Chemical bonding in oblatonido ditantalaboranes and related compounds.

The recently discovered ditantalaboranes CpTaBH ( n = 4, 5) are isoelectronic with the previously discovered dimetallaboranes CpMBH of the group 6 metals Cr, Mo, and W where Cp = η-cyclopentadienyl or substituted cyclopentadienyl. Their oblatonido polyhedral structures can be derived from the oblate (flattened) deltahedra of the oblatocloso dirhenaboranes CpReBH by removal of an equatorial BH vertex with adjustment of the skeletal electron count by changing the metal atoms and adding hydrogen atoms. In these oblatocloso dirhenaborane deltahedra, the approximately antipodal rhenium atoms are close enough together to form a formal Re=Re double bond with lengths in the range 2.69-2.82 Å. Similarly, short M=M distances are maintained in the related oblatonido derivatives CpTaBH ( n = 4, 5) and CpMBH (M=Cr, Mo, W). However, the synthesis of CpTaBH ( n = 4, 5) from CpTaCl + LiBH/BH also gives a less-reduced product CpTaClBH with a longer Ta-Ta distance of ~3.2 Å. This may be regarded as a formal single bond bridged by one of the hydrogen atoms. Vertices of degree 5 (excluding terminal atoms/groups but not edge-bridging hydrogens) are sites of highest stability/lowest chemical reactivity not only in metal-free boranes but also in the dimetallaboranes discussed in this paper. For example, all four boron vertices in CpTaBH have the favorable degree or 5. Graphical Abstract: The recently discovered ditantalaboranes CpTaBH ( n = 4, 5) are isoelectronic with the previously discovered dimetallaboranes CpMBH (M=Cr, Mo, W). They have oblatonido structures derived by removal of one BH vertex from the flattened oblatocloso polyhedra of the dirhenaboranes CpReBH. All of these structures have internal M=M formal double bonds inside the deltahedron. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]