The synthesis and molecular structures of the cobalt (II) N-heterocyclic carbene (NHC) complexes [(NHC)-Co{N(SiMe3)(2)}(2)], where NHC = 1,3-bis(diisopropylphenyl)-imidazolylidene (IPr) (6), 1,3-bis(mesityl)imidazolylidene (IMes) (7), and 1,3-bis(tert-butyl)imidazol-2-ylidene ((IBu)-Bu-t) (8), are reported. Complexes 6-8 are rare examples of three-coordinate cobalt NHC complexes. The steric congestion within the coordination environments of the cobalt(II) centers in 6 and 7 results in the longest Co-C(NHC) distances currently known. Investigating the thermal stability of 6-8, we have found that the steric congestion in 6 is such that heating the complex to reflux in toluene prompts a rearrangement from the normal, C2-bonding mode of the IPr ligand to the corresponding "abnormal" or mesoionic bonding mode. The rearrangement results in formation of [(aIPr)Co{N(SiMe3)(2)}(2)] (9), which is the first cobalt complex of an abnormal NHC ligand. The Co C bond in 9 is 0.06 angstrom shorter than the analogous bond in 6, suggesting that, although the rearrangement occurs due to the spatial demands of the IPr ligand, there is also a thermodynamic driving force in terms of the Co C bond. In contrast to the case for 6, complex 7 is stable with respect to the normal-to-abnormal rearrangement. Refluxing complex 8 in toluene results in activation of a tert-butyl substituent, which is eliminated as isobutene, followed by formation of the 1-tert-butylimidazole complex [((t)BuIm)Co{N-(SiMe3)(2)}(2)] (10).