The solvatochromic indicator [Cu(acac)(tmen)(H2O)] ·PF6 (1·H2O) has been synthesized and crystallographically characterized. 1·H2O binds an H 2O molecule at the Cu(II) axial site, while the PF6 - anion is coordination free. The binding properties of [Cu(PF 6)(acac)(tmen)] (1) and [Cu(BF4)(acac)(tmen)] (2) have been investigated in solution and the solid state. The donor number of the PF6- anion (DNPF6) was determined from the UV-vis spectra of 1 in 1,2-dichloroethane. The value of DNPF6 of the PF6- anion is slightly larger than that of the tetraphenylborate anion (BPh4-), which is known as a noncoordinating anion. In the solid state, 1 and 2 reversibty bind and release H2O molecules at the Cu(II) axial sites. The coordinated H 2O molecules in 2 are more easily removed than those in 1 because of the strong Lewis basicity of the BF4- anion compared to the PF6- ion. The lower melting point of 1 versus 2 is attributed to the loose binding of the PF6- anions to the Cu(II) centers, which induces the dynamic nature of the crystal.