We employ quantum chemical calculations to discover how 'frustrated Lewis pairs (FLP) catalyze the reduction of CO, by ammonia borane (AB); specifically, we examine how the Lewis acid (LA) and Lewis base (LB) of an FLP activate CO, for reduction. We find that the LA (trichloroaluminum, AlCl3) alone catalyzes hydride transfer (HT) to CO, while the LB (trimesitylenephosphine, PMes(3)) actually hinders HT; inclusion of the LB increases the HT barrier by 8 kcal/mol relative to the reaction catalyzed by LAs only The LB hinders HT by donating its lone pair to the WMO of CO,,, increasing the electron density on the C atom and thus lowering its hydride affinity. Althbugh. the LB hinders HT, it nonetheless plays a crucial role by stabilizing 'the active FLP-CO, complex relative to the LA dinner, free CO2, and free LB. This greatly increases the concentration of the reactive complex in the form FLP-CO, and thus increases the rate of reaction. We. expect that the principles we describe will aid in understanding other catalytic CO, reductions.