S = 1 Spin State of Divalent Iron. I. Magnetic Properties of Phthalocyanine Iron (II)

The magnetic properties of phthalocyanine iron (II) have been investigated in the temperature ranges 1.25°–20°K and 100°–300°K. Between 100° and 300°K the magnetic susceptibility obeys the Curie–Weiss law accurately, but the measured value of μ_eff at 25°C is ${\text{3.71 μ}}_B$ , which is intermediate between the theoretical spin‐only values for $\text{S\hspace{0.17em}=\hspace{0.17em}1}$ and $\text{S\hspace{0.17em}=\hspace{0.17em}2}$ states. However, in the range 1.25°–20°K the susceptibility is virtually independent of temperature, and this result has been used to show that the ground state of the central iron atom is an orbital singlet, with a real spin triplet state. This is split by second‐order spin–orbit coupling into a singlet ground state and a doublet state at 70 cm⁻¹, to give $g_{\Vert }{\text{\hspace{0.17em}=\hspace{0.17em}1.9, g}}_{\perp }\text{\hspace{0.17em}=\hspace{0.17em}2.9}$ .