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R2 R3Electrochemical investigations on a selection of methylenephosphines 1 and related systems are reported. They are shown to possess an irreversible oxidation and reduction wave.An exception is diaminodiphosphene, which reversibly forms a radical anion. A relative ordering of frontier orbitals is possible, based on the redox properties of these systems. The remarkable elaboration of the synthetic utility of the chemistry of P(II1) double-bond systems, methylenephosphines 1') and iminophosphines 22), forms one of the most pleasing chapters in modern inorganic chemistry. The P(II1) double-bond systems, 1 -3, reveal an ambident behaviour, due to two energetically closely spaced occupied frontier orbitals3). i. e., their electron demand*'. This problem will be dealt with in the present report. We present in detail the first electrochemical investigations of a selection of phosphorus doublebond systems, especially of methylenephosphines 1. We will relate our findings to substituted ethenes, i. e., to typical electron-rich (enamines) and electron-poor (tetracyanoethene) olefins. Results and Discussions Electrochemical InvestigationsA typical cyclic voltammogram of a methylenephosphine 1 is shown in Figure 1. The anodic oxidation as well as the cathodic reduction were irreversible. This is a common feature of all methylenephosphines 1. Even at temperatures of -100°C the redox reactions remain irreversible. The peak potentials for various substituted methylenephosphines 1 are collected in Table 1.This can be seen in the plethora of metal complexes of methylenephosphines 1, iminophosphines 2, and diphosphenes 34-61. In addition, iminophosphines 2 show distinct self-addition behaviour7' alternatively leading either to For completeness we include the first vertical ionization potentialsg) in Table 1.To a first-order approximation, the reduction and oxidation potentials are a quantitative measure of the frontierorbital energies of HOMO and LUMO, even

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