The N,N',S-donor ligand 4-methoxy-3,5-dimethyl-2-((3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)meth yl)pyridine (L) was prepared from 2-(chloromethyl)-4-methoxy-3,5-dimethylpyridine hydrochloride and 3-(2-(methylthio)phenyl)-1H-pyrazole. The Cu(I) complexes [Cu-2(L)(2)CH3CN][Cu(L)CH3CN](BF4)(3) (1), [Cu(L)PPh3]BF4 (2), and [Cu-6(L)(2)(C6F5S)(6)] (3) were prepared and characterized by X-ray crystallography (PPh3 = triphenylphosphine, C6F5S- = pentafluorothiophenolate). The unit cell of compound 1 consists of cocrystallized mononuclear and dinuclear entities in which all of the copper atoms exhibit distorted tetrahedral coordination. Compound 2 is monomeric with L bound in the kappa(3)-N,N',S mode and a PPh3 molecule that completes the coordination environment. Compound 2 presents a fluxional behavior in CDCl3 solution due to the boat inversion of the six-membered N,N' chelate ring (Delta H-double dagger = +43.6(3) kJ mol(-1), Delta S-double dagger = -16(1) J mol(-1) K-1). Crystallization of 3 in acetonitrile leads to a polynuclear structure that contains a CH3CN molecule coordinated to one of the copper atoms: [Cu-6(L)(2)(C6F5S)(6)CH3CN] (3a). The core of 3a partially resembles a {Cu4S6} adamantane-like moiety, the only difference being that the Cu-NCCH3 interaction leads to the opening of the cluster by disrupting a Cu-Cu interaction. Part of this assembly is found in the yeast metallothionein copper(I)-cysteinate core whose crystal structure has recently been reported. Two additional [Cu(L)](+) peripheral moieties interact with the cluster by means of bridging thiolates. ESI-mass spectrometry, conductivity measurements, and H-1/F-19 pulsed gradient spin echo (PGSE) NMR experiments suggest that 3a dissociates in acetonitrile solution: 3a + CH3CN -> [Cu-4(C6F5S)(6)](2-) + 2[Cu(L)CH3CN](+). The stability of the cluster with respect to the hypothetical. mononuclear species, [Cu(L)(C6F5S)], is confirmed by DFT calculations (B3LYP), which illustrate the exergonic character of the reaction: 6[Cu(L')(C6H5S)] -> [Cu-6(L')(2)(C6H5S)(6)] + 4L' (Delta G(298) = -58.6 kJ mol-1, where L' and C6H5S- are simplified models for L and C6F5S-, respectively). The energetics pertinent to the ionic dissociation of the cluster in acetonitrile is computed using the polarizable continuum model (PCM) approach.