Na2Mg2NiH6 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 5-coordinate geometry to one H(3), two equivalent H(2), and two equivalent H(4) atoms. In the second Na site, Na(2) is bonded to one H(1), one H(3), two equivalent H(2), and two equivalent H(4) atoms to form distorted NaH6 pentagonal pyramids that share corners with four equivalent Mg(1)H6 octahedra, corners with two equivalent Ni(1)H4 tetrahedra, edges with four equivalent Mg(1)H6 octahedra, and  an edgeedge with one Ni(1)H4 tetrahedra. The corner-sharing octahedral tilt angles range from 8-81°. Mg(1) is bonded to one H(1), one H(3), two equivalent H(2), and two equivalent H(4) atoms to form distorted MgH6 octahedra that share corners with four equivalent Mg(1)H6 octahedra, corners with two equivalent Na(2)H6 pentagonal pyramids,  an edgeedge with one Mg(1)H6 octahedra, edges with two equivalent Na(2)H6 pentagonal pyramids, and edges with two equivalent Ni(1)H4 tetrahedra. The corner-sharing octahedral tilt angles range from 24-33°. Ni(1) is bonded to one H(1), one H(3), and two equivalent H(2) atoms to form NiH4 tetrahedra that share corners with two equivalent Na(2)H6 pentagonal pyramids, edges with four equivalent Mg(1)H6 octahedra, and  an edgeedge with one Na(2)H6 pentagonal pyramid. There are four inequivalent H sites. In the first H site, H(3) is bonded to one Na(1), one Na(2), two equivalent Mg(1), and one Ni(1) atom to form a mixture of distorted edge and corner-sharing HNa2Mg2Ni trigonal bipyramids. In the second H site, H(4) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), and two equivalent Mg(1) atoms. In the third H site, H(1) is bonded in a distorted rectangular see-saw-like geometry to one Na(2), two equivalent Mg(1), and one Ni(1) atom. In the fourth H site, H(2) is bonded to one Na(1), one Na(2), two equivalent Mg(1), and one Ni(1) atom to form a mixture of distorted edge, corner, and face-sharing HNa2Mg2Ni square pyramids.
Na2Mg2NiH6 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 5-coordinate geometry to one H(3), two equivalent H(2), and two equivalent H(4) atoms. The Na(1)-H(3) bond length is 2.33 Å. Both Na(1)-H(2) bond lengths are 2.57 Å. Both Na(1)-H(4) bond lengths are 2.64 Å. In the second Na site, Na(2) is bonded to one H(1), one H(3), two equivalent H(2), and two equivalent H(4) atoms to form distorted NaH6 pentagonal pyramids that share corners with four equivalent Mg(1)H6 octahedra, corners with two equivalent Ni(1)H4 tetrahedra, edges with four equivalent Mg(1)H6 octahedra, and  an edgeedge with one Ni(1)H4 tetrahedra. The corner-sharing octahedral tilt angles range from 8-81°. The Na(2)-H(1) bond length is 2.33 Å. The Na(2)-H(3) bond length is 2.32 Å. Both Na(2)-H(2) bond lengths are 2.37 Å. Both Na(2)-H(4) bond lengths are 2.48 Å. Mg(1) is bonded to one H(1), one H(3), two equivalent H(2), and two equivalent H(4) atoms to form distorted MgH6 octahedra that share corners with four equivalent Mg(1)H6 octahedra, corners with two equivalent Na(2)H6 pentagonal pyramids,  an edgeedge with one Mg(1)H6 octahedra, edges with two equivalent Na(2)H6 pentagonal pyramids, and edges with two equivalent Ni(1)H4 tetrahedra. The corner-sharing octahedral tilt angles range from 24-33°. The Mg(1)-H(1) bond length is 2.14 Å. The Mg(1)-H(3) bond length is 2.21 Å. There is one shorter (2.15 Å) and one longer (2.16 Å) Mg(1)-H(2) bond length. Both Mg(1)-H(4) bond lengths are 1.98 Å. Ni(1) is bonded to one H(1), one H(3), and two equivalent H(2) atoms to form NiH4 tetrahedra that share corners with two equivalent Na(2)H6 pentagonal pyramids, edges with four equivalent Mg(1)H6 octahedra, and  an edgeedge with one Na(2)H6 pentagonal pyramid. The Ni(1)-H(1) bond length is 1.58 Å. The Ni(1)-H(3) bond length is 1.58 Å. Both Ni(1)-H(2) bond lengths are 1.59 Å. There are four inequivalent H sites. In the first H site, H(3) is bonded to one Na(1), one Na(2), two equivalent Mg(1), and one Ni(1) atom to form a mixture of distorted edge and corner-sharing HNa2Mg2Ni trigonal bipyramids. In the second H site, H(4) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), and two equivalent Mg(1) atoms. In the third H site, H(1) is bonded in a distorted rectangular see-saw-like geometry to one Na(2), two equivalent Mg(1), and one Ni(1) atom. In the fourth H site, H(2) is bonded to one Na(1), one Na(2), two equivalent Mg(1), and one Ni(1) atom to form a mixture of distorted edge, corner, and face-sharing HNa2Mg2Ni square pyramids.