Synthesis and Characterization of [Fe 3 (As 3 ) 3 (As 4 )] 3- , a Binary Fe/As Zintl Cluster With an Fe 3 Core.

We report here the synthesis and structural characterization of the first binary iron arsenide cluster anion, [Fe 3 (As 3 ) 3 (As 4 )] 3- , present in both [K([2.2.2]crypt)] 3 [Fe 3 (As 3 ) 3 (As 4 )] (1) and [K(18-crown-6)] 3 [Fe 3 (As 3 ) 3 (As 4 )]⋅en (2). The cluster contains an Fe 3 triangle with three short Fe-Fe bond lengths (2.494(1) Å, 2.459(1) Å and 2.668(2) Å for 1, 2.471(1) Å, 2.473(1) Å and 2.660(1) Å for 2), bridged by a 2-butene-like As 4 unit. An analysis of the electronic structure using DFT reveals a triplet ground state with direct Fe-Fe bonds stabilizing the Fe 3 core.