The crystal structure of tetrakis(tetracarbonyliron)tin has been elucidated by single-crystal X-ray analysis. The crystals are monoclinic: a= 10·78, b= 19·13, c= 11·77 Å, β= 90° 40′, Z= 4, space-group P2₁/n(C_2h⁵). The approximate molecular symmetry in the crystalline state is 2m(D_2d). The tetrahedral valencies of the tin atom are tetragonally distorted, giving two different pairs of iron–iron distances, one 2·87 the other 4·65 Å. The tin–iron bond length is 2·54 Å. The configuration about each iron atom may be described as approximately octahedral, the vertices of the octahedron being defined by the tin atom, another iron atom, and four carbonyl groups. The Fe–C–O bonds are non-linear in the crystalline state (ca. 171°), the mean values of the Fe–C and C–O bond lengths being 1·75 and 1·18 Å, respectively.