Homoleptic tetranuclear osmium carbonyls: from the rhombus via the butterfly to the tetrahedron

Abstract

The structures of the tetranuclear osmium carbonyl derivatives Os₄(CO)_n (n = 16, 15, 14, 13, 12) have been investigated using the density functional theory method MPW1PW91 with the SDD effective core potential basis set, found to be effective in previous work for the study of Os₃(CO)₁₂. The Os₄ clusters in the lowest energy structures for Os₄(CO)₁₆, Os₄(CO)₁₅, and Os₄(CO)₁₄ are found to be rhombi, butterflies, and tetrahedra with four, five, and six Os–Os bonds, respectively, in accord with structures determined by X-ray diffraction as well as the 18-electron rule. The fluxionality of tetrahedral Os₄(CO)₁₄, suggested by experimental work of Johnston, Einstein, and Pomeroy, is confirmed by our DFT studies, which find four Os₄(CO)₁₄ structures within 1.5 kcal mol⁻¹ of each other with similar tetrahedral Os₄ frameworks but with different arrangements of bridging and semibridging carbonyl groups. The lowest energy structures for the more unsaturated Os₄(CO)₁₃ and Os₄(CO)₁₂ are also based on Os₄ tetrahedra but with shorter Os–Os edge lengths than in Os₄(CO)₁₄ suggesting delocalized multiple bonding in the more highly unsaturated systems. Thus the global minimum for Os₄(CO)₁₂ is predicted to have tetrahedral symmetry, with all terminal carbonyl groups analogous to the experimentally known structure of (µ₃-H)₄Re₃(CO)₁₂, but without the face-bridging hydrogen atoms.