An open form of a 2,4‐diphosphabicyclobutane is one way to describe the unprecedented structure of the P2C2 heterocycle 1 (Ar = 2,4,6‐tBu3C6H2). Ab initio calculations on 1 show that a partially delocalized π system exists with a considerable diradical contribution. Dark red 1 is accessible from ArP  CCl2 and nBuLi.

The cation 4 can be obtained in an amazingly simple raction sequence from phosphorus trichloride 1 and the arylamine 2 via the iminophosphane 3. Spectroscopic findings—δ(31P) = 79.3—and structural data—PNC angle = 177°, PN distance = 147.5 pm—suggest the presence of a triple bond, whereby 4 is isoelectronic with :SiN–aryl.

Dedicated to Professor R o y Appel on the occasion of his 70th birthdayThe large number of known stable compounds in which phosphorus has a low coordination number makes it clear that such compounds can no longer be regarded as "exotic" in main group chemistry. While the rich chemistry of P-C multiply bonded systems makes clear their affinity to their organic congeners, iminophosphanes in particular are also of increasing importance. The linkage of a phosphinidine fragment with an imine fragment via a multiple bond gives rise to a class of compounds with an unusually wide range of structural types. This in turn leads to a broad spectrum of chemical behavior which makes iminophosphanes extremely useful synthetic building blocks in organoelement chemistry.

The electronic structure of the 2.4-diphosphacyclobutanediyl-1.3 and its substituted compounds were investigated by various electron-correlated quantum chemical ab initio calculations, at the MCSCF, MP2, and MR-MP2 levels, and compared with the results of RHF level calculations. The structure is in essence a biradical (biradicaloid) species with partial π-delocalization within the ring system. A problem occurs with the description of this species by the MCSCF approach. A limited active configuration space overestimates the biradical nature of the structure. A satisfactory description is given by the MP2 procedure which also takes proper account for the dynamical part of the correlation energy correction to the resulting wave function. A bicyclic structure is more stable than a ring structure. A phosphino-carbene analogous structure is only slightly less stable than the ring structure. A detailed study of the substituent effects indicates that silyl groups at carbon tend to increase considerably the stability of a singlet ground state over a triplet ground state. Bonding within the ring system is discussed in terms of biradical or cyclic delocalized canonical structures.

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