σ bond donates electron density to unoccupied Mn 4s orbital.

12. N: → Mn Donation

13. η 5 Bonding of Pentazolate to Mn

14. Pentazolato → Mn Electron Donation

15. N: → Mn Donation

16. Mn → N Backbonding

17. Bonding of C≡O to Mn

18. O≡C: → Mn σ Donation

19. Mn → C≡O Backbonding

20. Conclusions  Manganese η 2 –pentazolato tricarbonyl is more stable than manganese η 5 – pentazolato tricarbonyl by 12 kcal/mol (in the gas phase).  The NBO program is useful for visualizing the interactions of various orbitals.

21. Acknowledgments  Dr. Milletti (Eastern Michigan Univ.)  Dr. Schlegel  Dr. Winter  Barbara Munk  Elizabeth Rais