| Abstract
| - The remarkable polymorphism exhibited by the linear tricobalt compounds, Co3(μ3-dpa)4Cl2 andCo3(μ3-dpa)4Br2, can be explained using a model involving three distinct electronic states. At hightemperatures, symmetric and unsymmetric forms arise from the population of doublet (2A) and quartet (4B)states, respectively, the latter containing a localized high-spin CoII center. In the unsymmetric form, areduction in temperature leads to a spin-crossover to a second quite distinct doublet state, 2B, where,uniquely, the dx2-y2 character on the localized CoII center is distributed between the occupied and vacantmanifolds. The variable population of the Co dx2-y2 orbital gives rise to the continuous change in Co−Coand Co−N bond lengths as the temperature is decreased.
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