Co2As2O9 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Co sites. In the first Co site, Co(1) is bonded to one O(1), one O(2), one O(3), one O(6), one O(7), and one O(8) atom to form distorted CoO6 octahedra that share corners with two equivalent As(2)O4 tetrahedra, corners with three equivalent As(1)O4 tetrahedra, and edges with two equivalent Co(2)O6 octahedra. In the second Co site, Co(2) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(9) atom to form distorted CoO6 octahedra that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra, and edges with two equivalent Co(1)O6 octahedra. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AsO4 tetrahedra that share corners with two equivalent Co(2)O6 octahedra, corners with three equivalent Co(1)O6 octahedra, and  a cornercorner with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-62°. In the second As site, As(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form AsO4 tetrahedra that share corners with two equivalent Co(1)O6 octahedra, corners with three equivalent Co(2)O6 octahedra, and  a cornercorner with one As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 47-61°. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a bent 120 degrees geometry to one Co(2) and one As(2) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Co(1), one Co(2), and one As(2) atom. In the third O site, O(7) is bonded in a distorted trigonal planar geometry to one Co(1), one Co(2), and one As(2) atom. In the fourth O site, O(8) is bonded in a single-bond geometry to one Co(1) atom. In the fifth O site, O(1) is bonded in a distorted trigonal planar geometry to one Co(1), one Co(2), and one As(1) atom. In the sixth O site, O(9) is bonded in a single-bond geometry to one Co(2) atom. In the seventh O site, O(2) is bonded in a bent 120 degrees geometry to one Co(1) and one As(1) atom. In the eighth O site, O(3) is bonded in a distorted trigonal planar geometry to one Co(1), one Co(2), and one As(1) atom. In the ninth O site, O(4) is bonded in a bent 120 degrees geometry to one As(1) and one As(2) atom.
Co2As2O9 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Co sites. In the first Co site, Co(1) is bonded to one O(1), one O(2), one O(3), one O(6), one O(7), and one O(8) atom to form distorted CoO6 octahedra that share corners with two equivalent As(2)O4 tetrahedra, corners with three equivalent As(1)O4 tetrahedra, and edges with two equivalent Co(2)O6 octahedra. The Co(1)-O(1) bond length is 2.06 Å. The Co(1)-O(2) bond length is 1.93 Å. The Co(1)-O(3) bond length is 2.04 Å. The Co(1)-O(6) bond length is 2.19 Å. The Co(1)-O(7) bond length is 2.16 Å. The Co(1)-O(8) bond length is 1.61 Å. In the second Co site, Co(2) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(9) atom to form distorted CoO6 octahedra that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra, and edges with two equivalent Co(1)O6 octahedra. The Co(2)-O(1) bond length is 2.27 Å. The Co(2)-O(3) bond length is 2.15 Å. The Co(2)-O(5) bond length is 1.96 Å. The Co(2)-O(6) bond length is 2.01 Å. The Co(2)-O(7) bond length is 2.06 Å. The Co(2)-O(9) bond length is 1.72 Å. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AsO4 tetrahedra that share corners with two equivalent Co(2)O6 octahedra, corners with three equivalent Co(1)O6 octahedra, and  a cornercorner with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-62°. The As(1)-O(1) bond length is 1.74 Å. The As(1)-O(2) bond length is 1.69 Å. The As(1)-O(3) bond length is 1.73 Å. The As(1)-O(4) bond length is 1.80 Å. In the second As site, As(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form AsO4 tetrahedra that share corners with two equivalent Co(1)O6 octahedra, corners with three equivalent Co(2)O6 octahedra, and  a cornercorner with one As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 47-61°. The As(2)-O(4) bond length is 1.78 Å. The As(2)-O(5) bond length is 1.69 Å. The As(2)-O(6) bond length is 1.73 Å. The As(2)-O(7) bond length is 1.73 Å. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a bent 120 degrees geometry to one Co(2) and one As(2) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Co(1), one Co(2), and one As(2) atom. In the third O site, O(7) is bonded in a distorted trigonal planar geometry to one Co(1), one Co(2), and one As(2) atom. In the fourth O site, O(8) is bonded in a single-bond geometry to one Co(1) atom. In the fifth O site, O(1) is bonded in a distorted trigonal planar geometry to one Co(1), one Co(2), and one As(1) atom. In the sixth O site, O(9) is bonded in a single-bond geometry to one Co(2) atom. In the seventh O site, O(2) is bonded in a bent 120 degrees geometry to one Co(1) and one As(1) atom. In the eighth O site, O(3) is bonded in a distorted trigonal planar geometry to one Co(1), one Co(2), and one As(1) atom. In the ninth O site, O(4) is bonded in a bent 120 degrees geometry to one As(1) and one As(2) atom.