The magnetic properties of FeCl2 are investigated in detail from the standpoint of the one-pion approximation. The lowest orbital state of the ferrous ion in the crystalline field of trigonal symmetry is assumed to be doublet, in which the component of the orbital angular momentum along the trigonal axis is not quenched. The spin-orbit coupling energy produces a splitting of spin levels belonging to the lowest doublet. In the vicinity of, or below, the Néel temperature (24°K) where only the lowest spin level is populated, we have effectively an Ising model, in which the x, y components of the spins are completely quenched. The metamagnetic behaviour of this substance at liquid hydrogen temperatures can quite naturally be interpreted by this model. Also the susceptibility near the room temperature is discussed and it is shown that the paramagnetic Curie temperature is strongly modified by the large splitting of the spin levels due to the spin-orbit coupling energy. The crystalline field is calculated, and the result seems to support the assumed orbital level scheme. FeCO3, CoCl2, NiCl2, FeBr2 and FeI2 are also discussed briefly.