"Distorted" (eta(6)-arene)tricarbonylchromium complexes: Correlation of structural parameters with the electronegativity chi(G) and Hammett constantssigma(p) of arene substituents

The molecular structures of 34 (eta(6)-arene)tricarbonylchromium complexes have been retrieved from the Cambridge Structural Data Base and investigate d by focusing essentially on the distortions of the arene ligand caused by the electronic properties of the substituents connected to it. Two addition al meta-disubstituted and one ortho-disubstituted arene complexes have been synthesized and their structure established by X-ray diffraction analysis. Two types of geometrical parameters have been chosen. The first one is the deviation Delta(i) of the value of the endocyclic angle alpha(i) at carbon ipso with respect to the ideal value alpha(0). The second one encompasses the chromium to ipso-carbon and the chromium to ring distances, d(ipso) and d(0) respectively. It is shown that as long as no resonance effects interf ere, Delta(i) correlates with the group electronegativity for a wide variet y of substituents whose central atom belongs to the first, second or even t he third row of the periodic table. Similarly, we find that for mono-substi tuted arene complexes, d(ipso) as well as d(0) correlate with the Hammett c onstants sigma(p) and sigma(p)(+). It is further evident that for a large v ariety of mono- and di-substituted (mu(6)-arene) tricarbonylchromium comple xes, d(ipso) correlates linearly with the sigma(p)(+) constant, thus indica ting that d(ipso) is a specific geometric constraint imposed by the electro nic properties (e.g. inductive and mesomeric) of a given substituent connec ted to the arene ligand.