METAMAGNETISM IN TETRACHLOROBIS(N-PHENYLACETAMIDINE)RHENIUM(IV)

Single-crystal magnetisation and polarised neutron diffraction (PND) m easurements have been made for the complex cis-[ReCl4(NH=CMeNHPh)(2)]. At higher temperatures the magnetic behaviour is typical of such a rh enium-(IV) complex, with three unpaired spins (mu = 3.6 mu(B)), an alm ost isotropic g-tensor of 1.86(1) and a large zero-field splitting, 20 , of -20(2) cm(-1). At very low temperatures the magnetisation behavio ur as a function of temperature and field strength and direction indic ates two magnetically ordered structural phases when H parallel to b, but only one for a and c, with T-N = 9.7(1) K. A tricritical point at T = 5.8(2) K and H = 4.40(5) T was established on the transition line in H vs. T space between the two phases when H parallel to b. This beh aviour is metamagnetic. The two-dimensional form of the stronger magne tic exchange implied, with weaker interlayer interaction, can be corre lated with the observed structural feature of ab planes of ReCl4 units separated by layers of phenyl rings. The PND experiment showed that a bout half of the spin of the system is delocalised away from the Re by covalent bonding to the Cl atoms and the amidine ligand, accompanied by considerable effects resembling spin polarisation. The magnetic spa ce groups are P2(1)/c at low magnetic fields and Pc2(1)n at high field s along b, compared with the nuclear structural space group of Peen.