A complex of an N-heterocyclic carbene I^tBu (1,3-di-tert-butylimidazolin-2-ylidene) and aluminum hydride was observed to isomerize into an abnormal carbene complex aI^tBu·AlH₃ (aI^tBu = 1,3-di-tert-butylimidazolin-4-ylidene) in a polar solvent, and, for the first time, in the solid state. I^tBu·AlH₃ and aI^tBu·AlH₃ were structurally characterized by single crystal X-ray diffraction analysis. NMR studies and DFT computations indicate that the polarity of the solvent promotes the isomerization process. The possible pathways for the isomerization are discussed on the basis of the DFT computational studies.