F-19 NUCLEAR-MAGNETIC-RESONANCE STUDIES OF MIXED BIS(ETA-6-ARENE)CHROMIUM(0) COMPLEXES

F-19 NMR studies have been conducted on mixed bis(eta-6-arene)chromium(0) complexes containing (CH3)m (m = 1, 2 or 3) substituents in one of the partner arenes and CF3 or F(n) (n = 1 or 2) substituents in the other arene. There is deshielding of the F-19 in the (C6H6-m(CH3)m)-(C6H5CF3)Cr complex (I) relative to that in (C6H5CF3)2Cr(II). However, increased shielding of F-19 is observed in the (C6H6-m(CH3)m)-(C6H6-nF(n))Cr complex (III) relative to that in (C6H6-nF(n)2Cr (IV). It appears that the fluorines in CF3 take on an unique behaviour which is affected by a complex series of electron charge movements mainly due to more back-donation towards the (C6H6-m(CH3)m arene and relaying of effects of substituents of one arene on to those of the partner arene by the chromium centre. On the other hand, the upfield shift of the F-19 resonance in III, is rationalized partly on the basis of greater back-donation to the F(n)-containing arene and electron-releasing effect of the (CH3)m that is transmitted by the metal centre. Evidence from two different methyl proton chemical shifts in (1,2,4-C6H3(CH3)3)(1,2-C6H4F2)Cr suggests that the through space dipole interactions are an insignificant factor in the electron charge perturbations in these mixed-arene complexes.