It could well be in the fingerprint region, unfortunately vibrational modes in IR are not independent. The P-B stretch is probably not characteristic as it will interact with the other bonds, much like the P-C stretch. Be sure to go to quite low wavenumbers as the vibrational transition is probably fairly low in energy.

Obtain IR spectra for Ph3P and B(C6F5) separately then compare them to an admixture of the two. Overlay the spectra and compare the differences.

Thank a lot Eric and Samer. Yes, Eric I have already made the comparison between the IR spectra of a classical Lewis adduct , it was PPh3 with BCF as you mention, also CY3P with BCF which also leads to a classical adduct. The comparison with the spectra of a pure FLP such as PMes3 with BCF didn't show me something particular in a fingerprint region , as you mention there are lot of vibrations, and I think it under one of them ;.. Ya Samer, for the NMR all is right I have made the multi nuclei NMR so it's easy to compare the starting material with the adduct vs FLPs. But I would be sure for IR spectra the vibration is not clear. 

Have you considered looking at the related Raman spectroscopy?

Be sure to check very low wavenumbers approaching the far IR spectrum. Empirically the stretching frequency is related to the relative bond energy which will be considerably lower for a lewis adduct compared to the standard covalent bond. It will definitely be under 1000cm-1 but you may need to move into the far IR spectrum

Perhaps this elongation is very small if it is because it's the bond between to heteroatoms so into the IR spectrum where I the fingerprint  region , I think it could be under a huge strengths !

Relative signal intensity in an IR spectra is due to the change in dipole moment with respect to distance and is a function of the spectroscopic selection laws for which a P-B stretch should be allowed. The relatively weak bond strength primarily affects the wavenumber of the absorption band, hence you should be looking at lower frequencies. If you consider a system of increasing steric bulk the P-B bond length should elongate to a point where it is no longer stabilising, resulting in a frustrated lewis pair. If you were to run an IR analysis this means that your frequency for the IR band absorption is going to get lower as the bond weakens and so will the intensity.

Raman spectroscopy has different selection laws, sometimes weaker IR intensities will show up strongly under Raman spectroscopy.

See articles

Phosphorus Complexes of Group III Acids. II. Infrared Spectra of Boron Complexes of Polycyclic Phosphites

C. W. Heitsch , J. G. Verkade

Inorg. Chem., 1962, 1 (4), pp 863–865

DOI: 10.1021/ic50004a032;

Simultaneous determination of the total content of boron and phosphorus in high‐resistivity silicon by ir spectroscopy at low temperatures

B. O. Kolbesen1

+ View Affiliations

Appl. Phys. Lett. 27, 353 (1975); http://dx.doi.org/10.1063/1.88474;

Infrared and Raman Spectra of Inorganic and Coordination Compounds

Sample Characterization and Spectral Data Processing

Spectra–Structure Correlations

Kazuo Nakamoto

Published Online: 15 AUG 2006

DOI: 10.1002/0470027320.s4104;

Spectra and structure of phosphorus—boron compounds Part XXV. Infrared and Raman spectra, conformational stability and normal coordinate analysis of ethyldimethylphosphine—borane ☆

J.R. Durig, T.J. Hizer1, 2, J.D. Odom, doi:10.1016/0022-2860(87)85009-3 and references herein.

Really interesting papers Pavel, the first time I can read values about P-B strengths, thanks a lot!!!!!

Good luck, Bolley!

Anaëlle it's better !! there are many Bolley I think but indeed they perhaps don't make chemistry!

Mr. Anaëlle,

The region within 750-550 cm-1 has been shown in [ref. 1]. There is determined also possible band at 868 cm-1 for nP-B stretching mode.

[ref. 1] L. Thomas, Interpretation of the infrared spectra of organophosphorou compounds, Heyden, London, 1974, pp. 1-276