THE NIDO-PB-9(4-) AND THE JAHN-TELLER DISTORTED CLOSO-PB-9(3-) ZINTL ANIONS - SYNTHESES, X-RAY STRUCTURES, AND THEORETICAL-STUDIES

The isolation and structural characterization of the nonaplumbide Zintl anion nido-Pb-9(4-) is reported, completing the series of nine-atom homopolyatomic anions of the group IV elements, M(9)(4-) (M = Ge, Sn, ph). The nido-Pb-9(4-) anion was obtained by extraction of the alloy KPb2.05 in ethylenediamine in the presence of a stoichiometric deficit of 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane). The K+(2,2,2-crypt-K+)(3)-Pb-9(4-) salt was obtained as a dark red crystalline compound. The X-ray crystal structure of K+(2,2,2-crypt-K+)(3)-Pb-9(4-) was determined at -100 degrees C; it crystallizes in the triclinic system, space group P (1) over bar, with two molecules in a unit cell of dimensions a 15.811(5) Angstrom, b = 16.173(5) Angstrom, c = 20.180(6) Angstrom, alpha = 98.600)degrees, beta = 104.59(2)degrees, and gamma 118.32(2)degrees with R = 0.0673 for 11833 observed (I > 2 sigma(I)) reflections. The use of an excess of 2,2,2-crypt allowed the isolation of (2,2,2-crypt-K+)(3)Pb-9(3-) containing the Pb-9(3-) anion which proved to be identical to the recently reported Pb-9(3-) anion in (2,2,2-crypt-K+)(3)Pb-9(3-). 0.5en. The (2,2,2-crypt-K+) salts contain the nine-atom homopolyatomic cluster anions nido-Pb-9(4-) and closo-Pb-9(3-) which have monocapped square antiprismatic (C-4 nu point symmetry) and distorted tricapped trigonal prismatic (C-2 nu point symmetry) geometries, respectively. Uncomplexed potassium atoms bridge the waist edges of the Pb-9(4-) anion in the K+(2,2,2-crypt-K+)(3)Pb-9(4-) salt forming infinite -K-(Pb-9)-K-(Pb-9)-K- chains. The structures of nido-Pb-9(4-) and related group IV analogs are predicted by Wade's rules and possess 2n + 4 skeletal electrons whose structures are monocapped square antiprisms of approximate C-4 nu point symmetry derived from 10-vertex polyhedra. The observed geometry of the closo-Pb-9(3-) can also be accommodated by Wade's rules and is regarded as a closo-structure which is Jahn-Teller distorted to C-2 nu-symmetry. Results from local and nonlocal density functional calculations show that for Pb-9(4-) and Pb-9(3-), the C-4 nu, and C-2 nu symmetries are true minima and are in agreement with their experimentally determined structures.