A carbon–carbon double bond consists of a σ bond and a π bond. Recently, the concept of a π single bond, where a π bond is not accompanied by a σ bond, has been proposed in diradicals containing carbon and heteroatom radical centers. Here we report a closed-shell compound having a silicon–silicon π single bond. 1,2,2,3,4,4-Hexa-tert-butylbicyclo[1.1.0]tetrasilane has a silicon−silicon π single bond between the bridgehead silicon atoms. The X-ray crystallographic analysis shows that the silicon−silicon π single bond (2.853(1) Å) is far longer than the longest silicon−silicon bond so far reported. In spite of this unusually long bond length, the electrons of the 3p orbitals are paired, which is confirmed by measurement of electron paramagnetic resonance, and magnetic susceptibility and natural bond orbital analysis. The properties of the silicon−silicon π single bond are studied by UV/Vis and²⁹Si NMR spectroscopy, and theoretical calculations.

Photophysical and singlet oxygen sensitization processes of 5,10,15,20-tetrakis(4-trimethylsilylphenyl)porphyrin have been studied. It was found out that the quantum yield (ΦΔ) of singlet oxygen production increased by the silylation of 5,10,15,20-tetraphenylporphyrin (TPP). The silylation does not affect the photophysical processes of TPP but enhances the sensitization efficiency of singlet oxygen by the triplet sensitizer (fΔT). This enhancement can be explained by the suppression of the process from the triplet encounter complex composed of the triplet sensitizer and ground-state oxygen to the ground-state sensitizer and oxygen.

The planar tetrapalladium cluster Pd₄{Si(ⁱPr)₂}₃(CN^tBu)₄ (4) was synthesised in 86% isolated yield by the reaction of palladium(0) bis(isocyanide) Pd(CN^tBu)₂ with octaisopropylcyclotetrasilane (3). In the course of this reaction, the palladium atoms are clustered via insertion into the Si–Si bonds of 3, followed by extrusion of one SiⁱPr₂ moiety and reorganisation to afford 4 with a 54-electron configuration. The CN^tBu ligand in 4 was found to be easily replaced by N-heterocyclic carbene (ⁱPr₂IM^Me) to afford the more coordinatively unsaturated cluster Pd₄{Si(ⁱPr)₂}₃(ⁱPr₂IM^Me)₃ (5) having the planar Pd₄Si₃ core. On the other hand, the replacement of CN^tBu with a sterically compact ligand trimethylolpropane phosphite {P(OCH₂)₃CEt} led to a planar tripalladium cluster Pd₃{Si(ⁱPr)₂}₃{P(OCH₂)₃CEt}₃ (6) and Pd{P(OCH₂)₃CEt}₄ in 1:1 molar ratio as products.