A Three-State Model for the Polymorphism in Linear Tricobalt Compounds

The remarkable polymorphism exhibited by the linear tricobalt compounds, Co₃(μ₃-dpa)₄Cl₂ and Co₃(μ₃-dpa)₄Br₂, can be explained using a model involving three distinct electronic states. At high temperatures, symmetric and unsymmetric forms arise from the population of doublet (²A) and quartet (⁴B) states, respectively, the latter containing a localized high-spin Co^II center. In the unsymmetric form, a reduction in temperature leads to a spin-crossover to a second quite distinct doublet state, ²B, where, uniquely, the d_x²-y² character on the localized Co^II center is distributed between the occupied and vacant manifolds. The variable population of the Co d_x²-y² orbital gives rise to the continuous change in Co−Co and Co−N bond lengths as the temperature is decreased.