K6U2Mo4O21 crystallizes in the triclinic P-1 space group. There are three inequivalent K sites. In the first K site, K(1) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(5), one O(7), one O(8), one O(9), and two equivalent O(3) atoms. In the second K site, K(2) is bonded in a 5-coordinate geometry to one O(10), one O(3), one O(8), and two equivalent O(6) atoms. In the third K site, K(3) is bonded in a 6-coordinate geometry to one O(10), one O(4), one O(6), one O(8), and two equivalent O(2) atoms. U(1) is bonded to one O(1), one O(10), one O(11), one O(2), one O(5), one O(7), and one O(9) atom to form distorted UO7 pentagonal bipyramids that share  a cornercorner with one U(1)O7 pentagonal bipyramid, corners with two equivalent Mo(1)O4 tetrahedra, and corners with two equivalent Mo(2)O4 tetrahedra. There are two inequivalent Mo sites. In the first Mo site, Mo(1) is bonded to one O(3), one O(5), one O(6), and one O(7) atom to form MoO4 tetrahedra that share corners with two equivalent U(1)O7 pentagonal bipyramids. In the second Mo site, Mo(2) is bonded to one O(1), one O(4), one O(8), and one O(9) atom to form MoO4 tetrahedra that share corners with two equivalent U(1)O7 pentagonal bipyramids. There are eleven inequivalent O sites. In the first O site, O(11) is bonded in a linear geometry to two equivalent U(1) atoms. In the second O site, O(1) is bonded in a distorted trigonal planar geometry to one K(1), one U(1), and one Mo(2) atom. In the third O site, O(2) is bonded in a distorted single-bond geometry to one K(1), two equivalent K(3), and one U(1) atom. In the fourth O site, O(3) is bonded in a 2-coordinate geometry to one K(2), two equivalent K(1), and one Mo(1) atom. In the fifth O site, O(4) is bonded in a linear geometry to one K(3) and one Mo(2) atom. In the sixth O site, O(5) is bonded in a 3-coordinate geometry to one K(1), one U(1), and one Mo(1) atom. In the seventh O site, O(6) is bonded in a 4-coordinate geometry to one K(3), two equivalent K(2), and one Mo(1) atom. In the eighth O site, O(7) is bonded in a 1-coordinate geometry to one K(1), one U(1), and one Mo(1) atom. In the ninth O site, O(8) is bonded in a 4-coordinate geometry to one K(1), one K(2), one K(3), and one Mo(2) atom. In the tenth O site, O(9) is bonded in a 3-coordinate geometry to one K(1), one U(1), and one Mo(2) atom. In the eleventh O site, O(10) is bonded in a distorted single-bond geometry to one K(2), one K(3), and one U(1) atom.
K6U2Mo4O21 crystallizes in the triclinic P-1 space group. There are three inequivalent K sites. In the first K site, K(1) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(5), one O(7), one O(8), one O(9), and two equivalent O(3) atoms. The K(1)-O(1) bond length is 2.71 Å. The K(1)-O(2) bond length is 3.19 Å. The K(1)-O(5) bond length is 2.86 Å. The K(1)-O(7) bond length is 2.88 Å. The K(1)-O(8) bond length is 2.83 Å. The K(1)-O(9) bond length is 2.77 Å. There is one shorter (3.20 Å) and one longer (3.26 Å) K(1)-O(3) bond length. In the second K site, K(2) is bonded in a 5-coordinate geometry to one O(10), one O(3), one O(8), and two equivalent O(6) atoms. The K(2)-O(10) bond length is 2.74 Å. The K(2)-O(3) bond length is 2.66 Å. The K(2)-O(8) bond length is 2.80 Å. There is one shorter (2.69 Å) and one longer (2.81 Å) K(2)-O(6) bond length. In the third K site, K(3) is bonded in a 6-coordinate geometry to one O(10), one O(4), one O(6), one O(8), and two equivalent O(2) atoms. The K(3)-O(10) bond length is 2.69 Å. The K(3)-O(4) bond length is 2.59 Å. The K(3)-O(6) bond length is 2.75 Å. The K(3)-O(8) bond length is 2.76 Å. There is one shorter (2.67 Å) and one longer (2.69 Å) K(3)-O(2) bond length. U(1) is bonded to one O(1), one O(10), one O(11), one O(2), one O(5), one O(7), and one O(9) atom to form distorted UO7 pentagonal bipyramids that share  a cornercorner with one U(1)O7 pentagonal bipyramid, corners with two equivalent Mo(1)O4 tetrahedra, and corners with two equivalent Mo(2)O4 tetrahedra. The U(1)-O(1) bond length is 2.38 Å. The U(1)-O(10) bond length is 1.83 Å. The U(1)-O(11) bond length is 2.15 Å. The U(1)-O(2) bond length is 1.83 Å. The U(1)-O(5) bond length is 2.41 Å. The U(1)-O(7) bond length is 2.46 Å. The U(1)-O(9) bond length is 2.39 Å. There are two inequivalent Mo sites. In the first Mo site, Mo(1) is bonded to one O(3), one O(5), one O(6), and one O(7) atom to form MoO4 tetrahedra that share corners with two equivalent U(1)O7 pentagonal bipyramids. The Mo(1)-O(3) bond length is 1.74 Å. The Mo(1)-O(5) bond length is 1.79 Å. The Mo(1)-O(6) bond length is 1.75 Å. The Mo(1)-O(7) bond length is 1.80 Å. In the second Mo site, Mo(2) is bonded to one O(1), one O(4), one O(8), and one O(9) atom to form MoO4 tetrahedra that share corners with two equivalent U(1)O7 pentagonal bipyramids. The Mo(2)-O(1) bond length is 1.80 Å. The Mo(2)-O(4) bond length is 1.73 Å. The Mo(2)-O(8) bond length is 1.75 Å. The Mo(2)-O(9) bond length is 1.81 Å. There are eleven inequivalent O sites. In the first O site, O(11) is bonded in a linear geometry to two equivalent U(1) atoms. In the second O site, O(1) is bonded in a distorted trigonal planar geometry to one K(1), one U(1), and one Mo(2) atom. In the third O site, O(2) is bonded in a distorted single-bond geometry to one K(1), two equivalent K(3), and one U(1) atom. In the fourth O site, O(3) is bonded in a 2-coordinate geometry to one K(2), two equivalent K(1), and one Mo(1) atom. In the fifth O site, O(4) is bonded in a linear geometry to one K(3) and one Mo(2) atom. In the sixth O site, O(5) is bonded in a 3-coordinate geometry to one K(1), one U(1), and one Mo(1) atom. In the seventh O site, O(6) is bonded in a 4-coordinate geometry to one K(3), two equivalent K(2), and one Mo(1) atom. In the eighth O site, O(7) is bonded in a 1-coordinate geometry to one K(1), one U(1), and one Mo(1) atom. In the ninth O site, O(8) is bonded in a 4-coordinate geometry to one K(1), one K(2), one K(3), and one Mo(2) atom. In the tenth O site, O(9) is bonded in a 3-coordinate geometry to one K(1), one U(1), and one Mo(2) atom. In the eleventh O site, O(10) is bonded in a distorted single-bond geometry to one K(2), one K(3), and one U(1) atom.