The structure described by Mg(NiO2)2 crystallizes in the orthorhombic Pmmn space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form distorted MgO6 pentagonal pyramids that share corners with six equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with four equivalent Ni(2)O6 octahedra, and edges with two equivalent Mg(2)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 3-17°. In the second Mg site, Mg(2) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form distorted MgO6 pentagonal pyramids that share corners with four equivalent Ni(1)O6 octahedra, corners with eight equivalent Ni(2)O6 octahedra, edges with two equivalent Mg(1)O6 pentagonal pyramids, and faces with two equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-52°. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form NiO6 octahedra that share corners with two equivalent Mg(2)O6 pentagonal pyramids, corners with three equivalent Mg(1)O6 pentagonal pyramids, edges with two equivalent Ni(1)O6 octahedra, edges with four equivalent Ni(2)O6 octahedra,  an edgeedge with one Mg(1)O6 pentagonal pyramid, and  a faceface with one Mg(2)O6 pentagonal pyramid. In the second Ni site, Ni(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form NiO6 octahedra that share corners with four equivalent Mg(2)O6 pentagonal pyramids, edges with two equivalent Ni(2)O6 octahedra, edges with four equivalent Ni(1)O6 octahedra, and edges with two equivalent Mg(1)O6 pentagonal pyramids. There are three inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ni(1), and two equivalent Ni(2) atoms. In the second O site, O(2) is bonded to one Mg(2), one Ni(1), and two equivalent Ni(2) atoms to form OMgNi3 trigonal pyramids that share corners with four equivalent O(3)Mg2Ni3 trigonal bipyramids, corners with three equivalent O(2)MgNi3 trigonal pyramids, and edges with four equivalent O(3)Mg2Ni3 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(1), one Mg(2), one Ni(2), and two equivalent Ni(1) atoms to form OMg2Ni3 trigonal bipyramids that share corners with five equivalent O(3)Mg2Ni3 trigonal bipyramids, corners with two equivalent O(2)MgNi3 trigonal pyramids, edges with four equivalent O(3)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(2)MgNi3 trigonal pyramids. is represented by the Crystallographic Information File (CIF) [CIF]
data_Mg(NiO2)2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   4.916
_cell_length_b   5.748
_cell_length_c   10.079
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   284.765
_cell_formula_units_Z   4
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Mg  Mg0  1  0.335  0.000  0.500  1.0
  Mg  Mg1  1  0.665  0.500  0.000  1.0
  Mg  Mg2  1  0.999  0.000  0.000  1.0
  Mg  Mg3  1  0.001  0.500  0.500  1.0
  Ni  Ni4  1  0.013  0.500  0.239  1.0
  Ni  Ni5  1  0.013  0.500  0.761  1.0
  Ni  Ni6  1  0.987  0.000  0.261  1.0
  Ni  Ni7  1  0.987  0.000  0.739  1.0
  Ni  Ni8  1  0.500  0.250  0.250  1.0
  Ni  Ni9  1  0.500  0.750  0.750  1.0
  Ni  Ni10  1  0.500  0.750  0.250  1.0
  Ni  Ni11  1  0.500  0.250  0.750  1.0
  O  O12  1  0.358  0.500  0.149  1.0
  O  O13  1  0.358  0.500  0.851  1.0
  O  O14  1  0.642  0.000  0.351  1.0
  O  O15  1  0.642  0.000  0.649  1.0
  O  O16  1  0.710  0.500  0.351  1.0
  O  O17  1  0.710  0.500  0.649  1.0
  O  O18  1  0.290  0.000  0.149  1.0
  O  O19  1  0.290  0.000  0.851  1.0
  O  O20  1  0.177  0.250  0.364  1.0
  O  O21  1  0.177  0.750  0.636  1.0
  O  O22  1  0.823  0.750  0.136  1.0
  O  O23  1  0.823  0.250  0.864  1.0
  O  O24  1  0.823  0.250  0.136  1.0
  O  O25  1  0.823  0.750  0.864  1.0
  O  O26  1  0.177  0.750  0.364  1.0
  O  O27  1  0.177  0.250  0.636  1.0
[/CIF]
.
