The title compounds are obtained from reactions of KI, RI3, R, and Os at 750-850 degrees C in sealed Nb or Ta containers. The structure was established by single-crystal X-ray diffraction studies (tetragonal, P4/mnc, Z = 2; a = 10.044(4), 9.879(3) Angstrom and c = 21.825(4), 21.86(1) Angstrom for La, Pr, respectively) for three samples, La and Pr at 23 degrees C and La at -100 degrees C. This particular combination of large R, interstitial Z, and halogen results in the highest X:R ratio known among rare-earth-metal cluster halides and an unprecedented structure type. The cluster units are interconnected according to the pattern [(R6Os)(I8I4/2i-aI4/2a-iI2a)-I-i](4-) in which four R in the waist of each cluster, the Os, and four bridging I-i-a,I-a-i pairs generate a planar 2D network of geometrically nearly ideal, 16-electron R6Os octahedra. This contrasts with the tetragonal compression found for many other 16-e(-) examples with the parallel disappearance of Curie-Weiss paramagnetism. Powdered alpha-K4La6I14Os exhibits an exceptionally low resistivity for a cluster halide (similar to 120 mu Omega.cm) over 110-280 K and a small paramagnetism over about 35 to similar to 250 K associated with a probable Mott-Hubbard state that changes to a temperature-independent (presumably Pauli) paramagnetism above about 300 K. EHTB band calculations show that a significant broadening of the HOMO (similar to t(1u)) states takes place in the plane of the intercluster bridging. The unique structure, properties, and band picture for the compound indicate a transition to a delocalized (band) state.