ESR and NMR investigation of the distribution of the spin density in bisarenechromium compounds

Oxidatively induced isomerisation of vinylidene ligands to alkynes: ESR spectra of paramagnetic vinylidene and alkyne arene metal complexes

“…The d z 2 character of the SOMO is confirmed for the alkyne complexes by the appearance of hyperfine coupling to the arene ring protons. The proton couplings observed in the ESR spectra of [Cr(η-C 6 H 6 ) 2 ] ϩ (3.46 G), 19 [Mo(η-C 6 H 6 ) 2 ] ϩ (4.45 G), 20 and related species are positive in sign 24 and have been interpreted as resulting from direct delocalisation of the predominantly metal d z 2 SOMO into the σ-orbitals of the arene; 25 the usual mechanism whereby proton couplings in arene radical ions arise from spin polarisation by π spin density is of negligible importance in these systems. If we assume that the arene proton coupling is proportional to the metal d z 2 character, the observed 3.46 G coupling for [Cr(η-C 6 H 6 ) 2 ] ϩ , together with the EHMO calculations, suggests a coupling of 1.6 G for [Cr(CO) 2 (η-HC᎐ ᎐ ᎐ CH)(η-C 6 H 6 )] ϩ and 1.4 G for [Cr(CO) 2 (C᎐ ᎐ CH 2 )(η-C 6 H 6 )] ϩ .…”