Stable homopolyatomic anions of the post-transition metals. "Zintl ions". The synthesis and structure of a salt containing the heptantimonide(3-) anion

Abstract

The phenomenon of the formation of homopolyatomic or the so-called Zintl anions on solutions of many alloys in liquid ammonia is considered, and a general route to the isolation of stable homopolyatomic anion is illustrated in the case of antimony. The dark brown crystalline (4,7,13,16,21,24-hexaoxa-1, 10-diazabicyclo(8.8.8)hexacasanc) sodium heptantimonide (3-), (C/sub 18/H/sub 36/N/sub 2/O/sub 6/ . Na/sup +/)/sub 3/Sb/sub 7//sup 3 -/, has been prepared from the reaction of a sodium--antimony alloy with C/sub 18/H/sub 36/N/sub 2/O/sub 6/(2,2,2-crypt) in ethylenediamine. Lattice constants a = 23.292 A, b = 13.791 A, c = 25.355 A, and ..beta.. = 108.56/sup 0/ with four molecules per unit cell in space group P2/sub 1//n were deduced from three-dimensional x-ray data collected from a single crystal of the compound on a four-circle diffractometer using MoK/sub ..cap alpha../ radiation. The problem was phased using direct methods. Stepwise full-matrix least-squares refinement of the ten heavy atoms with anisotropic thermal parameters and of the 78 light atoms of the crypts with isotropic temperature factors resulted in converged atomic parameter with R = 0.111 and R/sub w/ = 0.134. Three cryptated sodium cations in which the sodium ion occupies the central cavity of the macrobicycle occur together with the Sb/submore » 7//sup 3 -/ cluster anion, the first example of a well-characterized polyatomic anion for a metallic element. The polyantimony anion exhibits approximate C/sub 3..nu../ symmetry as an end-capped trigonal prism with the capped face substantially expandable, analogous to the isoelectric P/sub 4/S/sub 3/. Bond distances in the cluster range from 2.693 to 2.880 A. Possible geometries for several other Zintl ions of the group 5 elements are proposed on the basis of known isoelectric ions or molecules.« less