Using DFT methods, the Zn---H or Mg---H distances in models for 5 and 6b can be stretched or compressed to understand the equilibrium bond length and the energetic cost for deviation from equilibrium values (Figure ). The results of these calculations can be plotted on a 4-D potential energy surface (PES). For 5, the only minimum on the PES is the tris(sigma) complex. Stretching of each of the Zn-H bonds comes with a small but real energy cost. The highest energy structures on the PES involve the elongation of two Zn-H bonds while keeping one compressed. The model of 6b essentially shows the opposite trend. The only minimum on the PES is the hexagonal planar complex. The equilibrium Mg---H distances are extremely long and compressing them costs energy.