Crystal structures of the phosphorus-boron adducts n-Pr3P center dot BBr3 and I3P center dot BBr3

The structures of the phosphorus-boron adducts n-Pr3P . BBr3 (trigonal, spa ce group P (4) over bar 4c1. Z=4, n=11.5423(6), b=11.5423(6) and c=13.8066( 7) Angstrom) and I3P . BBr3 (orthorhombic, space group Pnma, Z=4, n=12.761( 2), b=11.427(1), c=7.3728(7)A) were determined by X-ray crystallography. Th e P-B distance of 2.01(1) Angstrom in I3P . BBr3 is significantly longer th an the P-B bond in n-Pr3P . BBr3 (1.95(1) Angstrom). The different Lewis ba sicity of phosphorus halides, PX3 (X = Cl, Pr, I), and alkylphosphines is d iscussed. The charge transfer and the bond situation in these donor-accepto r complexes is studied on the basis of NBO analysis. Selected frequencies o f n-Pr3P . BBr3 obtained by Raman and infrared spectroscopy are assigned an d compared with the normal modes of I3P . BBr3.