Complexes of Re and Tc with 1,4,7-trithiacyclononane (9S3) differ from their later transition metal analogues in that their d⁶ 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 π-back donation by Re and Tc, compared to later metals, into ligand C–S σ* 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 d⁶ analogues, extended Hückel 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 σ* 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 π-back donation into C–S σ* orbitals, and correctly predicts that ethene loss occurs more readily from rhenium and technetium d⁵ and especially d⁶ complexes than from later transition metal analogues.