The structures and energetics of species with the stoichiometries (Me₂CNO)M(CO)_nCp (M = Cr, Mo, W; n = 3, 2, 1; Cp = η⁵-C₅H₅) have been studied by density functional theory. The experimental structure Mo2-1S with an NO-dihapto Me₂CNO ligand found by X-ray crystallography for (Me₂CNO)Mo(CO)₂Cp is shown to be the lowest energy isomer by a substantial margin. The expected initial Me₂CNOMo(CO)₃Cp product from the reaction of NaMo(CO)₃Cp with Me₂C(NO)Br used to synthesize (Me₂CNO)Mo(CO)₂Cp is predicted to undergo facile CO dissociation based on ΔH and ΔG values. The energetics of the currently unknown analogous chromium and tungsten systems appear to be similar to that of the molybdenum system. The structures Me₂NC(CO)OM(CO)₂Cp (M = Cr, Mo, W) with a dimethylcarbamate ligand are much lower energy (Me₂CNO)M(CO)₃Cp isomers but are clearly not accessible from reactions of the anions CpM(CO)₃⁻ with Me₂C(NO)Br. The lowest energy structures for the monocarbonyl (Me₂CNO)M(CO)Cp are of the type (Me₂CN)M(O)(CO)Cp with separate dimethylimino and oxo ligands formed by oxidative addition of the Me₂CNO unit to the central metal atom.