STEREOCHEMICAL NONRIGIDITY IN SOME TRANSITION METAL COMPLEXES CONTAINING TRIS(1-PYRAZOLYL)BORATE LIGANDS

The temperature dependence of the proton nmr spectra of some transition metal complexes containing tris(l-pyrazolyl)borate ligands has been studied with emphasis on compounds having structure RB(pz)3M(CO)2-π-allyl as well as substituted analogs (pz stands for 1-pyrazolyl; M is Mo or W). Three types of fluxional behavior were observed: (1) “tumbling” or rapid intramolecular exchange of the coordinated pyrazolyl group which makes all pyrazolyl groups in, e.g., Ba[B(pz)4]2 or B(pz)4Pd-π-C3H5, spectroscopically equivalent; (2) rotation of the tridentate RB(pz)3 ligand around the B-M axis; (3) rotation of substituent R, when it is 1-pyrazolyl, around the N-B bond. Well-defined limiting spectra were obtained for processes 2 and 3. There is no evidence for rotation of the π-allyl moiety. Introduction of methyl groups at the 3,5 positions raises the rotational energy barrier, as does substitution on the π-allyl group. Surprisingly, the barrier to rotation is lowest with π-CH2CMeCH2 rather than with an unsubstituted π-allyl group. © 1969, American Chemical Society. All rights reserved.