S Supporting Information B orides are an important class of nonmolecular solids, although they often display substantial molecular character due to the strong tendency of B to self-bond and thus form molecule-like clusters such as B 6 octahedra, B 12 icosahedra (as found in elemental boron), and B 24 cube-truncated octahedra. On decreasing the relative B content in intermetallic borides, the tendency of boron to self-bond remains and leads to boron networks with low dimensionally, such as broken honeycombs, chains, and dimers, before isolated B atoms are encountered. 1,2 Due to the light mass of boron, which yields high vibrational frequencies, borides are expected to display high superconducting transition temperatures; MgB 2 (T c = 39 K), with its honeycomb lattice of boron, and YB 6 (T c = 7 K), with its B 6 clusters, are important examples. 3−10 Superconductivity is generally considered to fall within the realm of materials physics, but new superconducting compounds, often found by materials chemists, can have a significant impact in the direction of the field. When physicists search for new superconductors, their outlook is based on k-space and "Fermiology", which are not intuitive concepts for most chemists. Here we approach the problem of superconductor design differently. Motivated by the fragment formalism widely used when viewing the structures of inorganic and organometallic molecules, we here describe the discovery of a new superconductor made through manipulating the valence electron concentration in boridesnot through substitution of individual dopant atoms to adjust the Fermi level and therefore the Fermi surface, 11 but by doping through the combination of electron-donating and electron-accepting structural fragments, a distinctly chemical concept.Superconductivity is very unpredictable for new materials. This is because it results from electronic instabilities that are delicately balanced with other factors such as electron−lattice coupling. 12 Although no predictive tools have yet been devised to account for all factors, we show here that the fragment formalism is a new way to predict potential new superconductors from a chemical perspective. We use this principle to search for new superconducting compounds. We find that the metal-rich boride, NbRuB, whose existence and structure have also recently been described, 13 is superconducting. We observe BCS-like superconductivity in this compound with a T c = 3.1 K. This structure may be considered as being built up by two types of boron-centered trigonal prisms: an uncapped boron-centered trigonal prism with the formula Ru 3 B whose structure is as found in Re 3 B, a known superconductor with a T c of 4.8 K, and a B−B dimer-containing face-sharing double trigonal prism of formula Nb 3 B 2 . Given these fragments and the electron count of the Re 3 B superconductor, we start with the hypothesis that in NbRuB the structural fragments will be combined to yield (Ru 3 B)(−3e − )Nb 3 B 2 (+3e − ) (i.e., Nb 3 Ru 3 B 3 ) in the optimal case; Ru 3 B has three...