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.