Use Wade's rules with the metal included as a skeletal vertex. Each BH vertex supplies two skeletal electrons, each CH supplies three, and the two additional hydrogen atoms supply two more. The fragment supplies skeletal electrons after the metal's non-skeletal orbitals have been accounted for. ThusThe six-vertex cluster is therefore nido, with a pentagonal-pyramidal skeleton: Ru is the apex, bonded to the five vertices of the planar base, and the three carbonyl ligands project away from that base. This is the six-vertex fragment obtained by removing an equatorial vertex from a seven-vertex pentagonal bipyramid. The five ring atoms carry their terminal B–H or C–H bonds; the two additional hydrogens belong to the hydrogen-rich open-face bonding framework. The skeletal electron count does not, by itself, locate every bridging hydride, so a unique detailed hydrogen arrangement should not be inferred from it.
There are two carbon-placement isomers, represented by the cyclic base sequences and . The carbon vertices are adjacent in the first and separated by one boron vertex in the second. Rotation or reversal of a sequence does not produce another constitutional isomer.
Each arrangement has a reflection symmetry that exchanges a pair of boron sites while leaving the third distinct. Consequently each can give two environments, with site populations , in a proton-decoupled NMR spectrum. This assumes that the hydrogen and carbonyl arrangements preserve, or rapidly average to, the corresponding skeletal symmetry. Two peaks alone neither choose between the two skeletons nor establish a unique hydrogen arrangement; accidental overlap or fluxionality can also reduce the number of observed signals.
The usual qualitative prediction is the carbon-separated isomer. It has four B–C ring edges and one B–B edge, whereas the adjacent-carbon isomer has two B–C, two B–B and one C–C edge. Separating the more electron-rich carbon vertices distributes skeletal electron density more evenly and avoids concentrating both carbon vertices on one edge, a useful carborane stability heuristic.
This is a thermodynamic suggestion, not a consequence of the peak count. hydrogen placement, metal–ring interactions and entropy can affect the ordering. Establishing the preferred isomer experimentally would require reversible interconversion and an equilibrium composition, rather than simply identifying the product formed fastest.
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