The Walsh diagram for the simultaneous symmetric compression of three Mg---H units (variation of θ -Figure ) in a model of 6a provides further insight into the changes in bonding between hexagonal planar (D3h) and trigonal planar (C3h) geometries (Figure ). The stabilisation of the key e' set (HOMO-5 and HOMO-6) upon distortion can be rationalised in terms of the formation of bonding σ-(Mg-H) interactions. The antibonding e' set (LUMO and LUMO+1) is destabilised due to the formation of the corresponding antibonding interactions. The hexagonal planar geometry has few MOs with direct Mg-H interactions. For example, the HOMO-5 and HOMO-6 have negligible Mg s AO contributions when θ = 60°. The Mg s AO contribution increases for this e' set as θ decreases. This can be understood qualitatively by a visual inspection of snapshots of the MOs from points along the pathway of deformation (Figure ). These snapshots show that deformation from the hexagonal planar to trigonal planar geometry increases