Stabilization of thallium(I) Pd9TlPd3 sandwich monocation with octahedral-based [Pd9(CO)9(PMe3)6] and [Pd3(CO)3(PMe3)3] entities versus unstable Pd3Tl(I)Pd3 sandwich monocation with [Pd3(CO)3(PEt3)3] entities: Comparative computational implications

In contrast to an unstable 'classical' {Tl[Pd3(CO)3(PEt3)3]2]}+ sandwich cation, Tl(I) in the {[Pd9(CO)9(PMe3)6]Tl[Pd3(CO)3(PMe3)3]}+ sandwich is stabilized and retains its structure in the solution due to the combined effects of increased 6s2 Tl(I) electron-pair donation and increased back-donation from its palladium cluster counterparts to the empty Tl(I) 6p valence orbitals.209