Phase transitions caused by the charge instability between the neutral and ionic phases of compounds, i.e., NâI phase transitions, provide avenues for switching the intrinsic properties of compounds related to electron/spin correlation and dipole generation as well as charge distribution. However, it is extremely difficult to control the transition temperature (T
c) for the NâI phase transition, and only chemical modification based on the original material have been investigated. Here, a design overview of the tuning of NâI phase transition by interstitial guest molecules is presented. This study reports a new chain coordinationâpolymer [Ru2(3,4âCl2PhCO2)4TCNQ(EtO)2]âDCE (1âDCE; 3,4âCl2PhCO2
â = 3,4âdichlorobenzoate; TCNQ(EtO)2 2,5âdiethoxyâ7,7,8,8âtetracyanoquinodimethane; and DCE = 1,2âdichloroethane) that exhibits a oneâstep NâI transition at 230 K (= T
c) with the Nâ and Iâstates possessing a simple paramagnetic state and a ferrimagnetically correlated state for the highâ and lowâtemperature phases, respectively. The T
c continuously decreases depending on the content of DCE, which eventually disappears with the complete evacuation of DCE, affording solventâfree compound 1 with the Nâstate in the entire temperature range (this behavior is reversible). This is an example of tuning the in situ T
c for the NâI phase transition via the control of the interstitial guest molecules.