Pentamethyl- and 1,2,4-tri(tert-butyl)cyclopentadienyl containing p-block complexes - differences and similarities

The sterically encumbered cyclopentadienyl ligand 1,2,4-(Me₃C)₃C₅H₂ (Cp''') was used to stabilize efficiently the main group metals of Al, Ga, In, Ge and Sn, respectively. The σ-bonded gallium compounds [η¹-Cp'''Ga(μ-X)X]₂ (X = Cl, 2; X = I, 3) and indium compound [η¹-Cp'''In(μ-Br)ⁿBu]₂ (7) exhibit dimers through halogen bridges. Reduction of 2 with 2 equivalents of KC₈ leads almost to the same amount of η¹-Cp'''Ga(THF)Cl₂ (4) and η⁵-Cp'''Ga (5), respectively. The exception is compound 5, which is obtained by reducing 2 or 3 with 4 equivalents of KC₈. Compound 5 as Lewis base reacts with GaI₃ readily forming the Lewis acid-base adduct product η⁵-Cp'''Ga → GaI₃ (6). Moreover, compounds with the Cp''' ligand stabilize heavier low-valent group 14 elements for example [η⁵-Cp'''E^II]⁺[E^IICl₃]^- (E = Ge 8, Sn 9), which are π-bonded ionic compounds that possess a low-valent cation and an anion. In the cation of [η⁵-Cp'''E^II]⁺, the Cp''' ligand adopts an η⁵-coordination mode with germanium and tin, respectively, which present half-sandwich complexes. While the E^II fragment interacts with five π electrons from the Cp''' unit to generate an electron-octet arrangement at the respective element. All new reported structures are comparing well with the corresponding compounds containing the pentamethylcyclopentadienyl (Cp*) ligand.