Analysis of η2 vs. η5 Bonding of the Pentazolato Ligand to Manganese

Monika Wiedmann

April 25, 2005

Introduction

Experimentally, the pentazolate anion is unknown.

Pentazolate vs. Cyclopentadienyl Ion

The Pentazolato Ligand on Mn

Is it η5? Or is it η2?

Calculations Calculations were done on Mn(N5)(CO)3 with the

pentazolato ligand in both the η5 and η2 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.

The Pentazolato Ligand on Mn

Is it η5? Or is it η2?

12 kcal/mol lower in energy

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.

η2 Bonding of Pentazolate to Mn

Occupied 3d Orbitals on Mn

Unoccupied 3d and 4s Orbitals on Mn

Nitrogen → Mn Electron Donation

Filled N-N σ bond donates electron density to unoccupied Mn 4s orbital.

N: → Mn Donation

η5 Bonding of Pentazolate to Mn

Pentazolato → Mn Electron Donation

N: → Mn Donation

Mn → N Backbonding

Bonding of C≡O to Mn

O≡C: → Mn σ Donation

Mn → C≡O Backbonding

Conclusions

Manganese η2–pentazolato tricarbonyl is more stable than manganese η5–pentazolato tricarbonyl by 12 kcal/mol (in the gas phase).

The NBO program is useful for visualizing the interactions of various orbitals.

Acknowledgments

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