Diverse reactivities of aromaticity–unsaturation coexisted metalladithiolene rings

This review paper summarizes the reactivities of metal dithiolene complexes based on the ‘coexistence of aromaticity and unsaturation’ in the five-membered metallacycle, the so-called metalladithiolene ring (MS2C2). The 16-electron [LM(dithiolene)] (LM = CpMIII, Cp*MIII, (C6R6)MII) complexes are coordinatively unsaturated and usually show M–S centered cycloaddition reactions with nucleophiles (e.g. diazoalkanes, organic azides, quadricyclane) and electrophiles (e.g. tetracyanoethylene oxide, activated acetylene). The resulting metalladithiolene cycloadducts, which have three-membered M–S–C or M–S–N rings, further react with protic acids or PR3 to undergo the ring-opening reactions involving the M–C bond, M–S bond or M–N bond cleavages. Furthermore, diverse adduct dissociations are observed by thermal, photochemical or electrochemical redox reactions. Such reactions normally produce the original [LM(dithiolene)] complexes (non-adduct) and the eliminated fragments. Among them, the Co–S centered imido adduct [CpCo(dithiolene)(NR)] (R = Ts, Ms) reacted under thermal conditions in the presence of PR3 to undergo the intramolecular imido migration reaction to the Cp ligand, giving [(C5H4–NHR)Co(dithiolene)] complexes. The M–S centered multinuclear cluster complexes are obtained by the reaction of [LM(dithiolene)] with low valent M(CO)n complexes. The square-planar bis(dithiolene) complexes [M(dithiolene)2]0 (M = Ni, Pd, Pt) or tris(dithiolene) complexes [M(dithiolene)3]0 yield cycloaddition products with olefins. These reactions are due to ligand centered reactions made possible by a molecular orbital overlap between dithiolene LUMO and olefin HOMO. Similar ligand centered adducts are obtained by the reaction of dianionic [M(dithiolene)2]2− with haloalkanes or dihaloalkanes. Also these adducts of bis(dithiolene) complex are dissociated photochemically and electrochemically. This paper also describes the reactivities of organometallic o-carborane dithiolate complexes, which are generally formulated as [LM(S2C2B10H10)] (LM = CpCo, Cp*Rh, Cp*Ir, (p-cymene)Ru and (p-cymene)Os). Diverse addition reactions are reported; in particular, the reaction with acetylene involves B–H bond activation in the carborane moiety.