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SiB6 is Calcium hexaboride structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional and consists of one silicon molecule and one B framework. In the B framework, B is bonded in a 5-coordinate geometry to five equivalent B atoms. There is one shorter (1.65 Å) and four longer (1.75 Å) B–B bond length.
MgB12Si2 crystallizes in the orthorhombic Pnma space group. The structure is zero-dimensional and consists of sixteen boron molecules and four Mg(B4Si)2 clusters. In each Mg(B4Si)2 cluster, Mg2+ is bonded in a distorted bent 150 degrees geometry to two Si4- atoms. There are one shorter (2.45 Å) and one longer (2.64 Å) Mg–Si bond lengths. There are six inequivalent B+0.50+ sites. In the first B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.08 Å. In the second B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.07 Å. In the third B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.17 Å. In the fourth B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.00 Å. In the fifth B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.00 Å. In the sixth B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.07 Å. There are two inequivalent Si4- sites. In the first Si4- site, Si4- is bonded in a 5-coordinate geometry to one Mg2+ and four B+0.50+ atoms. In the second Si4- site, Si4- is bonded in a 4-coordinate geometry to one Mg2+ and four B+0.50+ atoms.