A 1:1:1 reaction between complex (Tp)-(ArN=)Mo(H)(PMe(3)) (3), silane PhSiD(3), and carbonyl substrate established that hydrosilylation catalyzed by 3 is not accompanied by deuterium incorporation into the hydride position of the catalyst, thus ruling out the conventional hydride mechanism based on carbonyl insertion into the M-H bond. An analogous result was observed for the catalysis by (O=)(PhMe(2)SiO)Re(PPh(3))(2)(I)(H) and (Ph(3)PCuH)(6).