AbstractThe synthesis of 2,2′‐bis(bis(dimethylamino)‐phosphino)‐3,3′5,5′‐tetra‐tert‐butylbiphennyl (5) is described. It was extensively studied by1H,13C, and31P NMR spectroscopy. Furthermore, the X‐ray analysis of5is reported. Crystals of5are tetragonal, space group P¯421c,a = b =24.770 (3) Å,c= 12.658 (4) Å, Z = 8. The surprising reaction of5with proton acids leading to the formation of various phosphorus containing five‐ and six‐membered ring compounds is discussed. On reaction of one of the six‐membered ring compounds (9) with magnesium in THF, a λ3, λ5‐diphosp