Mg-H bonding in the equatorial plane. The Walsh diagram suggests that the trigonal planar geometry should be favoured based on covalent bonding arguments. The key e' bonding orbitals are lower in energy in this geometry than in hexagonal planar. This covalent (orbital) contribution is more important for 5 than for 6a/6b. Natural resonance theory analysis, conducted as part of NBO calculations, suggests that models of these complexes have higher covalent contribution to the bonding when M = Zn, compared to M = Mg. We speculate that, hexagonal planar configurations might be more likely to emerge when the ionic contribution to the bonding is large. This may reduce the energy cost in deforming the complex to θ = 60°; the weakened covalent interactions between ligands should allow the Mg atoms to translate to a position where they can maximise electrostatic interactions to the transition metal fragment.