The structure described by Mg(NiO2)2 crystallizes in the orthorhombic Pmmn space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form distorted MgO6 pentagonal pyramids that share corners with six equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with four equivalent Ni(2)O6 octahedra, and edges with two equivalent Mg(2)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 3-17°. In the second Mg site, Mg(2) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form distorted MgO6 pentagonal pyramids that share corners with four equivalent Ni(1)O6 octahedra, corners with eight equivalent Ni(2)O6 octahedra, edges with two equivalent Mg(1)O6 pentagonal pyramids, and faces with two equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 45-52°. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form NiO6 octahedra that share corners with two equivalent Mg(2)O6 pentagonal pyramids, corners with three equivalent Mg(1)O6 pentagonal pyramids, edges with two equivalent Ni(1)O6 octahedra, edges with four equivalent Ni(2)O6 octahedra,  an edgeedge with one Mg(1)O6 pentagonal pyramid, and  a faceface with one Mg(2)O6 pentagonal pyramid. In the second Ni site, Ni(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form NiO6 octahedra that share corners with four equivalent Mg(2)O6 pentagonal pyramids, edges with two equivalent Ni(2)O6 octahedra, edges with four equivalent Ni(1)O6 octahedra, and edges with two equivalent Mg(1)O6 pentagonal pyramids. There are three inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ni(1), and two equivalent Ni(2) atoms. In the second O site, O(2) is bonded to one Mg(2), one Ni(1), and two equivalent Ni(2) atoms to form OMgNi3 trigonal pyramids that share corners with four equivalent O(3)Mg2Ni3 trigonal bipyramids, corners with three equivalent O(2)MgNi3 trigonal pyramids, and edges with four equivalent O(3)Mg2Ni3 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(1), one Mg(2), one Ni(2), and two equivalent Ni(1) atoms to form OMg2Ni3 trigonal bipyramids that share corners with five equivalent O(3)Mg2Ni3 trigonal bipyramids, corners with two equivalent O(2)MgNi3 trigonal pyramids, edges with four equivalent O(3)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(2)MgNi3 trigonal pyramids. is represented by the Crystallographic Information File (CIF) [CIF]
data_Mg(NiO2)2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   4.916
_cell_length_b   5.748
_cell_length_c   10.079
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   284.765
_cell_formula_units_Z   4
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Mg  Mg0  1  0.335  0.000  0.500  1.0
  Mg  Mg1  1  0.665  0.500  0.000  1.0
  Mg  Mg2  1  0.999  0.000  0.000  1.0
  Mg  Mg3  1  0.001  0.500  0.500  1.0
  Ni  Ni4  1  0.013  0.500  0.239  1.0
  Ni  Ni5  1  0.013  0.500  0.761  1.0
  Ni  Ni6  1  0.987  0.000  0.261  1.0
  Ni  Ni7  1  0.987  0.000  0.739  1.0
  Ni  Ni8  1  0.500  0.250  0.250  1.0
  Ni  Ni9  1  0.500  0.750  0.750  1.0
  Ni  Ni10  1  0.500  0.750  0.250  1.0
  Ni  Ni11  1  0.500  0.250  0.750  1.0
  O  O12  1  0.358  0.500  0.149  1.0
  O  O13  1  0.358  0.500  0.851  1.0
  O  O14  1  0.642  0.000  0.351  1.0
  O  O15  1  0.642  0.000  0.649  1.0
  O  O16  1  0.710  0.500  0.351  1.0
  O  O17  1  0.710  0.500  0.649  1.0
  O  O18  1  0.290  0.000  0.149  1.0
  O  O19  1  0.290  0.000  0.851  1.0
  O  O20  1  0.177  0.250  0.364  1.0
  O  O21  1  0.177  0.750  0.636  1.0
  O  O22  1  0.823  0.750  0.136  1.0
  O  O23  1  0.823  0.250  0.864  1.0
  O  O24  1  0.823  0.250  0.136  1.0
  O  O25  1  0.823  0.750  0.864  1.0
  O  O26  1  0.177  0.750  0.364  1.0
  O  O27  1  0.177  0.250  0.636  1.0
[/CIF]
.