The recently discovered ditantalaboranes Cp 2Ta 2B nH n+6 (n = 4, 5) are isoelectronic with the previously discovered dimetallaboranes Cp 2M 2B nH n+4 of the group 6 metals Cr, Mo, and W where Cp = η 5-cyclopentadienyl or substituted cyclopentadienyl. Their oblatonido polyhedral structures can be derived from the oblate (flattened) deltahedra of the oblatocloso dirhenaboranes Cp 2Re 2B n+1H n+1 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 Cp 2Ta 2B nH n+6 (n = 4, 5) and Cp 2M 2B nH n+4 (M=Cr, Mo, W). However, the synthesis of Cp 2Ta 2B nH n+6 (n = 4, 5) from CpTaCl 4 + LiBH 4/BH 3 also gives a less-reduced product Cp 2Ta 2Cl 2B 5H 11 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 Cp 2Ta 2B 4H 10 have the favorable degree or 5. © 2012 Springer-Verlag.