Metal-metal bond length variability in Co3(dipyridylamide)4Cl2:: Bond-stretch isomerism, crystal field effects, or spin transition process?: A DFT study

The unprecedented structural behavior Of Co-3(dipyridylamide)(4)Cl-2, characterized in two crystalline forms in which the tricobalt framework is either symmetric or highly nonsymmetric at room temperature is investigated by means of gradient-corrected DFT calculations. The isolated molecule is assigned a single energy minimum associated with a low-spin (doublet) electronic configuration. The optimal geometry closely reproduces the X-ray structure observed for the isomer displaying equivalent metal-metal distances. However, the groundstate potential energy surface is extremely shallow with respect to a distortion of the Co-3 framework. A "weak" distortion, similar to that observed for the unsymmetrical complex at low temperature (Deltad(Co-Co)) = 0.08 Angstrom at 110 K) induces a destabilization of 1.1 kcal.mol(-1) only. The distortion observed at room temperature (Deltad(Co-Co) = 0.17 Angstrom) destabilizes the isolated complex by 4.2 kcal.mol(-1). These results are rationalized in terms of the "three-electron three-center" concept applied to the sigma -bonding electrons of the cobalt framework. A phenomenological model based upon the Heisenberg Hamiltonian successfully reproduces the calculated potential energy curve and assigns the relative stability of the symmetric structure to local forces (Pauli repulsion, ligand bite, etc.) distinct from delocalized sigma bonding. In view of these results, the two structures characterized from X-rays cannot be termed "bond-stretch isomers" according to the strict definition given by Parkin. To investigate the origin of the distorted form, an electric field was applied to the isolated molecule, but it did not shift the equilibrium position toward asymmetry, despite a strong polarization of the electron density. Finally, the I quartet state of lowest energy ((4)A state)has an optimal structure that is distorted and that reproduces most of the distinctive features observed in the nonsymmetric structure. Despite the high relative energy calculated for this quartet state, we assign the occurrence of the nonsymmetric form and its extreme variability with temperature to a progressive population of this excited state as temperature increases.