Metal-metal bond length variability in Co-3(dipyridylamide)(4)Cl-2: Bond-stretch isomerism, crystal field effects, or spin transition process? A DFT study

The unprecedented structural behavior Of Co-3(dipyridylamide)(4)Cl-2, chara cterized in two crystalline forms in which the tricobalt framework is eithe r symmetric or highly nonsymmetric at room temperature is investigated by m eans of gradient-corrected DFT calculations. The isolated molecule is assig ned 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. Howev er, the groundstate potential energy surface is extremely shallow with resp ect to a distortion of the Co-3 framework. A "weak" distortion, similar to that observed for the unsymmetrical complex at low temperature (Deltad(Co-C o)) = 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 An gstrom) destabilizes the isolated complex by 4.2 kcal.mol(-1). These result s are rationalized in terms of the "three-electron three-center" concept ap plied to the sigma -bonding electrons of the cobalt framework. A phenomenol ogical 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 or 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 or igin of the distorted form, an electric field was applied to the isolated m olecule, but it did not shift the equilibrium position toward asymmetry, de spite a strong polarization of the electron density. Finally, the I quartet state of lowest energy ((4)A state)has an optimal structure that is distor ted and that reproduces most of the distinctive features observed in the no nsymmetric structure. Despite the high relative energy calculated for this quartet state, we assign the occurrence of the nonsymmetric form and its ex treme variability with temperature to a progressive population of this exci ted state as temperature increases.