KMgNi2H3(SeO5)2 crystallizes in the monoclinic Cm space group. K(1) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(7), one O(8), and two equivalent O(4) atoms. Mg(1) is bonded to one O(2), one O(6), one O(7), and two equivalent O(3) atoms to form distorted MgO5 trigonal bipyramids that share corners with two equivalent Ni(1)O6 octahedra,  a cornercorner with one Se(2)O4 tetrahedra, and edges with two equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 52°. Ni(1) is bonded to one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted NiO6 octahedra that share corners with two equivalent Se(2)O4 tetrahedra,  a cornercorner with one Mg(1)O5 trigonal bipyramid, edges with two equivalent Ni(1)O6 octahedra, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. There are three inequivalent H sites. In the first H site, H(1) is bonded in a bent 150 degrees geometry to one O(1) and one O(5) atom. In the second H site, H(2) is bonded in a single-bond geometry to one O(6) atom. In the third H site, H(3) is bonded in a single-bond geometry to one O(5) atom. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted trigonal non-coplanar geometry to one O(1) and two equivalent O(3) atoms. In the second Se site, Se(2) is bonded to one O(2), one O(8), and two equivalent O(4) atoms to form SeO4 tetrahedra that share corners with four equivalent Ni(1)O6 octahedra and  a cornercorner with one Mg(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 49-50°. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one K(1), one H(1), and one Se(1) atom. In the second O site, O(2) is bonded in a 2-coordinate geometry to one K(1), one Mg(1), and one Se(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ni(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one K(1), one Ni(1), and one Se(2) atom. In the fifth O site, O(5) is bonded in a distorted water-like geometry to two equivalent Ni(1), one H(1), and one H(3) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to one Mg(1), two equivalent Ni(1), and one H(2) atom. In the seventh O site, O(7) is bonded in a 3-coordinate geometry to one K(1), one Mg(1), and two equivalent Ni(1) atoms. In the eighth O site, O(8) is bonded in a distorted rectangular see-saw-like geometry to one K(1), two equivalent Ni(1), and one Se(2) atom.
KMgNi2H3(SeO5)2 crystallizes in the monoclinic Cm space group. K(1) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(7), one O(8), and two equivalent O(4) atoms. The K(1)-O(1) bond length is 2.53 Å. The K(1)-O(2) bond length is 2.98 Å. The K(1)-O(7) bond length is 2.80 Å. The K(1)-O(8) bond length is 2.74 Å. Both K(1)-O(4) bond lengths are 2.77 Å. Mg(1) is bonded to one O(2), one O(6), one O(7), and two equivalent O(3) atoms to form distorted MgO5 trigonal bipyramids that share corners with two equivalent Ni(1)O6 octahedra,  a cornercorner with one Se(2)O4 tetrahedra, and edges with two equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 52°. The Mg(1)-O(2) bond length is 2.22 Å. The Mg(1)-O(6) bond length is 2.13 Å. The Mg(1)-O(7) bond length is 2.09 Å. Both Mg(1)-O(3) bond lengths are 2.04 Å. Ni(1) is bonded to one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted NiO6 octahedra that share corners with two equivalent Se(2)O4 tetrahedra,  a cornercorner with one Mg(1)O5 trigonal bipyramid, edges with two equivalent Ni(1)O6 octahedra, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. The Ni(1)-O(3) bond length is 2.07 Å. The Ni(1)-O(4) bond length is 2.14 Å. The Ni(1)-O(5) bond length is 2.38 Å. The Ni(1)-O(6) bond length is 2.18 Å. The Ni(1)-O(7) bond length is 1.93 Å. The Ni(1)-O(8) bond length is 2.35 Å. There are three inequivalent H sites. In the first H site, H(1) is bonded in a bent 150 degrees geometry to one O(1) and one O(5) atom. The H(1)-O(1) bond length is 1.54 Å. The H(1)-O(5) bond length is 1.03 Å. In the second H site, H(2) is bonded in a single-bond geometry to one O(6) atom. The H(2)-O(6) bond length is 0.98 Å. In the third H site, H(3) is bonded in a single-bond geometry to one O(5) atom. The H(3)-O(5) bond length is 0.98 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted trigonal non-coplanar geometry to one O(1) and two equivalent O(3) atoms. The Se(1)-O(1) bond length is 1.69 Å. Both Se(1)-O(3) bond lengths are 1.76 Å. In the second Se site, Se(2) is bonded to one O(2), one O(8), and two equivalent O(4) atoms to form SeO4 tetrahedra that share corners with four equivalent Ni(1)O6 octahedra and  a cornercorner with one Mg(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 49-50°. The Se(2)-O(2) bond length is 1.67 Å. The Se(2)-O(8) bond length is 1.71 Å. Both Se(2)-O(4) bond lengths are 1.68 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one K(1), one H(1), and one Se(1) atom. In the second O site, O(2) is bonded in a 2-coordinate geometry to one K(1), one Mg(1), and one Se(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ni(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one K(1), one Ni(1), and one Se(2) atom. In the fifth O site, O(5) is bonded in a distorted water-like geometry to two equivalent Ni(1), one H(1), and one H(3) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to one Mg(1), two equivalent Ni(1), and one H(2) atom. In the seventh O site, O(7) is bonded in a 3-coordinate geometry to one K(1), one Mg(1), and two equivalent Ni(1) atoms. In the eighth O site, O(8) is bonded in a distorted rectangular see-saw-like geometry to one K(1), two equivalent Ni(1), and one Se(2) atom.