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MnB2 is hexagonal omega structure structured and crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Mn is bonded to twelve equivalent B atoms to form a mixture of edge and face-sharing MnB12 cuboctahedra. All Mn–B bond lengths are 2.27 Å. B is bonded in a 9-coordinate geometry to six equivalent Mn and three equivalent B atoms. All B–B bond lengths are 1.73 Å.
Ta(MnB2)2 crystallizes in the orthorhombic Immm space group. The structure is three-dimensional. Ta5+ is bonded to twelve B3- atoms to form a mixture of edge and face-sharing TaB12 cuboctahedra. There are eight shorter (2.39 Å) and four longer (2.47 Å) Ta–B bond lengths. Mn+3.50+ is bonded in a 7-coordinate geometry to seven B3- atoms. There are a spread of Mn–B bond distances ranging from 2.14–2.26 Å. There are two inequivalent B3- sites. In the first B3- site, B3- is bonded in a 9-coordinate geometry to four equivalent Ta5+, two equivalent Mn+3.50+, and three B3- atoms. There is two shorter (1.80 Å) and one longer (1.88 Å) B–B bond length. In the second B3- site, B3- is bonded in a 9-coordinate geometry to two equivalent Ta5+, five equivalent Mn+3.50+, and two equivalent B3- atoms.