To^MMgHB(C₆F₅)₃ (1, To^M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) catalyzes the 1,4-hydrosilylation of α,β-unsaturated esters. This magnesium hydridoborate compound is synthesized by the reaction of To^MMgMe, PhSiH₃, and B(C₆F₅)₃. Unlike the transient To^MMgH formed from the reaction of To^MMgMe and PhSiH₃, the borate adduct 1 persists in solution and in the solid state. Crystallographic characterization reveals tripodal coordination of the HB(C₆F₅)₃ moiety to the six-coordinate magnesium center with a ∠Mg–H–B of 141(3)°. The pathway for formation of 1 is proposed to involve the reaction of To^MMgMe and a PhSiH₃/B(C₆F₅)₃ adduct because the other possible intermediates, To^MMgH and To^MMgMeB(C₆F₅)₃, react to give an intractable black solid and To^MMgC₆F₅, respectively. Under catalytic conditions, silyl ketene acetals are isolated in high yield from the addition of hydrosilanes to α,β-unsaturated esters with 1 as the catalyst.