A series of bimetallic, trigonal bipyramidal clusters of type {[Co(N-N)(2)](3)[Fe(CN)(6)](2)} are reported. The reaction of {Co(tmphen)(2)}(2+) with [Fe(CN)(6)](3)(-) in MeCN affords {[Co(tmphen)(2)](3)[Fe(CN)(6)](2)} (1). The cluster can exist in three different solid-state phases: a red crystalline phase, a blue solid phase obtained by exposure of the red crystals to moisture, and a red solid phase obtained by desolvation of the blue solid phase in vacuo. The properties of cluster 1 are extremely sensitive to both temperature and solvent content in each of these phases. Variable-temperature X-ray crystallography; (57)Fe Mossbauer, vibrational, and optical spectroscopies; and magnetochemical studies were used to study the three phases of 1 and related compounds, Na{[Co(tmphen)(2)](3)[Fe(CN)(6)](2)}(ClO(4))(2) (2), {[Co(bpy)(2)](3)[Fe(CN)(6)](2)}[Fe(CN)(6)](1/3) (3), and {[Ni(tmphen)(2)](3)[Fe(CN)(6)](2)} (4). The combined structural and spectroscopic investigation of 1-4 leads to the unambiguous conclusion that 1 can exist in different electronic isomeric forms, {Co(III)(2)Co(II)Fe(II)(2)} (1A), {Co(III)Co(II)(2)Fe(III)Fe(II)} (1B), and {Co(II)(3)Fe(III)(2)} (1C), and that it can undergo a charge-transfer-induced spin transition (CTIST). This is the first time that such a phenomenon has been observed for a Co/Fe molecule.

A charge-transfer-induced spin transition (CTIST) is observed in the discrete cyanide-bridged complex, {[Co(tmphen)2]3[Fe(CN)6]2}. Single-crystal X-ray diffraction, 57Fe Mössbauer spectroscopy, and magnetic susceptibility were used collectively to describe the oxidation states of the Co and Fe ions in this cluster as a function of temperature. This pentanuclear complex represents the first example of a CTIST at the discrete molecular level.

We perform inelastic neutron scattering measurements on the molecular nanomagnet Mn 12 -acetate to measure the excitation spectrum up to 45 meV ͑500 K͒. We isolate magnetic excitations in two groups at 5 -6.5 meV ͑60-75 K͒ and 8 -10.5 meV ͑95-120 K͒, with higher levels appearing only at 27 meV ͑310 K͒ and 31 meV ͑360 K͒. From a detailed characterization of the transition peaks we show that all of the lowenergy modes appear to be separate S = 9 excitations above the S = 10 ground state, with the peak at 27 meV ͑310 K͒ corresponding to the first S = 11 excitation. We consider a general model for the four exchange interaction parameters of the molecule. The static susceptibility is computed by high-temperature series expansion and the energy spectrum, matrix elements, and ground-state spin configuration by exact diagonalization. The theoretical results are matched with experimental observation by inclusion of cluster anisotropy parameters, revealing strong constraints on possible parameter sets. We conclude that only a model with dominant exchange couplings J 1 ϳ J 2 ϳ 5.5 meV ͑65 K͒ and small couplings J 3 ϳ J 4 ϳ 0.6 meV ͑7 K͒ is consistent with the experimental data.Mn 12 -acetate 4,16-18 is a mixed-valence ͑Mn 3+ /Mn 4+ ͒ compound where the magnetic ions are arranged in two groups: a central core composed of a tetrahedron of four Mn 4+ ions ͑S =3/2͒ and an external ring, or crown, of eight Mn 3+ ions ͑S =2͒. Figure 1 shows the Mn 12 -acetate cluster viewed along the c axis. The point group of the Mn 12 molecule in the crystal structure is S 4 . To simplify the analysis we make the additional assumption of fourfold rotation and reflection symmetry of the individual molecular clusters, and return later to a discussion of this approximation. Each cluster is only very weakly coupled to its neighbors, which are separated from each other by molecules of water and acetic acid, such that no long-range magnetic order has been found for temperatures as low as the mK range. Consequently, most of the experimental work performed at temperatures exceeding 1 K may be interpreted in terms of the properties of a single molecule. Neighboring Mn ions within a cluster are coupled in an intricate pattern by different types of -oxo bridge and by acetate bridges, as a result of which both antiferromagnetic (AFM) and ferromagnetic (FM) exchange interactions may be present in the system. A schematic representation of the exchange couplings is shown in Fig. 2. Within the approximation of fourfold cluster symmetry there are three inequivalent Mn sites with four different exchange couplings between neighboring Mn ions: J 1 (involving two -oxo bridges) and J 2 = J 2a Ϸ J 2b (one -oxo bridge) between Mn 3+ and Mn 4+ ions, J 3 between Mn 4+ ions inside the core tetrahedron (two -oxo bridges), and J 4 = J 4a Ϸ J 4b between Mn 3+ ions around the external ring (one -oxo bridge and two carboxylate groups). Inspection of the Mn-Mn distances and Mn-O-Mn angles presented in Table I suggests that the approximations J 2a Ϸ J 2b and J 4a Ϸ J 4b are emi...