ELECTRON-SPIN-RESONANCE SUPER-HYPERFINE SPLITTING STUDY OF POLYCRYSTALLINE NIOBOCENE AND VANADOCENE DICHLORIDES

The electron spin resonance (ESR) spectra of niobocene and vanadocene dichlorides were studied on the title compounds prepared as magnetical ly diluted species in polycrystalline form. The resolved super-hyperfi ne splitting of anisotropic ESR spectra of both studied compounds was firstly observed. Computer simulation confirmed that this super-hyperf ine splitting is due to interaction of unpaired electron with nuclear spin of two chlorine ligands. Average value of super-hyperfine couplin g constant is 13.3 MHz for niobocene dichloride. For vanadocene dichlo ride, it is estimated to be in the range of 6-10 MHz. It corresponds a pproximately to 10% delocalization of metal unpaired electron spin den sity onto chlorine ligands in case of niobocene dichloride and to 4-7% delocalization in case of vanadocene dichloride.