Experimental and Theoretical Charge Density Study of Chemical Bonding in a Co Dimer Complex

The charge density of Co2(CO)6(HC⋮CC6H10OH) (1) in the crystalline state has been determined
using multipolar refinement of single-crystal X-ray diffraction data collected (i) with a synchrotron source at
very low temperatures (15 K) and (ii) using a conventional source with the crystal at intermediate temperature
(100 K). The X-ray charge density model is augmented by complete active space and density functional
theory calculations. Topological analyses of the different charge distributions show that the two Co atoms
are not bonded to each other in the quantum theory of atoms in molecules (QTAIM) sense of the word.
However, the behavior of the source function and the total energy density indicate that there is some bond-like character in the Co−Co interaction. The bridging alkyne fragment provides an unusual bonding situation,
with extremely small electron density differences between the two Co−C bond critical points and the “CoC2”
ring critical point. Thus, the structure is close to a topological catastrophe point. Comparison of the results
obtained from the two diffraction data sets and ab initio theory suggests that the topology of the experimental
electron density in this special atomic environment is highly sensitive to subtle effects of measurement
errors and potential shortcomings of the multipole model, or to effects of the crystal field. Thus, even the
two identical molecules in the asymmetric unit show altered bonding patterns.