Several manganese(I) and rhenium(I) carbonyl complexes of the chelating diphosphanylborane ligands Ph2PCH(PPh2)CH2B(C8H14) (1) and Ph2PCH2CH[B(C8H14)]PPh2 (2) were prepared to study Lewis acid assisted stoichiometric CO reductions. The phosphanylborane complexes fac,cis-Mn(CO)3(1)(Br) (3a), fac,cis-Re(CO)3(1)(Br) (4a), fac,cis-Mn(CO)3(2)(Br) (5a), and fac,cis-Re(CO)3(2)(Br) (6a) were obtained from the reactions of Mn(CO)5Br or Re(CO)5Br with diphosphanylborane ligands 1 and 2. Further treatment of all four complexes with one equivalent of AgBF4 in CH2Cl2 in the dark followed by 1 atm of CO afforded the corresponding monocationic complexes [Mn(CO)4(1)][BF4] (3b), [Re(CO)4(1)][BF4] (4b), [Mn(CO)4(2)][BF4] (5b), and [Re(CO)4(2)](BF4) [6b]. An X-ray diffraction study confirmed the expected molecular structure of 3b. In addition to NMR and elemental analysis characterizations, the structures of these complexes could be unambiguously assigned based on two major ν(CO) IR bands, which established the cis-(CO)4 local ligand environment. The cationic complexes 3b, 4b, 5b, and 6b were tested for stoichiometric CO reductions by reaction with NaHBEt3 in chlorobenzene. However, the reductions of the complexes 3b and 4b produced mixtures of formyl and hydride species. The reaction of complexes 5b and 6b led to the formation of the stable formyl derivatives [Mn(CHO)(CO)3(2)] (5c) and [Re(CHO)(CO)3(2)] (6c), which were isolated in pure form and characterized by spectroscopic means. In addition, 5c could be characterized by a single-crystal X-ray study, which revealed the formyl structure and a stabilizing coordination of the pendant Lewis acidic B(C8H14) group to the formyl oxygen atom.