Treatment of aromatic N-substituted N-heterocyclic carbenes (NHCs) with trimethyl-gallium and -indium yielded the new Lewis acid–base adducts, IMes·GaMe₃ (1), SIMes·GaMe₃ (2), IPr·GaMe₃ (3), SIPr·GaMe₃ (4), IMes·InMe₃ (5), SIMes·InMe₃ (6), IPr·InMe₃ (7), and SIPr·InMe₃ (8), with all complexes being identified by X-ray diffraction, IR, and multinuclear NMR analyses. Complex stability was found to be largely dependent on the nature of the constituent NHC ligands. Percent buried volume (%V_Bur) and topographic steric map analyses were employed to quantify and elucidate the observed trends. Additionally, a detailed bond snapping energy (BSE) decomposition analysis focusing on both steric and orbital interactions of the M–NHC bond (M = Al, Ga and In) has been performed.