Structures, magnetic properties, and reactivity studies of salts containing the dinuclear anion [M2Cl6](2-) (M = Mn, Fe, Co)

Salts of the edge-sharing bitetrahedral anion [M2Cl6](2-) with M = Mn, Fe, and Co were prepared in high yields by reaction of MCl2 with 1 equiv of chl oride ion. The anion [Fe2Cl6](2-) was isolated with five different cations, namely, [PPh4](+) (1), [Et4N](+) (2), [ppn](+) (3), [AsPh4](+) (4), and [H -TMPP](+) (5) ([H-TMPP](+) = tris(2,4,6-trimethoxyphenyl)phosphonium; [ppn] (+) = bis(triphenylphosphonium)iminium chloride). The Mn and Co compounds w ere isolated as [ppn](+) salts, [ppn](2)[Co2Cl6] (6) and [ppn](2)[Mn2Cl6] ( 7). The compounds were characterized by single-crystal X-ray analysis and s ubjected to variable-temperature and field-dependent magnetic measurements. These magnetic data were analyzed from a spin Hamiltonian that contains an isotropic exchange term supplemented by a zero-field-splitting term to acc ount for the single-ion anisotropy of the interacting spins. By using this approach, the [Fe2Cl6](2-) salts (1-5) were shown to exhibit magnetic behav ior dominated by single-ion anisotropy of the spin S = 2 of the tetrahedral Fe(II) ions (D similar to 5 cm(-1)). The Co(II) compound (6) was found to exhibit considerable single-ion anisotropy of the spin S = 3/2 tetrahedral Co(II) centers (D = 29 cm(-1)) and stronger antiferromagnetic coupling than the Fe compounds. In the absence of complicating factors such as ZFS effec ts, as is the case for the Mn(II) derivative (7) it was possible to discern weak intermolecular as well as intramolecular antiferromagnetic interactio ns. Reactions between 2,2'-bipyrimidine (2,2'-bpym) and salts of [Fe2Cl6](2 -) yielded two neutral compounds, namely, Fe2Cl4(2,2'-bpym)(3) (8) and [Et4 N]Cl .[Fe2Cl4(MeOH)(4)(mu-2,2'-bpym)] (9). In contrast to the previously de scribed Fe(LI) compounds, the Fe(II) center is octahedral in 8 and 9. The e lectronic ground state of this ion is orbitally degenerate and therefore hi ghly anisotropic. An analysis of the magnetic data confirmed these predicti ons and revealed that the bridging ligand mediates antiferromagnetic exchan ge of similar to 1 cm(-1) and that the D parameter is large and negative (D similar to -17 cm(-1)).