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* 2 Re 2 B n- 2 H n - 2 (Cp* = Me 5 C 5 ; n = 8-12) of the third row group 7 element rhenium are not favored for analogous dimetallaboranes Cp 2 Mn 2 B n - 2 H n - 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 Cp 2 Mn 2 B n - 2 H n - 2 ( n = 8-13) structures typically have a central MnB n - 2 closo deltahedron with one face capped by the second CpMn unit. However, for the 14-vertex Cp 2 Mn 2 B 12 H 12 system the lowest-energy structures consist of B 12 icosahedra with faces capped by both CpMn units. The thermochemistry of cluster buildup reactions of the type Cp 2 Mn 2 B n - 2 H n - 2 + BH → Cp 2 Mn 2 B n - 1 H n - 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 Cp 2 Mn 2 B n - 2 H n - 2 ( n = 8-14) suggests possible applications in novel magnetic materials.