Organosilicon-phosphorus compounds are versatile building blocks in contemporary organophosphorus chemistry [1] and for the synthesis of metal phosphide semiconducting materials or respective molecular clusters. [2] Likewise, cyclic silicon-phosphorus compounds can serve as chelating electron-rich ligands in transitionmetal complexes, modeling unusual coordination modes of metal centers in catalysts.[3] The formation of siliconphosphorus compounds is commonly achieved by salt metathesis reactions using silicon halides and main-groupmetal phosphides (for example, lithium phosphides). [1, 4] However, the latter method is hardly suitable for the synthesis of strained silicon-phosphorus cycles and cages because of facile skeletal rearrangement reactions (for example, intermolecular ring enlargement) under the reaction conditions.[5] Hence, other synthetic methods are desirable to gain access to welldefined, strained silicon-phosphorus cycles and cages. A promising alternative strategy represents the utilization of unsaturated silicon compounds and white phosphorus (P 4 ) as shown by the gentle reaction of disilenes (R 2 Si=SiR 2 ) and phosphasilenes (R 2 Si=PR') with P 4 at 40 8C, leading to the unusual butterfly-shaped Si 2 P 2 and SiP 3 heterobicyclo-[1.1.0]butanes, [6a, 7] respectively. The mechanism of this remarkably mild P 4 activation and degradation process, initiated by low-valent silicon, is not clearly understood as yet.[6b] The latter results prompted us to investigate whether stable silylenes (R 2 SiD) are also capable of P 4 activation to give novel strained silicon-phosphorus cycles. Herein we report the first SiP 4 and Si 2 P 4 cage compounds which result from consecutive P 4 activation with the stable silylene 1 (Scheme 1).Although several stable silylenes have been described thus far, to our knowledge, both the highly electrophilic silylene (CH 2 ) 2 [C(SiMe 3 ) 2 ] 2 SiD [8] and the silicon analogues of nucleophilic Arduengo-type carbenes [9] seemed to be unsuitable for P 4 activation. In contrast, the zwitterionic silylene 1[10a] reacts gently with P 4 in the molar ratio of 1:1 at ambient temperature to give solely the compound 2. Remarkably, the germylene analogue of 1, [10b] in turn, is resistant towards P 4 even in boiling toluene owing to the lower reduction potential of Ge II versus Si II (inert-pair effect). Compound 2 can be isolated in the form of colorless crystals in 60 % yield. [11] According to EI-MS spectra and combustion analysis, 2 is a 1:1 adduct of 1 and P 4 . Its 1 H NMR spectrum proves the integrity of the C 3 N 2 chelate ligand at the silicon atom. As shown by 31 P NMR spectroscopy, 2 bears three chemically different sorts of 31 P nuclei (A, B, and X), giving rise to temperature-invariant resonance signals at d X = 131.9, d A = À342.4, and d B = À348.0 ppm. The low-field resonance signal at d X splits into a doublet of doublets ( 1 J(P X ,P A ) = 146.8, 1 J(P X ,P B ) = 144.7 Hz) while each of the two high-field signals exhibits a doublet of triplets ( 1 J(P X ,P A ) = 144....
