Virtual Free Ion Magnetism and the Absence of Jahn−Teller Distortion in a Linear Two-Coordinate Complex of High-Spin Iron(II)

Mössbauer spectroscopy and dc magnetization measurements have been used to characterize the low temperature magnetism of a rigorously linear, two-coordinate complex of high-spin Fe(II), Fe[(C(Si(CH3)3]2 (1). The local C−Fe−C chromophore of 1 exhibits novel slow, single-ion paramagnetic relaxation and fully resolved magnetic hyperfine splitting of its zero field Mössbauer spectrum over the range ∼100 to ∼50K. The hyperfine field at 4.2 K is 152 T! This is the largest magnetic hyperfine field observed for iron to date regardless of spin, oxidation state, or coordination environment. This observation is attributable to the large unquenched orbital angular momentum corresponding to the degenerate ground (d xy , d x 2 - y 2 ) orbital pair of 1 in local D ∞ h symmetry. Maintenance of the ground-state degeneracy is required by the Jahn−Teller theorem leading to the unprecedented result that the magnitude of the magnetic moment of 1's 5 Δ g ground state is essentially that of the parent free ion ( 5 D 4 ) ground term.