The linear trinuclear compound Co 3 (dpa) 4 Cl 2 (1; dpa ) di(2-pyridyl)amide anion) crystallizes from CH 2 Cl 2 solution in two forms simultaneously, namely, an orthorhombic form 1‚CH 2 Cl 2 and a tetragonal form 1‚2CH 2 Cl 2 . The three linearly arranged cobalt atoms in 1 are supported by four dpa ligands in a spiral configuration. The chain of cobalt atoms is symmetrical in 1‚CH 2 Cl 2 , but unsymmetrical in 1‚2CH 2 Cl 2 . Both crystal structures have been studied at various temperatures. A reversible second-order phase transition (165 K) from orthorhombic (Pnn2) to monoclinic (Pn) symmetry for the crystal of 1‚CH 2 Cl 2 has been documented by X-ray studies at 296, 168, and 109 K as well as a neutron diffraction study at 20 K. The linear tricobalt unit in 1‚CH 2 Cl 2 becomes slightly unsymmetrical at low temperature although the two Co-Co bonds remain statistically equivalent (Co-Co ≈ 2.32 Å) throughout the experimental temperature range. No phase transition was observed for the tetragonal form 1‚2CH 2 Cl 2 at low temperature, but the Co-Co distances in 1 changed from 2.299(1) and 2.471(1) Å at 298 K to 2.3035(7) and 2.3847(8) Å at 20 K. Magnetic susceptibility measurements indicate that the two compounds are in an S ) 1 / 2 ground state at low temperature and exhibit gradual spin-crossover at higher temperature.

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