A cyclic alltyl(ainino) carbene (cAAC) stabilized dimer [(cAAC)Si(P-Tip)](2) (2) (Tip = 2,4,6-triisopropylphenyl) is reported. 2 can be considered as a dimer of the heavier ketenimine (R2C=C=N-R) analogue. The dark-red rod-shaped crystals of 2 were synthesized by reduction of the precursor, cAAC-dichlorosilylene-stabilized phosphinidene (cAAC)SiCl2 -> P-Tip with sodium napthalenide. The crystals of 2 are storable at room temperature for several months and stable up to 215 degrees C under an inert atmosphere. X-ray single-crystal diffraction revealed that 2 contains a cyclic nonplanar four-membered SiPSiP ring: Magnetic susceptibility measurements confirmed the singlet spin ground state of 2. Cyclic voltammetry of 2 Showed a quasi-reversible one-electron reduction indicating the formation of the corresponding radical anion 2(center dot-), which was further characterized by EPR measurements in solution. The electronic structure and bonding of 2 and 2(center dot-) were studied by theoretical calculations. The experimentally obtained data are in good agreement with the calculated values.