Several coordinatively unsaturated pseudotetrahedral iron(II) precursors, [PhBP3Pr]Fe-R ([PhBP(Pr)3] = [PhB((CH2PPr2)-Pr-/)(3)](-); R = Me (2), R = CH2Ph (3), R = CH2CMe3 (4)) have been prepared from [PhBP3/Pr)FeCl (1) that serve as precatalysts for the room-temperature hydrogenation of unsaturated hydrocarbons (e.g., ethylene, styrene, 2-pentyne) under atmospheric H-2 pressure. The solid-state crystal structures of 2 and 3 are presented. To gain mechanistic insight into the nature of these hydrogenation reactions, a number of [PhBP3/Pr]-supported iron hydrides were prepared and studied. Room-temperature hydrogenation of alkyls 2-4 in the presence of a trapping phosphine ligand affords the iron(IV) trihydride species [PhBP3/Pr]Fe(H)(3)(PR3) (PR3 = PMe3 (5); PR3 = PEt3 (6); PR3 = PMePh2 (7)). These spectroscopically well-defined trihydrides undergo hydrogen loss to varying degrees in solution, and for the case of 7, this process leads to the structurally identified Fe(II) hydride product [PhBP3/Pr]Fe(H)(PMePh2) (9). Attempts to prepare 9 by addition of LiEt3BH to 1 instead lead to the Fe(I) reduction product [PhBP3/Pr]Fe(PMePh2) (10). The independent preparations of the Fe(II) monohydride complex [PhBP3/Pr]Fe-II(H)(PMe3) (II) and the Fe(I) phosphine adduct [PhBP3/Pr]Fe(PMe3) (8) are described. The solid-state crystal structures of trihydride 5, monohydride 11, and 8 are compared and demonstrate relatively little structural reorganization with respect to the P3Fe-P' core motif as a function of the iron center's formal oxidation state. Although paramagnetic 11 (S = 1) is quantitatively converted to the diamagnetic trihydride 5 under H-2, the Fe(I) complex 8 (S = (3)/(2)) is inert toward atmospheric H-2. Complex 10 is likewise inert toward H-2. Trihydrides 5 and 6 also serve as hydrogenation precatalysts, albeit at slower rates than that for the benzyl complex 3 because of a rate-contributing phosphine dependence. That these hydrogenations appear to proceed via well-defined olefin insertion steps into an Fe-H linkage is indicated by the reaction between trihydride 5 and ethylene, which cleanly produces the ethyl complex [PhBP3/Pr]Fe(CH2CH3) (13) and an equivalent of ethane. Mechanistic issues concerning the overall reaction are described.