The tetracapped truncated tetrahedron in 16-vertex tetrametallaborane structures: spherical aromaticity with an isocloso rather than a closo skeletal electron count

Abstract

Density functional theory studies on the experimentally known Cp*₃Rh₃B₁₂H₁₂Rh(B₄H₉RhCp*) as well as the model compounds Cp₄Rh₄B₁₂H₁₂ and Cp₃Rh₃B₁₂H₁₂Rh(η³-C₃H₅) indicate low energy structures with central Rh₄B₁₂ tetracapped tetratruncated tetrahedra (TTT) for these 32 Wadean skeletal electron systems. This skeletal electron count corresponds to 2k² (k = 4) skeletal electrons suggesting a spherical aromatic system with filled 1s + 1p + 1d + 1f molecular orbitals as well as an isocloso 2n (= 32 for n = 16) skeletal electron count. Similar TTT structures are found for the valence isoelectronic 32 skeletal electron systems [Cp₄M′′₄B₁₂H₁₂]⁴⁺ (M′′ = Ni, Pd, Pt) and [Cp₄M′₄B₁₂H₁₂]⁴⁻ (M′ = Fe, Ru, Os). The preferred structures of the 34 skeletal electron systems [Cp₄M₄B₁₂H₁₂]²⁻ (M = Co, Rh, Ir), [Cp₄M′′₄B₁₂H₁₂]²⁺ (M′′ = Ni, Pd, Pt) are not the most spherical TTT despite their 2n + 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 Cp₄M′′₄B₁₂H₁₂ (M′′ = Pd, Pt) are derived from triple square antiprisms with two open M′′₂B₂ 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 Cp₄Ni₄B₁₂H₁₂ and [Cp₄Co₄B₁₂H₁₂]⁴⁻. However, this polyhedron is not found in the 36 skeletal electron [Cp₄M₄B₁₂H₁₂]⁴⁻ (M = Rh, Ir), that instead have symmetrical central M₄B₁₂ TTTs. For some 16-vertex [Cp₄M₄B₁₂H₁₂]^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 [Cp₄M₄B₁₂H₁₂]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 η⁴-C₅H₆ rings.