A neutral heteroleptic Fe(III) complex 1 derived from a pi-extension of the parent complex 2 was prepared and characterized. Complex 1 exhibited an abrupt spin crossover (SCO) transition exactly at room temperature (T-SCO = 298 K). A crystal structure analysis of 1 revealed that the Fe(III) complex molecules formed a three-dimensional pi-stacking interaction network. To thermodynamically clarify the mechanism of the SCO transition, the thermodynamic parameters of the SCO transitions for 1 and 2 were deduced from the temperature dependence of the magnetic susceptibility in the solid and solution states using the regular solution model. A comparison of the SCO enthalpy difference between the solid and molecule for 1 and 2 revealed that the lattice enthalpy difference would largely contribute to the SCO transition enthalpy difference. A computational evaluation of intermolecular interactions and lattice energies before and after the SCO transitions in 1 and 2 disclosed the significant contribution of the next-nearest neighbor dispersion interactions to the lattice enthalpy differences. This finding indicates that not only conventional nearest neighbor intermolecular interactions but also next-nearest neighbor dispersion interactions should be taken into account to understand the fundamental mechanism of a phase transition in molecular solids.