A straightforward strategy allows for the synthesis of storable bicyclic (alkyl)(amino)carbenes (BICAACs), which feature enhanced σ-donating and π-accepting properties compared to monocyclic (alkyl)(amino)carbenes (CAACs). Due to the bicyclo[2.2.2]octane skeleton, the steric environment around the carbene center is different from that of CAACs and similar to that observed in classical N-heterocyclic carbenes. The different electronic properties of BICAACs as compared to CAACs allow for ligand exchange reactions not only at a metal center, but also at main group elements.

We report a new class of stable mesoionic N‐heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground‐state structure cannot be described through an uncharged mesomeric Lewis‐structure, thereby structurally distinguishing them from traditional N‐heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N′‐diaryl‐1,2,3‐triazolium and N,N′‐diaryl‐imidazolium salts, respectively. In their reactivity, they represent strong σ‐donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs.

A room-temperature stable (phosphino)-phosphinidene reacts with carbon monoxide, stable singlet carbenes, including the poor π-accepting imidazol-2-ylidene, and phosphines giving rise to the corresponding phosphaketene, phosphinidene-carbene and phosphinidene-phosphine adducts, respectively. Whereas the electronic ground-state calculations indicate a PP multiple bond character in which the terminal phosphorus is negatively charged, the observed reactivity clearly indicates that (phosphino)phosphinidenes are electrophilic as expected for an electron-deficient species. This is further demonstrated by competition experiments as well as by the results of Fukui function calculations.

(Phosphino)phosphaketenes (>P-P═C═O) behave as (phosphino)phosphinidene-carbonyl adducts (>P-P←:C═O). CO scrambling was observed using C labeled CO, and exchange reactions with phosphines afford the corresponding (phosphino)phosphinidene-phosphine adducts (>P-P←:PR). The latter react with isonitriles and singlet carbenes giving (phosphino)phosphinidene-isonitrile (>P-P←:CNR) and -carbene adducts (>P-P←:C<). Based on experimental results and DFT calculations, it is shown that these "ligand" exchange reactions occur via an associative mechanism as classically observed with transition metal complexes.