A cyclic carbanionic valence isomer of a carbocation: Diphosphino analogs of diaminocarbocations

Diaminocarbocations (or amidinium salts) feature a three-center 4 pi electr on system with an open planar structure. Their 2 pi electron three-membered cyclic valence isomers, in which the carbon atom bears a negative charge, are predicted to be about 541 kilojoules per mate higher in energy than the open form. This isomer has not been identified yet. In contrast, the attem pted synthesis of a diphosphorus analog of amidinium salts Leads to the cyc lic carbanionic form. There is no precedent for such a transformation of a carbocationic center into a carbanionic center, but with the help of heavie r main-group elements, numerous examples can be imagined. This approach wil l enable the preparation of many unknown structural moieties that are diffi cult or even impossible to access in the corresponding carbon and nitrogen series.