Ligand field influence on the electronic and magnetic properties of quasi-linear two-coordinate iron(II) complexes

The 2 to 300 K magnetic susceptibilities of Fe{N(SiMe2Ph)2}2, 1, Fe{N(SiMePh2)2}2, 2, and the diaryl complex Fe(Ar(Pr(i)4))2, 3, where Ar(Pr(i)4) is C6H3-2,6(C6H3-2,6-Pr(i)2)2 have been measured. Initial fits of these properties in the absence of an independent knowledge of their ligand field splitting have proven problematic. Ab initio calculations of the CASSCF/RASSI/SINGLE-ANISO type have indicated that the orbital energies of the complexes, as well as those of Fe(Ar(Me6))2, 4, where Ar(Me6) is C6H3-2,6(C6H2-2,4,6-Me3)2), are in the order d(xy)≈ d(x(2)-y(2)) < d(xz) ≈ d(yz) < d(z(2)), and the iron(II) complexes in this ligand field have the (d(xy), d(x(2)-y(2)))(3)(d(xz), d(yz))(2)(d(z(2)))(1) ground electronic configuration with a substantial orbital contribution to their effective magnetic moments. An ab initio-derived ligand field and spin-orbit model is found to yield an excellent simulation of the observed magnetic properties of 1-3. The calculated ligand field strengths of these ligands are placed in the broader context of common coordination ligands in hypothetical two-coordinate linear iron(ii) complexes. This yields the ordering I(-) < H(-) < Br(-)≈ PMe3 < CH3(-) < Cl(-)≈ C(SiMe3)3(-) < CN(-)≈ SAr(Pr(i)6-) < Ar(Pr(i)4-) < Ar(Me6-)≈ N3(-) < NCS(-)≈ NCSe(-)≈ NCBH3(-)≈ MeCN ≈ H2O ≈ NH3 < NO3(-)≈ THF ≈ CO ≈ N(SiMe2Ph)2(-)≈ N(SiMePh2)2(-) < F(-)≈ N(H)Ar(Pr(i)6-)≈ N(SiMe3)Dipp(-) < OAr(Pr(i)4-). The magnetic susceptibility of the bridged dimer, [Fe{N(SiMe3)2}2]2, 5, has also been measured between 2 and 300 K and a fit of χMT with the isotropic Heisenberg Hamiltonian, Ĥ = -2JŜ1·Ŝ2 yields an antiferromagnetic exchange coupling constant, J, of -131(2) cm(-1).