KMgNi2H3(SeO5)2 crystallizes in the triclinic P1 space group. K(1) is bonded in a 8-coordinate geometry to one Se(1), one O(2), one O(3), one O(4), one O(5), one O(6), and two equivalent O(1) atoms. Mg(1) is bonded to one O(10), one O(3), one O(4), one O(8), and one O(9) atom to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one Ni(1)O5 square pyramid,  a cornercorner with one Ni(2)O5 square pyramid,  a cornercorner with one Se(2)O4 tetrahedra,  an edgeedge with one Ni(1)O5 square pyramid, and  an edgeedge with one Ni(2)O5 square pyramid. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(10), one O(6), one O(7), one O(8), and one O(9) atom to form distorted NiO5 square pyramids that share corners with two equivalent Se(2)O4 tetrahedra,  a cornercorner with one Mg(1)O5 trigonal bipyramid, edges with two equivalent Ni(2)O5 square pyramids, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. In the second Ni site, Ni(2) is bonded to one O(10), one O(5), one O(7), one O(8), and one O(9) atom to form distorted NiO5 square pyramids 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)O5 square pyramids, 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 distorted bent 150 degrees geometry to one O(1) and one O(7) atom. In the second H site, H(2) is bonded in a single-bond geometry to one O(8) atom. In the third H site, H(3) is bonded in a single-bond geometry to one O(7) atom. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one K(1), one O(1), one O(3), and one O(4) atom. In the second Se site, Se(2) is bonded to one O(10), one O(2), one O(5), and one O(6) atom to form SeO4 tetrahedra that share corners with two equivalent Ni(1)O5 square pyramids, corners with two equivalent Ni(2)O5 square pyramids, and  a cornercorner with one Mg(1)O5 trigonal bipyramid. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to two equivalent K(1), one H(1), and one Se(1) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to one K(1) and one Se(2) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one K(1), one Mg(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one K(1), one Mg(1), and one Se(1) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to one K(1), one Ni(2), and one Se(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one K(1), one Ni(1), and one Se(2) atom. In the seventh O site, O(7) is bonded in a distorted tetrahedral geometry to one Ni(1), one Ni(2), one H(1), and one H(3) atom. In the eighth O site, O(8) is bonded in a 4-coordinate geometry to one Mg(1), one Ni(1), one Ni(2), and one H(2) atom. In the ninth O site, O(9) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ni(1), and one Ni(2) atom. In the tenth O site, O(10) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ni(1), one Ni(2), and one Se(2) atom.
KMgNi2H3(SeO5)2 crystallizes in the triclinic P1 space group. K(1) is bonded in a 8-coordinate geometry to one Se(1), one O(2), one O(3), one O(4), one O(5), one O(6), and two equivalent O(1) atoms. The K(1)-Se(1) bond length is 3.45 Å. The K(1)-O(2) bond length is 2.66 Å. The K(1)-O(3) bond length is 2.76 Å. The K(1)-O(4) bond length is 2.73 Å. The K(1)-O(5) bond length is 2.95 Å. The K(1)-O(6) bond length is 2.91 Å. There is one shorter (3.06 Å) and one longer (3.14 Å) K(1)-O(1) bond length. Mg(1) is bonded to one O(10), one O(3), one O(4), one O(8), and one O(9) atom to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one Ni(1)O5 square pyramid,  a cornercorner with one Ni(2)O5 square pyramid,  a cornercorner with one Se(2)O4 tetrahedra,  an edgeedge with one Ni(1)O5 square pyramid, and  an edgeedge with one Ni(2)O5 square pyramid. The Mg(1)-O(10) bond length is 2.19 Å. The Mg(1)-O(3) bond length is 2.10 Å. The Mg(1)-O(4) bond length is 2.09 Å. The Mg(1)-O(8) bond length is 2.13 Å. The Mg(1)-O(9) bond length is 1.96 Å. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(10), one O(6), one O(7), one O(8), and one O(9) atom to form distorted NiO5 square pyramids that share corners with two equivalent Se(2)O4 tetrahedra,  a cornercorner with one Mg(1)O5 trigonal bipyramid, edges with two equivalent Ni(2)O5 square pyramids, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. The Ni(1)-O(10) bond length is 2.24 Å. The Ni(1)-O(6) bond length is 2.06 Å. The Ni(1)-O(7) bond length is 2.03 Å. The Ni(1)-O(8) bond length is 2.07 Å. The Ni(1)-O(9) bond length is 1.93 Å. In the second Ni site, Ni(2) is bonded to one O(10), one O(5), one O(7), one O(8), and one O(9) atom to form distorted NiO5 square pyramids 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)O5 square pyramids, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. The Ni(2)-O(10) bond length is 2.27 Å. The Ni(2)-O(5) bond length is 2.06 Å. The Ni(2)-O(7) bond length is 2.02 Å. The Ni(2)-O(8) bond length is 2.06 Å. The Ni(2)-O(9) bond length is 1.93 Å. There are three inequivalent H sites. In the first H site, H(1) is bonded in a distorted bent 150 degrees geometry to one O(1) and one O(7) atom. The H(1)-O(1) bond length is 1.04 Å. The H(1)-O(7) bond length is 1.58 Å. In the second H site, H(2) is bonded in a single-bond geometry to one O(8) atom. The H(2)-O(8) bond length is 0.98 Å. In the third H site, H(3) is bonded in a single-bond geometry to one O(7) atom. The H(3)-O(7) bond length is 0.98 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one K(1), one O(1), one O(3), and one O(4) atom. The Se(1)-O(1) bond length is 1.79 Å. The Se(1)-O(3) bond length is 1.71 Å. The Se(1)-O(4) bond length is 1.71 Å. In the second Se site, Se(2) is bonded to one O(10), one O(2), one O(5), and one O(6) atom to form SeO4 tetrahedra that share corners with two equivalent Ni(1)O5 square pyramids, corners with two equivalent Ni(2)O5 square pyramids, and  a cornercorner with one Mg(1)O5 trigonal bipyramid. The Se(2)-O(10) bond length is 1.74 Å. The Se(2)-O(2) bond length is 1.64 Å. The Se(2)-O(5) bond length is 1.69 Å. The Se(2)-O(6) bond length is 1.69 Å. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to two equivalent K(1), one H(1), and one Se(1) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to one K(1) and one Se(2) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one K(1), one Mg(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one K(1), one Mg(1), and one Se(1) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to one K(1), one Ni(2), and one Se(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one K(1), one Ni(1), and one Se(2) atom. In the seventh O site, O(7) is bonded in a distorted tetrahedral geometry to one Ni(1), one Ni(2), one H(1), and one H(3) atom. In the eighth O site, O(8) is bonded in a 4-coordinate geometry to one Mg(1), one Ni(1), one Ni(2), and one H(2) atom. In the ninth O site, O(9) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ni(1), and one Ni(2) atom. In the tenth O site, O(10) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ni(1), one Ni(2), and one Se(2) atom.