The tetracapped truncated tetrahedron in 16-vertex tetrametallaborane structures: spherical aromaticity with an isocloso rather than a closo skeletal electron count. Issue 39 (27th September 2019) - Attia, Amr A. A.
Abstract : DFT studies on the experimentally known Cp*3 Rh3 B12 H12 Rh(B4 H9 RhCp*) as well as two model compounds indicate low energy structures with central Rh4 B12 tetracapped tetratruncated tetrahedra for these 32 Wadean skeletal electron systems. Abstract : Density functional theory studies on the experimentally known Cp*3 Rh3 B12 H12 Rh(B4 H9 RhCp*) as well as the model compounds Cp4 Rh4 B12 H12 and Cp3 Rh3 B12 H12 Rh(η 3 -C3 H5 ) indicate low energy structures with central Rh4 B12 tetracapped tetratruncated tetrahedra (TTT) for these 32 Wadean skeletal electron systems. This skeletal electron count corresponds to 2 k 2 ( k = 4) skeletal electrons suggesting a spherical aromatic system with filled 1s + 1p + 1d + 1f molecular orbitals as well as an isocloso 2 n (= 32 for n = 16) skeletal electron count. Similar TTT structures are found for the valence isoelectronic 32 skeletal electron systems [Cp4 M′′4 B12 H12 ] 4+ (M′′ = Ni, Pd, Pt) and [Cp4 M′4 B12 H12 ] 4− (M′ = Fe, Ru, Os). The preferred structures of the 34 skeletal electron systems [Cp4 M4 B12 H12 ] 2− (M = Co, Rh, Ir), [Cp4 M′′4 B12 H12 ] 2+ (M′′ = Ni, Pd, Pt) are not the most spherical TTT despite their 2 n + 2 skeletal electron count (= 34 for n = 16) for a closo structure by the Wade–Mingos rules. Instead they are prolate (elongated) polyhedra with two degree 6 and two degree 5 metal vertices with a central M4 macrobutterfly having one long M⋯M distance of ∼5.0 Å between the wingtips. The preferred structures for the still electron richer 36 skeletal electron systems Cp4 M′′4 B12 H12 (M′′ = Pd, Pt) are derived from triple square antiprisms with two open M′′2 B2 square faces. A distorted version of this polyhedron is the deltahedral structure with four degree 5 metal vertices and four degree 6 boron vertices found in the valence isoelectronic 36 skeletal electron first row transition metal derivatives Cp4 Ni4 B12 H12 and [Cp4 Co4 B12 H12 ] 4− . However, this polyhedron is not found in the 36 skeletal electron [Cp4 M4 B12 H12 ] 4− (M = Rh, Ir), that instead have symmetrical central M4 B12 TTTs. For some 16-vertex [Cp4 M4 B12 H12 ] z systems deviating from the favored 32 skeletal electron count, low-energy structures are found in which hydrogen atoms migrate to bridge B–B edges or bend over to bridge M–B edges. In addition, the hypoelectronic hexacations [Cp4 M4 B12 H12 ] 6+ (M = Co, Rh, Ir; Ni, Pd, Pt) are found to have low-energy structures in which three of the four Cp rings are hydrogenated to give tetrahapto cyclopentadiene η 4 -C5 H6 rings.