Investigation of the synthesis of {Mo(η⁵-C₅H₅)(CO)₃}⁺ fragments partnered with the monoanionic carboranes [closo-CB₁₁H₁₁Br]^-, [closo-CB₁₁H₆Br₆]^- and [closo-HCB₁₁Me₁₁]^- by silver salt metathesis and hydride abstraction

Abstraction of iodide from [Cp(CO)₃MoI] (Cp = η⁵-C₅H₅) using the silver salt of the weakly coordinating anion [closo-CB₁₁H₁₁Br]^-, initially affords an intermediate dimeric complex, [MoCp(CO)₃I·Ag(CB₁₁H₁₁Br)]₂, which has a central {AgI}₂ core appended by two carborane anions in the solid state. Prolonged reaction does result in elimination of AgI to form [MoCp(CO)₃(CB₁₁ H₁₁Br)], but not cleanly. This complex can be isolated in good yield by hydride abstraction from [Cp(CO)₃MoH] using the new trityl salt [CPh₃][closo-CB₁₁H₁₁Br]. Similarly, reaction of this hydride source with [CPh₃][closo-CB₁₁ H₆Br₆] in CD₂Cl₂ results in hydride abstraction, but a mixture of products is formed, suggested to consist of the hydride bridged dimer, [{Cp(CO)₃Mo}₂(μ-H)][closo-CB₁₁ H₆Br₆], the anion coordinated complex [Cp(CO)₃Mo (closo-CB₁₁H₆Br₆)] and the solvent adduct [Cp(CO)₃Mo(ClCD₂Cl)] [closo-CB₁₁H₆Br₆]. Reaction of Ag[closo-HCB₁₁Me₁₁] with [Cp(CO)₃MoX] (X = Cl, I) results in a intermediate complex which has a [Cp(CO)₃MoX]₆Ag₂ central core (crystallographically characterised for X = I) with no interactions from the permethylated carborane anions. Prolonged reaction does result in partial AgX elimination to form the halide-bridged dimers [{CpMo(CO)₃}₂(μ-X)] [closo-HCB₁₁Me₁₁]. Hydride abstraction from [Cp(CO)₃MoH] using the new trityl salt [CPh₃][closo-HCB₁₁Me₁₁] resulted in the clean formation of the hydride-bridged dimer [{Cp(CO)₃Mo}₂(μ-H)] [closo-HCB₁₁Me₁₁]. The two anions [closo-CB₁₁H₆Br₆]^- and [closo-HCB₁₁Me₁₁]^- have been compared with [B(Ar_F′)₄^-] [Ar_F′ = 3,5-bis(trifluoromethyl)phenyl] in Bullock's ionic hydrogenation of 3-pentanone using [Cp(CO)₂(PPh₃)Mo]⁺ catalysts. The permethylated anion slightly outperforms its hexabrominated congener in catalysis, but is still slower than [B(Ar_F′)₄]^-.