Formation of M4Se4 cuboids (M = As, Sb, Bi) via secondary pnictogen-chalcogen interactions in the co-crystals MX3·Se&z.dbd;P(p-FC6H4)3 (M = As, X = Br; M = Sb, X = Cl; M = Bi, X = Cl, Br)Electronic supplementary information (ESI) available: Table S1: Short FF and FH contacts in the structures of 2, 3, 5 and 6. CCDC 881801-881804. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt31010d

The reactions of the group 15 trihalides, MX 3 (M = As, Sb, Bi; X = Cl, Br), with the phosphine selenide SeP( p -FC 6 H 4 ) 3 result in the formation of co-crystals of formula MX 3 ·SeP( p -FC 6 H 4 ) 3 . No reaction was observed with MI 3 (M = As, Sb, Bi). The structures of MX 3 ·SeP( p -FC 6 H 4 ) 3 (M = As, X = Br 2 ; M = Sb, X = Cl 3 ; M = Bi, X = Cl 5 ; M = Bi, X = Br 6 ) have been established, and are isomorphous, crystallising in the cubic I 23 space group. All the structures feature a primary MX 3 unit, which has three weak secondary MSe interactions to SeP( p -FC 6 H 4 ) 3 molecules. However, each of these SeP( p -FC 6 H 4 ) 3 molecules bridges three MX 3 molecules, resulting in the generation of an M 4 Se 4 (M = As, Sb, Bi) distorted cuboid linked by the pnictogen-chalcogen interactions. Four opposing corners of the cuboid are occupied by the M atom (M = As, Sb, Bi) of an MX 3 pyramid, and the other four by the selenium atom of the phosphine selenide. The reactions of the group 15 trihalides, MX 3 (M = As, Sb, Bi; X = Cl, Br), with SeP( p -FC 6 H 4 ) 3 result in the formation of co-crystals of formula MX 3 ·SeP( p -FC 6 H 4 ) 3 , which link up via MSe interactions to form distorted cuboids M 4 Se 4 of T symmetry.