The reaction of (L-Ar)Co(py) with (BuN3)-Bu-t afforded the isolable three-coordinate Co-imido complex (L-Ar)Co((NBu)-Bu-t), which is paramagnetic at room temperature. Variable-temperature (VT) H-1 NMR spectroscopy, VT crystallography, and magnetic susceptibility measurements revealed that (L-Ar)Co((NBu)-Bu-t) undergoes a thermally induced spin crossover from an S = 0 ground state to a quintet (S = 2) state. The reaction of (L-Ar)Co(py) with mesityl azide yielded an isolable S = 1 terminal imido complex that was converted into the metallacycloindoline (L-Ar)Co(kappa(2)-NHC6H2-2,4-Me-2-6-CH2) via benzylic C-H activation.