The crystal structures of the title compounds, [Sb(S₂C·NEt₂)₃], (I), and [Bi(S₂C·NEt₂)₃](II) have been determined from X-ray diffractometer data by the heavy-atom method and refined by least squares to R 0.07 (I), 3 [332 reflections] and 0.09 (II)[6 029 reflections]. Crystals of both are monoclinic, Z= 4, space group P2₁/a; (I): a= 14.665(5), b= 13.619(5), c= 12.642(4)Å, β= 99.86(4)°; (II): a= 14.825(4), b= 13.640(2), c= 12.605(3)Å, β= 100.01(3)°.

(I) and (II) are isostructural; the symmetry of the M(S₂C)₃ unit resembles the C₃ symmetry of the arsenic derivative, with three short M–S bonds fac, one from each ligand [M–S 2.487(4)–2.631(4)(I); 2.595(5)–2.775(5)Å(II)], and three long bonds [M–S 2.886(4)–2.965(4)(I); 2.956(5)–2.964(4)Å(II)], with a considerable gap in the co-ordination about the pseudo-C₃ axis suggesting a stereochemically active lone-pair. The inversion image of the molecule approaches near this direction also, with a weak intermolecular metal–sulphur interaction [M ⋯ S^I, 3.389(4)(I), 3.210(4)Å(II)].