Homoleptic Transition Metal Carbonyl Cations: Synthetic Approaches, Characterization and Follow-Up Chemistry

ConspectusCarbon monoxide, CO, is one of the most important ligands in organometallic chemistry. It is an excellent π-acceptor and a moderate σ-donor. Therefore, most of the known transition metal carbonyls (TMCs) exhibit a zerovalent or even negative metal oxidation state (OS) of up to -4. However, given the right conditions, the carbonyl ligand also forms homoleptic cationic complexes with one or more transition metal atoms, the transition metal carbonyl cations (TMCCs), known with an OS of up to +3. Despite their long-standing history upon discovery of the first [M(CO)₆]⁺ examples (M = Mn, Tc, Re) by E. O. Fischer in 1962 as well as their very fundamental nature, it took until the 1990s for the scope to be widened by Aubke, Strauss and Willner. Yet, many potential TMCC entries known from gas-phase mass spectrometry work remained unknown on preparative grounds. This is due to their high reactivity, which puts scientists to new challenges and encourages the development of suitable solvents, anions and oxidants, to cope with the demands of these fundamental salts─later referred to as pseudo-gas-phase conditions and innocent deelectronators and solvents.Hence, the utilization of extremely weakly coordinating perfluorinated alkoxyaluminates [Al(OR^F)₄]^- and [F{Al(OR^F)₃}₂]^- (OR^F = -OC(CF₃)₃) in combination with the polar but non- or weakly coordinating innocent solvents 1,2-difluorobenzene (oDFB) and 1,2,3,4-tetrafluorobenzene (TFB) yielded the first TMCC salts containing heptacoordinate [M(CO)₇]⁺ (M = Nb, Ta) as well as paramagnetic [M(CO)₆]^+· (M = Cr, Mo, W) or [Ni(CO)₄]^+·. However, the use of typical inorganic oxidants Ag⁺, [NO]⁺ and Ag⁺/0.5 I₂ regularly led to unwanted side reactions. For example, the Lewis acidic silver(I) cations form Lewis pairs with various Lewis basic TMCs yielding partly clustered [Ag{TMC}] complex salts, while nitrosonium cations may substitute for carbonyl ligands, forming [M(CO)(NO)]⁺ complexes. The synergistic oxidizing reagent Ag⁺/0.5 X₂ can add halonium ions X⁺ to the TMCs (X = Cl, Br, I). This prevented the synthesis of univalent group 8 TMCC salts. Yet, the application of radical cation salts of perfluorinated arenes as innocent deelectronators finally yielded salts of [Fe(CO)₅]^+· and [M₃(CO)₁₄]²⁺ (M = Ru, Os).TMCC salts are excellent starting materials, and the reaction of [Co(CO)₅]⁺ and [Ni(CO)₄]^+· with benzene led to the previously unknown bis(benzene) sandwich complexes [Co(benzene)₂]⁺ and [Ni(benzene)₂]^+·. Under the right conditions, even the very weakly bound oDFB-complex salts with [M(oDFB)₂]⁺ (M = Co, Ni) cations form, useful as naked metal(I) synthons and for small-molecule activation.