Penta-coordinated or -valent: the nature of the chemical bond of some Ti–C–Al compounds

Abstract

Detailed DFT calculations of the published [CpTi(μ²-Me)(μ²-NPPh₃)(μ⁵-C)(μ²-AlMe₂)₂(AlMe₂)(AlMe₃)] 1 revealed the triple-bond nature of the Ti–C bond and thus being a methide carbon with 5 surrounding ligands. This finding was further corroborated by the derivates [CpTi(μ²-Me)(μ²-NPPh₃)(μ⁴-C)(μ²-AlMe₂)₂(AlMe₂)] 3, which has one AlMe₃ ligand “removed” compared to 1, and [CpTi(μ²-NPPh₃)(SiMe₃)(μ³-C)(μ²-AlMe₂)(AlMe₂)] 4, where the –AlMe₂ moiety has been replaced by a non-coordinating SiMe₃ one. Detailed electronic investigations (QTAIM, NBO, ETS-NOVC) suggest a Ti–C triple bond, with one AlMe₂ moiety covalently bound to the carbon and the remaining three AlMe_x moieties interacting more in a Lewis acid–base fashion. Consequently, this is a penta-coordinated carbon and not, as the geometry would suggest, a penta-valent one.