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 CoII 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 x 2 - y 2 character on the localized CoII center is distributed between the occupied and vacant manifolds. The variable population of the Co d x 2 - y 2 orbital gives rise to the continuous change in Co−Co and Co−N bond lengths as the temperature is decreased.