Reversal of stability on metalation of pentagonal-bipyramidal (1-MB₆H₇^2-, 1-M-2-CB₅H₇^1-, and 1-M-2,4-C₂B₄H₇) and icosahedral (1-MB₁₁H₁₂^2-, 1-M-2-CB₁₀H₁₂^1-, and 1-M-2,4-C₂B₉H₁₂) boranes (M = Al, Ga, In, and Tl): energetics of condensation and relationship to binuclear metallocenes

Abstract

The usual assumption of the extra stability of icosahedral boranes (2) over pentagonal-bipyramidal boranes (1) is reversed by substitution of a vertex by a group 13 metal. This preference is a result of the geometrical requirements for optimum overlap between the five-membered face of the ligand and the metal fragment. Isodesmic equations calculated at the B3LYP/LANL2DZ level indicate that the extra stability of 1-M-2,4-C₂B₄H₇ varies from 14.44 kcal/mol (M = Al) to 15.30 kcal/mol (M = Tl). Similarly, M(2,4-C₂B₄H₆)₂^1- is more stable than M(2,4-C₂B₉H₁₁)₂^1- by 9.26 kcal/mol (M = Al) and by 6.75 kcal/mol (M = Tl). The preference for (MC₂B₄H₆)₂ over (MC₂B₉H₁₁)₂ at the same level is 30.54 kcal/mol (M = Al), 33.16 kcal/ mol (M = Ga) and 37.77 kcal/mol (M = In). The metal-metal bonding here is comparable to those in CpZn-ZnCp and H₂M-MH₂ (M= Al, Ga, and In).