Crystal Structure of Three Isomorphous Compounds of 2,5-Dibromopyridine with Tetrahalometalate(II) Ions

Abstract  Three salts of 2,5-dibromopyridinium with metal halides, (C₅H₄Br₂N)₂[CuCl₄], (I); (C₅H₄Br₂N)₂[CuBr₄], (II); and (C₅H₄Br₂N)₂[CdBr₄], (III), are isomorphous in C2/c with cell volumes 1864.5(8), 1974.6(4) and 2072.9(7) ?³, respectively. For the Cu cases (I) and (II) the discrete [MX₄]²⁻ anions are strongly distorted with max/min tetrahedral angles of 131.19(8)°/99.67(9)° and 130.24(5)°/99.36(11)°, respectively. For the Cd case (III) the distortion is much less with max/min tetrahedral angles of 115.97(3)°/105.99(4)°. The cations are planar. The short hal···hal contacts are for (I), aryl Br···anionic Cl 3.335(3) ? and 3.430(2) ?; for (II), aryl Br···anionic Br 3.473(2) ? and 3.567(2) ?; and for (III), aryl Br···anionic Br 3.452(1) ? and 3.675(1) ?. Longer aryl Br···aryl Br contacts (3.842(2) ? to 3.967(2) ? are present in the three isomorphs. Bifurcated hydrogen bonding to two anionic halides exist in the three structures with N–H···Cl 3.365(8) ? and 3.379(7) ? in (I), N–H···Br 3.539(11) ? and 3.425(11) ? in (II), and N–H···Br 3.426(5) ? and 3.387(5) ? in (III). Short ring contacts exist with the perpendicular distances between mean cation planes in (I), (II), and (III) of 3.68(6) ?, 3.56(1) ? and 3.72(1) ?, respectively. Index Abstract  An analysis is made of the competition between hydrogen bonding, halogen–halogen and halogen–halide interactions as well as differences between [CuX₄]²⁻ and [CdX_4] ²⁻ polydedra in the structures of the three isomorphs.