An investigation of C-S bond activation in transition metal crown thioether complexes using extended Huckel theory and electrospray mass spectrometry

Complexes of Re and Tc with 1,4,7-trithiacyclononane (9S3) differ from their later transition metal analogues in that their d(6) form ([M(9S3)(2)](+)) undergoes instantaneous C-S bond cleavage yielding ethene and [M(9S3)L](+) (L= SCH2CH2SCH2CH2S), a stable metal(III) thiolate complex, cleanly in aqueous solution. This contrast is interpreted as signifying increased pi-back donation by Re and Tc, compared to later metals, into ligand C-S sigma* orbitals. In order to validate this hypothesis within an established theoretical framework, and to compare the predicted relative C-S bond lability with relative experimental lability in a series of d(6) analogues, extended Huckel theory (EHT) was used to investigate the bonding (M=Mo, Tc, Ru, Rh or Pd) while electrospray mass spectrometry (ES-MS) was used to compare ethene loss, in a series of analogous complexes (M=Tc, Re, Ru or Os). The C-S overlap populations were smaller for M=Tc-II and Tc-I than for later metal(II) analogues, and were smaller for Tc-I than for Tc-II. Fragment molecular orbitals corresponding to C-S sigma* were more highly populated for M=Tc-II and Tc-I than for later analogues, and also more highly populated for Tc-I than for Tc-II. ES-MS showed that ethene loss from Tc/Re-I and Tc/Re-II complexes occurred at much lower energies than from the Ru/Os-II analogues. EHT supports the hypothesis that C-S activation is caused by pi-back donation into C-S sigma* orbitals, and correctly predicts that ethene loss occurs more readily from rhenium and technetium d(5) and especially d(6) complexes than from later transition metal analogues.