Metal cluster topology .18. Flattening of hypoelectronic indium and thallium deltahedra in intermetallic phases containing alkali metals

Recently characterized intermetallic phases of the alkali metals and the heaviest icosogens In and Tl contain unusual hypoelectronic icosogen deltahedra having from 5 to II vertices. In these deltahedra the apparent electron deficiency is relieved by a distortion consisting of flattening selected vertices to allow participation of vertex d orbitals in addition to the usual sp(3) valence orbital manifold. The shapes of these In and Tl deltahedra suggest that such flattening occurs preferentially at degree 4 vertices in accord with the four-fold symmetry of the relevant (n-1) d orbitals (e.g. d(x(2)-y(2))). Th, prevalence of globally delocalized flattened 10- and 1 I-vertex In deltahedra in phases such as K8In10Zn and K8In11 can be related to the strategic location of their degree 4 vertices, all of which are subject to flattening with retention of the symmetry of the undistorted deltahedron. A greater variety of structures are found in the Tl deltahedra where symmetry breaking of octahedral and tricapped trigonal bipyramidal Tl clusters can occur through selective flattening of degree 4 vertices of Tl-6(6-) in KTl and Tl-9(9-) in Na2K21Tl19, respectively. In addition, a variety of regular icosahedral Tl-12 clusters are known such as the Tl(Tl-12)(10-) anions in Na(4)M(6)Tl(13) (M = K, Rb, Cs), the Tl(Tl-12)(11-) anion in Na3K8Tl13, and the Na(Tl-12)(13-) anion in Na15K6Tl18H in which the apparent electron deficiencies are relieved by introduction of interstitial atoms (Tl or Na) rather than flattening in accord with the absence of degree 4 vertices in the regular icosahedron.