The methyl labelled C3‐bridged frustrated phosphane borane Lewis pair (P/B FLP) 2 b was prepared by treatment of Mes2PCl with a methallyl Grignard reagent followed by anti‐Markovnikov hydroboration with Piers’ borane [HB(C6F5)2)]. The FLP 2 b is inactive toward dihydrogen under typical ambient conditions, in contrast to the C2‐ and C4‐bridged FLP analogues. Dynamic NMR spectroscopy showed that this was not due to kinetically hindered P⋅⋅⋅B dissociation of 2 b. DFT calculations showed that the hydrogen‐splitting reaction of the parent compound 2 a is markedly endergonic. The PH+/BH− H2‐splitting product of 2 b was indirectly synthesized by a sequence of H+/H− addition. It lost H2 at ambient conditions and confirmed the result of the DFT analysis.