Metamagnetism in tetrachlorobis(N-phenylacetamidine)rhenium(IV)

Abstract

Single-crystal magnetisation and polarised neutron diffraction (PND) measurements have been made for the complex cis-[ReCl ₄ (NHCMeNHPh) ₂ ]. At higher temperatures the magnetic behaviour is typical of such a rhenium-(IV) complex, with three unpaired spins (µ = 3.6 µ _B ), an almost isotropic g-tensor of 1.86(1) and a large zero-field splitting, 2D, of -20(2) cm ^-1 . At very low temperatures the magnetisation behaviour as a function of temperature and field strength and direction indicates two magnetically ordered structural phases when H||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||b. This behaviour is metamagnetic. The two-dimensional form of the stronger magnetic exchange implied, with weaker interlayer interaction, can be correlated with the observed structural feature of ab planes of ReCl ₄ units separated by layers of phenyl rings. The PND experiment showed that about 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 space groups are P2 ₁ /c at low magnetic fields and Pc2 ₁ n at high fields along b, compared with the nuclear structural space group of Pccn.