  An analysis of the energy spectrum and the magnetization curve of
two-dimensional organic antiferromagnet F2PNNNO with a spin-one dimerized
structure shows that a behavior of the compound in an external magnetic field
can be explained within a lattice boson model with an extended Pauli's
exclusion principle, i.e. no more than two bosons per a dimer. The unusual
magnetization curve observed experimentally in the compound reflects a sequence
of phase transitions intrinsic for a lattice boson system with strong on-site
and inter-site repulsions due to a tuning of magnon density by the applied
magnetic field.
