Modular Design Enables Access to Onium‐Substituted Diphosphacyclobutadienyl Complexes

We report a modular approach to synthesise zwitterionic phosphorus analogues of cyclobutadienyl ligands in the coordination sphere of 3d transition metals. Specifically, cationic manganese and cobalt complexes featuring η 4 ‐coordinated, onium‐substituted LL′C 2 P 2 ligands [L, L′ = PPh 3 , IPr (1,3‐bis(2,6‐diisopropyl‐phenyl)imidazolium−2‐ylidene)] were accessed via reaction of halogenated phosphorus cycles RR’C 2 (PX) 2 (X = Cl, Br) with the corresponding carbonyl metalates. Here, the choice of the substituents L, L′ allows for precise control of the steric environment while retaining exceptional π‐donating properties exceeding those of [C 5 H 5 ] − . Furthermore, the carbonyl ligands serve as versatile entry points for further derivatization, as demonstrated by the light‐driven formation of the 2,3‐dimethylbutadiene (DMB) complex [Co{ η 4 ‐DMB}{ η 4 ‐(PPh 3 ) 2 C 2 P 2 }][OTf] and the acetonitrile complex [Mn(CO) 2 (MeCN){ η 4 ‐(PPh 3 ) 2 C 2 P 2 }][OTf].