Isolation of Elusive Electrophilic Phosphinidene Complexes with π-Donor N-Heterocyclic Vinyl Substituents.

Phosphinidene complexes of the general formula RPM(CO) (R = an alkyl or aryl group; M = a transition metal) are electrophilic and thermally unstable. Thus, the isolation of these elusive species for structural elucidations remains a challenge. Herein, we report the first terminal phosphinidene complexes [{(NHC)C(Ph)}P]Fe(CO) [NHC = IPr = C{(NDipp)CH} for ; Me-IPr = C{(NDipp)CMe} for ; Dipp = 2,6-PrCH; NHC = N-heterocyclic carbene] as red crystalline solids containing a π-donor N-heterocyclic vinyl (NHV) substituent at the phosphorus atom. Calculations reveal donor-acceptor type bonding between phosphorus and iron atoms in and . The P → Fe donation represents ∼70% of the orbital interaction, whereas the Fe → P π-back-donation corresponds to ∼15% of the orbital interaction. The phosphorus atoms in and carry charges of +0.65e and +0.64e, respectively, indicating the electrophilic character of the phosphinidene {(NHC)C(Ph)}P moiety. Accordingly, reacts with an NHC nucleophile (IMe) to yield the Lewis adduct [{(NHC)C(Ph)}P(IMe)]Fe(CO) () [IMe = C(NMeCMe)]. The coordination of an electron-rich NHC (IMe) to the phosphorus atom in precludes the π-electron density transfer from the NHV to the phosphorus atom. Thus, the C-C and C-P bonds of become shorter and longer, respectively, compared to those of .