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K2La6I12Os:  A New Structure Type for Isolated Clusters That Is Generated by Cation Requirements

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journal contribution
posted on 25.03.1998, 00:00 by S. Uma, John D. Corbett
Reaction of KI, La, LaI3, and Os in a welded niobium tube at 800 °C yields black, air-sensitive crystals of K2La6I12Os. The phase was characterized by single-crystal X-ray diffraction (monoclinic, P21/c, Z = 2, a = 9.890(3) Å, b = 19.319(5) Å, c = 10.047(4) Å, β = 108.69(3)° at 23 °C). The structure contains tetragonally compressed La6I12 clusters centered by Os and interbridged by Ia atoms according to the general pattern of (R6Z)Ii6Ii-a6/2Ia-i6/2 for all examples 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 Å tetragonal compression of the 16-electron cluster (relative to an optimal 18-e) splits the nominal t1u4 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, ∼6 × 10-4 emu mol-1.

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