Unexpected Reactivity Patterns of Ruthenium Alkylidenes with N‑Phosphino-Functionalized N‑Heterocyclic Carbene Ligands (NHCPs)

N-phosphino-functionalized N-heterocyclic carbene (NHCP) ligands have been evaluated as potential supporting ligands in ruthenium-catalyzed olefin metathesis. Initial density functional theory (DFT) calculations suggested that these NHCP ligands may allow access to neutral 14 valence electron (VE) speciesequivalents of the active 14 VE species formed by phosphine dissociation from Grubbs II precatalystsvia facile decoordination of the NHCP phosphino donor of the strained four-membered [RuPNC] chelate systems. Their attempted synthesis from NHCPs and Grubbs-type Ru carbenes revealed addition of an NHCP donor atom (P or C) to the alkylidene fragment, forming a new C–P or C–C bond in five-membered chelate structures. DFT investigations showed that these reactions are controlled kinetically and must not be neglected as important possible deactivation routes in olefin metathesis.