Nuclear motion in the σ-bound regime of metal-H₂ complexes: [Mg(H₂)(n=1-6)]²⁺

The dynamic, quantum structure of [Mg(H2)(n=1-6)](2+)complexes is investigated via ab initio path integral molecular dynamics simulations. These complexes represent the strong, σ-complex regime of metal-H2 interactions and are representative of bonding motifs found in metal-organic frameworks. Significant nuclear motion within the coordination sphere is observed, even though the ligands remain largely intact. Quantum effects are found to be important in the H-H and metal-H2 stretch coordinates, but the remaining motion in the molecule is well represented by classical simulations. Nearly free rotation of the dihydrogen moiety is observed in all complexes. Statistical averages and distributions of structural parameters are found to deviate nontrivially from the same parameters in static, equilibrium structures.