Reaction of KI, La, LaI3, and Os in a welded niobium tube at 800 degrees C yields black, air-sensitive crystals of K2La6I12Os. The phase was characterized by single-crystal X-ray diffraction (monoclinic, P2(1)/c, Z = 2, a = 9.890(3) Angstrom, b = 19.319(5) Angstrom, c = 10.047(4) Angstrom, beta = 108.69(3)degrees at 23 degrees C). The structure contains tetragonally compressed La6I12 clusters centered by Os and interbridged by I-a atoms according to the general pattern of (R(6)Z)(I6I6/2i-aI6/2a-i)-I-i for all tramples with X:R ratios of 2:1. However, the detailed pattern of three-bonded Ii-a and Ia-i bridging is new owing to the need to accommodate a relatively large, distorted cluster and two potassium ions in a more open structure. One type of Ii-a and Ia-i bridging defines zigzag chains of clusters with strain evident in the interconnections. EHMO calculations show that the observed 0.49 Angstrom tetragonal compression of the 16-electron cluster (relative to an optimal 18-e) splits the nominal t(lu)(4) HOMO into a two-below-one pattern and gives a closed-shell configuration. The compound accordingly exhibits only a small and typical temperature-independent paramagnetism, similar to 6 x 10(-4) emu mol(-1).