SYNTHESIS AND MAGNETIC-PROPERTIES OF 1-1 SALTS CONTAINING THE BIS(ETA-6-HEXAMETHYLBENZENE)COBALT CATION

The salts [Co(eta6-C6Me6)2]+[A]- (A = BPh4, NbF6, PF6 or SbF6) have be en prepared from anhydrous CoCl2. Solid-state magnetic susceptibility measurements on these salts indicate that they all obey the Curie-Weis s law in the temperature range 30-300 K with mu(eff) = 2.7, 2.8, 2.8 a nd 2.6 mu(B) with corresponding theta values of -1.4, 0.2, -4.1 and 0. 4 K for the BPh4-, NbF6-, PF6- and SbF6- salts respectively. Below 20 K, the magnetic susceptibility for all the salts deviate substantially from the Curie-Weiss law. The experimental susceptibility data for th e compounds [Co(eta6-C6Me6)2]+[BPh4]-and [Co(eta6-C6Me6)2]+[NbF6]- can be fitted to a model in which the 3A2g triplet ground state assigned to the [Co(eta6-C6Me6)2]+ moiety exhibits a positive zero-field splitt ing of 17.8 and 20.5 cm-1 respectively. In contrast, the magnetic susc eptibilities for [Co(eta6-C6Me6)2]+[A]- (A = PF6 or SbF6) cannot be fi tted to this model in the range 2-30 K due to more significant intermo lecular antiferromagnetic spin-spin interactions. All the salts were E PR silent at between 4 and 300 K both in the solid state and in frozen solution. Reaction of [Co(eta6-CMe6)2]+[PF6]- with either [NBu4][tcnq ] (tcnq = tetracyano-p-quinodimethane) or [NBu4][C3{C(CN)2}3] [C3{C(CN )2}3 = tris(dicyanomethylene)cyclopropane] gave insoluble paramagnetic precipitates with mu(eff) = 4.1 and 4.5 mu(B) and theta values of -12 .9 and -3.5 K respectively.