The effect of ylide substitution at the α position to the carbene carbon (C_c) atom on the stability and σ-donating ability of a number of cyclic carbenes has been studied theoretically. The stabilities of all of the carbenes were investigated from an evaluation of their singlet–triplet energy gaps and stabilization energies. All carbenes were found to have a stable singlet state. The energy of the σ-symmetric lone-pair orbital at the C_c atom increases as a result of the introduction of ylide centers near to the C_c atom. This indicates an enhanced σ-donating ability of the ylide-containing carbenes. The calculated carbonyl-stretching frequencies of the corresponding rhodium complexes, proton affinities, and nucleophilicity index values correlate well with the σ basicity of the carbenes.

Donor–acceptor interactions play a dominant role in descriptions of various chemical systems. The interactions of main-group Lewis bases with main-group Lewis acids has attracted interest for many years. In this article, donor–acceptor interactions in NHC–EX₃ (NHC = normal and abnormal N-heterocyclic carbene; E = B, Al, Ga; X = H, F, Cl, OH, NH₂, CH₃, CF₃) adducts have been investigated within the realms of DFT and atoms-in-molecules (AIM) theory. Substituents attached to the E atom have a profound effect on the strength and dissociation energies of the NHC–E bond. AIM analysis suggests that these donor–acceptor bonds have significant covalent character, which follows the order Al < Ga < B.

The chemistry of nitrogen fixation has been the subject of considerable research with a view to gaining a proper understanding of the mechanistic details. In this article, density functional calculations are performed on all the mechanistic possibilities for dinitrogen reduction mediated by the tripodal iron complexes [(SiP^Me₃)Fe^I] ([Fe_Si]) and [(BP^Me₃)Fe⁰] ([Fe_B]). Dinitrogen addition to the neutral bare complex is found to be thermodynamically more favorable than that to the anionic one. Both symmetric and asymmetric pathways, along with the possible intermediates and transition states, are considered in this study. For both systems, the symmetric path is found to be more likely, although the prospect of the asymmetric path cannot be ignored. Moreover, interconversions between these two pathways are found to be less likely. This study corroborates most experimental observations and provides theoretical insight into the possible existence of some hitherto unknown intermediates such as multiple-bonded FeN species in a trigonal bipyramidal geometry. Furthermore, in agreement with experimental observations, this study also highlights the possibility of hydrogen and hydrazine evolution during the complete reduction of dinitrogen.

Density functional calculations predict that 2,2′-bipyridyl carbenes have some degree of “hidden” carbon(0) character. This is supported by very high second proton affinity values and bond dissociation energies (BDEs) for the dissociation of one AuCl and one Ni(CO)₂ molecule from [(AuCl)₂(2,2′-bipyridyl carbene)] and [{Ni(CO)₂}₂(2,2′-bipyridyl carbene)]. An Arduengo type carbene also showed significant BDE values for gem-dimetallation, thus indicating the possible existence of carbon(0) character. All of the dimetallated derivatives showed metallophilic interactions.