Important intermediates in regioselective hydride reduction of NAD(P) model compounds were detected by various spectroscopic methods. The metal hydrido complexes [Ru(tpy)(bpy)H](+) and [Re(bpy)-(CO)(3)H] react with the NAD(P) models to give 1:1 adducts, which are cleaved to the 1,4-dihydro form of the models and the solvento complexes, quantitatively.