The new ruthenaborane, CP*3RU3B3H8, possesses a capped, five atom nido-cluster geometry rather than the more common octahedral geometry observed previously for seven skeletal electron pair M3B3 clusters. Comparison with the known, isoelectronic closo-1,2,3-{Cp*Co}(3)B3H5 shows that a more open capped geometry is favored as the number of endo-hydrogen atoms associated with the cluster framework increases.