The Nowotny chimney ladder phases: Whence the 14 electron rule?

The late transition metal Nowotny chimney ladder phases (NCLs, TtEm; T, groups 7-9; E, groups 13 and 14) follow a 14 electron rule: the total number of valence electrons per T atom is 14. In this paper, we extract a chemical explanation for this rule from extended Huckel calculations; we focus on RuGa2, the parent NCL structure. A gap between filled and unfilled bands arises from the occupation of two Ga-Ga bonding/Ru-Ga nonbonding orbitals per RuGa2, independent of k-point. In addition, the five Ru d levels are filled. Together this makes for 7 filled bands at each k-point, or 14 electrons per Ru. We discuss the connections between this 14 electron rule and the 18 electron rule of organometallic complexes.