Transition metal terminal carbides are exceedingly rare. The handful of examples known to date hail from Groups 6 and 8, including [{N(R)Ar}3Mo(≡C:)] -(Tp* = 3,5dimethyltris(pyrazolyl)borate) and [Tp*(CO)2M(≡CLi)] (M = W, Mo), 2,3,4,5 [M(≡C:)(L)2(X)2] (M = Ru, Os), and [P2Mo(≡C:)(CO)] +,0,-(P2 = a terphenyl-diphosphine ligand). For square-pyramidal and octahedral complexes, a "carbido wall" (analogous to the oxo wall) appears to apply at a d-electron of 2, corresponding to a dd 2 electronic configuration. Thus, an Ir(VII)-carbido corrole has been postulated as a stable species. Higher d-electron counts of up to 4 are conceivable for pseudotetrahedral complexes, corresponding to a dd 4 electronic configuration. Little, however, is known about the actual stability of such complexes or about their excited-state architectures. Our curiosity about this question was further piqued by reports of quadruple binding in the diatomic molecule RhB and subsequently in several additional diatomics. Might an isoelectronic quadruple-bonded metal-carbide fragment be isolable in the form of a synthetic complex? To shed light on the question, we have examined a series of d 4 pseudotetrahedral tris(N-heterocyclic-carbene) transition metal carbides and nitrides with density functional theory (DFT) calculations. The nitrido complexes examined are closely related to stable, experimentally well-characterized systems and have been included here as calibration for our calculations on the speculative carbido systems. Based on their singlet-triplet gaps and electron affinities, several of the terminal carbides examined are indeed expected to be thermodynamically stable and hence worthwhile synthetic targets. Scheme 1. Terminal carbides studied in this work.