Five dispirocyclic λ(3),λ(5)-tetraphosphetes [{R2Si(NR(1))(NR(2))P2}2] (R(1) = R(2) and R(1) ≠ R(2)) are easily prepared in almost quantitative yields via photolysis of the respective bis(trimethylsilyl)phosphanyldiazaphosphasiletidines with intense visible light. These deep-yellow low-coordinate phosphorus compounds can be considered as the first higher congeners of the well-known cyclodiphosphazenes. The tetraphosphetes are remarkably stable in air and show unexpected molecular properties related to the unique bonding situation of the central four-π-electron four-membered phosphorus ring. The extent of rhombic distortion of the central P4 ring is remarkable due to an unusually acute angle at the σ(2)-phosphorus atoms. All of the P-P bonds are approximately equal in length. The distances are in the middle of the range given by phosphorus single and double bonds. The anisotropic absorption of visible light that can easily be observed in the case of the yellow/colorless dichroic crystals of [{Me2Si(NtBu)(NtBu)P2}2] and the exceptional (31)P NMR chemical shift of the σ(2)-phosphorus atoms are the most remarkable features of the λ(3),λ(5)-tetraphosphetes. In the case of [{Me2Si(NtBu)(NtBu)P2}2], the Hansen-Coppens multipole model is applied to extract the electron density from high-resolution X-ray diffraction data obtained at 100 K. Static deformation density and topological analysis reveal a unique bonding situation in the central unsaturated P4 fragment characterized by polar σ-bonding, pronounced out-of-ring non-bonding lone pair density on the σ(2)-phosphorus atoms, and an additional non-classical three-center back-bonding contribution.

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The first λ3,λ5-tetraphosphete contains a 4π-electron four-membered ring as the central structural unit of a dispirocyclic system and can thus be classified as an analogue to diphosphetes and cyclodiphosphazenes. According to its crystal structure the central P4unit exhibits not only P–P bonds which are of equal length (P1–P2 2,139(1) Å, P1–P2A 2,142(1) Å), but also rhombic distortion (P1–P2–P1A 79,4(1)0, P2–P1–P2A 100,6(1)0).[1] Therefore its electronic structure cannot be described as 'Phosphacyclobutadiene' but either as a bis(ylide) or as a system with delocalized double bonds. After various quantum chemical calculations and an extensive examination of its reaction and coordination behavior failed to answer this question, we addressed the problem via a detailed analysis of its charge density distribution. The experimental charge density based on high resolution X-ray diffraction data collected at low temperature is determined by multipole least squares refinement using the program packageXD2006.[2] In a first step, the static deformation density exhibits charge density which is located mainly outside of the P4ring plane at the λ3-phosphorus atoms but simultaneously redistributed into the P–P bond area. In addition to that, a study of its topological properties and an inspection of the Laplacian of the electron density according to Bader's `Quantum Theory of Atoms in Molecules' (QTAIM)[3] further highlight the bonding features. They reveal polar Si–N, Si–C and P–N bonds with a decreasing amount of electrostatic contribution as well as four valence shell charge concentrations (thus sp3hybridization) at each of the phosphorus atoms. Finally supported by theoretical calculations, the results illustrate the unique bonding situation in the P4unit combining a high ylidic character with unusual not exclusively sigma-like P–P bonds.

Phosphorus Heterocycles Exceptional bonding and molecular properties are observed in mixed‐valent tetraphosphetes now accessible by a simplified synthetic protocol. Besides anisotropic absorption of visible light and remarkable 31P NMR chemical shifts, the dispirocyclic compounds combine bis(ylidic) properties with elements of non‐classical π‐bonding. For more details, see the Full Paper by V. Breuers, W. Frank and C. W. Lehmann on