Crystal Structures of the Phosphorus‐Boron Adducts n‐Pr3P · BBr3 and I3P · BBr3

The structures of the phosphorus‐boron adducts n‐Pr₃P · BBr₃ (trigonal, space group P 4 c1, Z = 4, a = 11.5423(6), b = 11.5423(6) and c = 13.8066(7) Å) and I₃P · BBr₃ (orthorhombic, space group Pnma, Z = 4, a = 12.761(2), b = 11.427(1), c = 7.3728(7) Å) were determined by X‐ray crystallography. The P–B distance of 2.01(1) Å in I₃P · BBr₃ is significantly longer than the P–B bond in n‐Pr₃P · BBr₃ (1.95(1) Å). The different Lewis basicity of phosphorus halides, PX₃ (X = Cl, Br, I), and alkylphosphines is discussed. The charge transfer and the bond situation in these donor‐acceptor complexes is studied on the basis of NBO analysis. Selected frequencies of n‐Pr₃P · BBr₃ obtained by Raman and infrared spectroscopy are assigned and compared with the normal modes of I₃P · BBr₃.