  Magnetic properties of the Co valence tautomers are considered in wide range
of magnetic field and temperature. Particularly, a model of the first order
structural phase transition accompanied by a magnetic moment jump is proposed.
The phase diagrams of the phase transition on a B-T plane (B is magnetic field
and T is temperature) are calculated for four ligand types. Dependence of the
critical magnetic field on physical characteristics known from experiments
(transition temperature at the absense of magnetic field, entropy change per
formula unit, energy difference between two phases) is found. The phase diagram
is parametrized for comparison with other phase transitions of this kind.
