Reactions of trans-[(dmpe)2MnH(C2H4)] (1) with BH3(NMe3), 9-BBN, and HBMes2 yielded the manganese(i) borohydride complexes [(dmpe)2Mn(μ-H)2BR2] (3: R = H, 4: R2 = C8H14, 5: R = Mes). The reaction of 1 with BH3(NMe3) proceeds via ethylene substitution. By contrast, a detuerium labelling study indicates that the reaction of 1 with HBMes2 involves initial isomerization of 1 to an unobserved 5-coordinate ethyl intermediate, [(dmpe)2MnEt], which reacts with the hydroborane to afford EtBR2 and [(dmpe)2MnH], followed by reaction with a second equivalent of hydroborane to generate 5 (an analogous pathway is likely followed for other base-free hydroboranes such as 9-BBN). Identification of 3-5 as κ2-borohydride complexes, as opposed to boryl dihydride or hydroborane hydride isomers, is supported by 11B NMR spectroscopy, X-ray diffraction, and Atoms in Molecules calculations. Two byproducts were observed in the syntheses of 3-5: [{(dmpe)2MnH}2(μ-dmpe)] (6) and [(dmpe)2MnH(κ1-dmpe)] (7). These complexes were independently prepared by exposure of 1 to free dmpe under an atmosphere of Ar or H2, and the generality of this synthetic route was demonstrated by the reaction of 1 with PMe3 (under H2) to form [(dmpe)2MnH(PMe3)] (8). Complexes 6-8 can exist as isomers with either a trans or a cis relationship between the hydride and κ1-coordinated phosphine ligands on manganese. trans to cis isomerization of 6-8 is photochemically induced, whereas the reverse reaction occurs under thermal conditions. X-ray crystal structures were obtained for 3-5, trans,trans-6, cis,cis-6, trans-7, and trans-8.