Dramatic tensor-axis non-coincidence effects in the electron spin resonance spectra of some low-spin manganese(II) complexes

Abstract

ESR spectra are reported for frozen CH₂Cl₂/ClCH₂CH₂Cl solutions of 14 low-spin manganese(II) cations: [(η⁵-C₅H₅)Mn(CO)L₂]⁺, L = 1/2 Ph₂PCH₂CH₂PPh₂(dppe), 1/2 Me₂PCH₂CH₂PMe₂(dmpe), 1/2 Ph₂PCH₂PPh₂(dppm), PMe₃, PPh₃; [(η⁵-MeC₅H₄)Mn(CO)dppe]⁺; [(η⁵-6-exo-PhC₆H₆)Mn(CO)L₂]⁺, L = 1/2 dppe, 1/2 dmpe, PMe₃; [(η⁵-6-exo-PhC₆HMe₅)Mn(CO)dppe]⁺; [(η⁵-6-exo-PhC₇H₈)Mn(CO)dppe]⁺; [(η⁵-C₅H₅-Mn(CO)₂PPh₃]⁺; and [(η⁵-6-exo-PhC₆H₆)Mn(CO)₂L]⁺, L = PMe₃, PⁿBu₃. The spectra show anisotropic ⁵⁵Mn hyperfine coupling and nearly isotropic ³¹P hyperfine coupling. There are dramatic departures from first-order line spacings which result from non-coincidence of the X and Z principal axes of the g and the ⁵⁵Mn hyperfine tensors. In most cases, the ESR parameters can be interpreted in terms of a semi-occupied molecular orbital (SOMO) primarily d_x²–y² in character, but rotated about the y axis to avoid an antibonding interaction with the dienyl ring. The spectrum of [(η⁵-6-exo-PhC₆H₆)Mn(CO)(PMe₃)₂]⁺ shows non-equivalent ³¹P couplings, suggesting an unsymmetrical dienyl ring conformation. This spectrum, and those of the monophosphine cations, are best explained in terms of a SOMO primarily d_yz in character.