The formation of a well-ordered organometallic sheet consisting of two-dimensional polymeric phthalocyanine (polyFePc) was recently reported by Abel et al. The growth was also demonstrated on an insulating sodium chloride film which decouples the polyFePc from the metal substrate and thus leads to a preservation of the intrinsic properties of the polymeric film. In this work we investigate theoretically the electronic and magnetic properties of a single layer of polyFePc by means of DFT and DFT+U calculations. DFT calculations suggest an antiferromagnetic ground-state which is at least 57 meV lower in energy than the ferromagnetic state, whereas the ferromagnetic arrangement is preferred within DFT+U - by at least 250 meV. This observation suggests that the polyFePc system represents a square-lattice arrangement of molecular spins which is stable at room temperature. Furthermore, we discuss the influence of different substrates on the electronic properties and the transport characteristics of the polyFePc system.