Triplet Spin-State Capped Deltahedral Structures Rather than Singlet Spin-State Oblatocloso Structures as Energetically Favored Dimanganaborane Structures

Density functional studies show that the singlet spin-state flattened oblatocloso deltahedral structures found experimentally in the dimetallaboranes Cp*2Re2Bn–2Hn–2 (Cp* = Me5C5; n = 8–12) of the third row group 7 element rhenium are not favored for analogous dimetallaboranes Cp2Mn2Bn–2Hn–2 (n = 8–14) of its first row congener manganese. Instead, the energetically preferred structures for the dimanganaboranes are higher spin-state triplet and quintet spin-state structures. This appears to be related to the lower ligand field splittings in complexes of the first row transition-metal manganese relative to analogous complexes of the third row transition-metal rhenium. The lowest-energy Cp2Mn2Bn–2Hn–2 (n = 8–13) structures typically have a central MnBn–2closo deltahedron with one face capped by the second CpMn unit. However, for the 14-vertex Cp2Mn2B12H12 system the lowest-energy structures consist of B12 icosahedra with faces capped by both CpMn units. The thermochemistry of cluster buildup reactions of the type Cp2Mn2Bn–2Hn–2 + BH → Cp2Mn2Bn–1Hn–1 suggests that the 11- and 13-vertex structures are likely to be favored products in reactions of cyclopentadienylmanganese derivatives with borane sources. The paramagnetism of the predicted triplet and quintet spin states for the lowest-energy dimanganaboranes Cp2Mn2Bn–2Hn–2 (n = 8–14) suggests possible applications in novel magnetic materials.