In Cat2 the negative and neutral ligands are carbanions and carbenes, respectively. Contrary to Cat1, the axial ligand (ligand 4) is now negative and the equatorial negative (ligands 3 and 5) and neutral ligands (ligands 6 and 7) are both trans. The ligand with single largest effect on the protonation is one of the equatorial negative ligands (ligand 5 in Figure ). Exchanging this ligand with NH - 2 increases the protonation energy from -1.4 to 11.1 kcal/mol. All subsequent substitutions yields structures with positive protonation energies.