SUBSTITUENT INTERACTIONS IN ETA-6-ARENE COMPLEXES .2. NONLINEAR SUBSTITUENT INTERACTIONS AND THE ELECTRON RICHNESS OF SUBSTITUTED (ETA-6-ARENE)CR(CO)3 COMPLEXES AS MEASURED BY IR AND C-13 NMR-SPECTROSCOPY AND CYCLIC VOLTAMMETRY - ROLE OF PI-DONOR AND PI-ACCEPTOR INTERACTIONS

The spectroscopic characterization and oxidative electrochemical behavior (stationary Pt-bead electrode, CH2Cl2, [n-Bu4N]PF6) of a series of 63 (eta-6-arene)Cr(CO)3 complexes having between zero and six substituents on the arene ring (i.e. NEt2, NMe2, NH2, OMe, F, Cl, Me, SiMe3, CO2Me, C(O)Me, CF3) with various substitution geometries are reported. The incremental effect of the substituents upon the electron richness of the complexes, as measured by the formal oxidation potentials, E-degrees', the IR carbonyl stretching frequencies and force constants, K(CO), and the C-13 NMR chemical shifts of the carbonyl ligands, delta(CO), is nonlinear and is found to be dependent upon both the electron richness of the chromium center and the substitution geometry about the arene ring. It is proposed that the origin of these nonlinearities lies principally in pi-donor and pi-acceptor interactions between the substituents on each arene and between each of the substituents on the arene and the chromium center in each complex. Evidence is also provided that the stability of the radical cations of these complexes is dependent upon the electron richness of the metal centers. A valence bond rationale for these effects is provided, and the relationship between these results and the degree of arene planarity and Cr-CO and CrC-O bond lengths of these complexes is discussed.