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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 μeff at 25°C is 3.71 μB, which is intermediate between the theoretical spin-only values for S = 1 and S = 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‖ = 1.9, g⊥ = 2.9.
Dale et al. (Tue,) studied this question.