Unusual Rearrangement of an <i>N</i>‑Donor-Functionalized <i>N</i>‑Heterocyclic Carbene Ligand on Group 8 Metals

We report an unexpected rearrangement of a deprotonated picolyl-functionalized N-heterocyclic carbene (NHC) ligand from N,C-chelate to N,N-chelate in three-legged piano-stool Fe­(II) and Ru­(II) complexes. The reaction mechanism has been explored for one of the Fe­(II) complexes. Experimental and computational studies suggest an unusual intermediate featuring a four-membered chelate ring, where the NHC and the α-carbon of one of the N-substituents coordinate to the Fe­(II) center. A possible Fe–alkylidene intermediate has also been predicted by computations.