Analysis of η 2 vs. η 5 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 η 5 ?  Or is it η 2 ?

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 G(3df) level.  NBO analysis was done to visualize the interactions between Mn and its ligands.

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

η 2 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

η 5 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 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.

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