Flattened deltahedral structures and bridging hydrogen atoms in hypoelectronic dimolybdaboranes and ditungstaboranes

The following four Cp2M2Bn−2Hn−2 structure types (M = Mo, W) have been found in the low-energy structures using density functional theory: (1) Flattened oblatocloso deltahedra related to the experimentally known Cp2Re2Bn−2Hn−2 structures but not as severely flattened with intradeltahedral M–M (M = Mo, W) distances of ∼2.8 to ∼3.2 Å, which are 0.15–0.30 Å less than those in their Tc and Re counterparts; (2) Structures based on closo or isocloso deltahedra with short Mtriple bond; length of mdashM distances (M = Mo, W) of ∼2.4 to ∼2.6 Å along an edge corresponding to deltahedral surface metal–metal triple bonds; (3) Three 11-vertex deltahedral Cp2M2B9H9 structures (M = Mo, W) structures having two non-adjacent degree 6 vertices occupied by the metal atoms; (4) Two 8-vertex Cp2M2B6H6 structures with multiple degree 3 vertices arising from the capping of smaller deltahedra. In many of these structures one or two of the hydrogen atoms bridge M–B or B–B edges of the deltahedron.