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These deep\u2010yellow low\u2010coordinate phosphorus compounds can be considered as the first higher congeners of the well\u2010known cyclodiphosphazenes. The tetraphosphetes are remarkably stable in air and show unexpected molecular properties related to the unique bonding situation of the central four\u2010\u03c0\u2010electron four\u2010membered phosphorus ring. The extent of rhombic distortion of the central P<jats:sub>4<\/jats:sub> ring is remarkable due to an unusually acute angle at the \u03c3<jats:sup>2<\/jats:sup>\u2010phosphorus atoms. All of the P\uf8ffP 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 [{Me<jats:sub>2<\/jats:sub>Si(N<jats:italic>t<\/jats:italic>Bu)(N<jats:italic>t<\/jats:italic>Bu)P<jats:sub>2<\/jats:sub>}<jats:sub>2<\/jats:sub>] and the exceptional <jats:sup>31<\/jats:sup>P\u2005NMR chemical shift of the \u03c3<jats:sup>2<\/jats:sup>\u2010phosphorus atoms are the most remarkable features of the \u03bb<jats:sup>3<\/jats:sup>,\u03bb<jats:sup>5<\/jats:sup>\u2010tetraphosphetes. In the case of [{Me<jats:sub>2<\/jats:sub>Si(N<jats:italic>t<\/jats:italic>Bu)(N<jats:italic>t<\/jats:italic>Bu)P<jats:sub>2<\/jats:sub>}<jats:sub>2<\/jats:sub>], the Hansen\u2013Coppens multipole model is applied to extract the electron density from high\u2010resolution X\u2010ray diffraction data obtained at 100\u2005K. Static deformation density and topological analysis reveal a unique bonding situation in the central unsaturated P<jats:sub>4<\/jats:sub> fragment characterized by polar \u03c3\u2010bonding, pronounced out\u2010of\u2010ring non\u2010bonding lone pair density on the \u03c3<jats:sup>2<\/jats:sup>\u2010phosphorus atoms, and an additional non\u2010classical three\u2010center back\u2010bonding contribution.<\/jats:p>","DOI":"10.1002\/chem.201406131","type":"journal-article","created":{"date-parts":[[2015,2,20]],"date-time":"2015-02-20T14:28:37Z","timestamp":1424442517000},"page":"4596-4606","source":"Crossref","is-referenced-by-count":18,"title":["Unusual Bonding and Properties in Main Group Element Chemistry: Rational Synthesis, Characterization, and Experimental Electron Density Determination of Mixed\u2010Valent 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