Bispyridinylidenes (BPYs) have emerged as an important class of neutralo rganic electron donors, with redox potentials that vary widely with choice of substituent. Methods to predict the effect of substitution on the redox potential are therefore highly desirable. Here we show that the redox potential of BPYs featuring iminophosphoranos ubstituents (R 3 P = N-), which representt he most reducing class of BPYs, can be predicted basedo n the well-knownT olman electronic parameter(TEP) for the respective phosphine fragment (R 3 P). Moreover,b uilding on earlier work relatingr edox potentials to Hammett-type substituent constants, it is now possible to quantitatively predict s p + values for iminophosphoranos ubstituents from TEP values. Theser esultsp rovide ap ath for precisely tailoringr edoxp otentials of iminophosphorano-substituted BPYs, but also give quantitative descriptors for how these highly versatile iminophosphorano substituents can impactt he properties of any molecular scaffold. Scheme1.Reductions of iodoester 4 to products 5 or 6 in varying isolated yields depending on the strengthoft he OEDused (1, 2,o r3).