The remarkable polymorphism exhibited by the linear tricobalt compounds, CO3(mu(3)-dpa)(4)Cl-2 and CO3(mu(3)-dpa)(4)Br-2, can be explained using a model involving three distinct electronic states. At high temperatures, symmetric and unsymmetric forms arise from the population of doublet (2 A) and quartet (4 B) states, respectively, the latter containing a localized high-spin Coll center. In the unsymmetric form, a reduction in temperature leads to a spin-crossover to a second quite distinct doublet state, 2 B, where, uniquely, the d(x)(-y)(2)(2) character on the localized Coll center is distributed between the occupied and vacant manifolds. The variable population of the Co d(x)(-y)(2)(2) orbital gives rise to the continuous change in Co-Co and Co-N bond lengths as the temperature is decreased.