PROTON NUCLEAR MAGNETIC RESONANCE STUDIES OF SPIN DELOCALIZATION IN LOW-SPIN CHELATES OF CHROMIUM(2) AND IRON(3) WITH SYMMETRICALLY SUBSTITUTED O-PHENANTHROLINES AND ALPHA,ALPHA'-BIPYRIDINES

Narrow, well-resolved proton nmr spectra have been observed for a series of substituted tris o-phen-anthroline and α,α-bipyridine chelates of Cr(II). The isotropic shifts are interpreted as arising from a combination of direct π spin delocalization of metal unpaired spins, and negative ó spin delocalization, which results from an exchange polarization interaction between the unpaired metal spins and the ligand donor ó electrons. The absence of sizable πdelocalization indicates that the Cr(II) chelates do not derive their marked stability from πbonding alone, and demonstrates that the “spin-only” magnetic moments for these compounds must be attributed to a strong trigonal distortion of the ligand field rather than to delocalization of magnetic electrons. A reinterpretation of the previously reported isotropic shifts for the analogous Fe(III) complexes suggests that they arise primarily from negative ó spin delocalization. This reinterpretation constitutes the most convincing evidence to date for the existence of the exchange polarization delocalization mechanism. It is also shown that this effect can be observed in other complexes in the literature. The relative contact shift magnitudes for the Cr(II) and Fe(III) chelates indicate that the former possess higher π covalency, while the latter exhibit larger ó covalency, which may be related to the higher energies of the Cr(II) d orbitals relative to Fe(III). © 1969, American Chemical Society. All rights reserved.