A model of nonradiative transitions in spin-crossover molecules is developed. The model takes into account linear and quadratic terms of electron-vibrational interaction. The frequency effect is shown to arise from the quadratic term of this interaction. With account of the frequency effect analytical expressions are derived for the multiphonon decay rate of the high-spin state of a spin-crossover molecule. In the framework of the suggested model for different temperatures the probability of the nonradiative decay of the high-spin Fe(II) state in the diluted [Zn1-xFex(ptz)6](BF4)2, x = 0.1 system is calculated. The theoretical results are in agreement with the experimental ones.