Kinetics for complex formation between Pd(H2O)(4)(2+) and thioethers of largely varying electronic and steric properties, viz. MeSCH(2)COOH, (n-Pr)(2)S, EtSCH(2)CH(2)OH, S(CH2CH2CH2OH)(2), S(CH2CH2OH)(2), EtSCH(2)COOH, S(CH2COOH)(2), S(CH2CH2COOH)(2), (i-Pr)(2)S, (s-Bu)(2)S, (t-Bu)(2)S, and protonated thiomorpholine, S(C2H4)(2)NH2+, has been studied by use of stopped-flow spectrophotometry in an acidic aqueous medium. Second-order rate constants k(1)(298) are 1.61 x 10(4), 8.0 x 10(4), 3.79 x 10(4), 3.69 x 10(4), 2.21 x 10(4), 1.84 x 10(4), 1.91 x 10(3), 1.34 x 10(4), 1.52 x 10(4), 7.75 x 10(3), 900, and 5.2 x 10(3) M(-1) s(-1), respectively. The reactivity toward Pd(H2O)(4)(2+) of all thioethers studied so far can be described as a function of their sigma-donor properties as expressed by the sum of the Taft constants, Sigma sigma*, and their steric requirements as defined by cone angles, theta, by use of the equation : log k(1) = (9.9 +/- 0.3) - (0.67 +/- 0.05)Sigma sigma* - (0.059 +/- 0.003)theta. Similarly, second-order rate constants k(298) reported previously for reactions between thioethers and Pd(dien)H2O2+ and Pt(dienBr+ are described by log k(298) = (10.5 +/- 0.6) - 0.67 Sigma sigma* - (0.081 +/- 0.006)theta and log k(298) = (4.6 +/- 0.6) - 0.72 Sigma sigma* - (0.080 +/- 0.006)theta, respectively. Hence, the reactivity trends of thioethers toward square-planar complexes can be given a general interpretation in terms of intrinsic, electronic, and steric parameters, by use of log k = gamma + alpha Sigma sigma* + beta theta. Large variations in both electronic and steric properties of the entering ligands indicate that there is no "duality behavior" in the reactions of thioethers with square-planar metal centers, as claimed in previous literature. No steric threshold is observed for these sterically unhindered systems. : log k(1) = (9.9 +/- 0.3) - (0.67 +/- 0.05)Sigma sigma* - (0.059 +/- 0.003)theta. Similarly, second-order rate constants k(298) reported previously for reactions between thioethers and Pd(dien)H2O2+ and Pt(dienBr+ are described by log k(298) = (10.5 +/- 0.6) - 0.67 Sigma sigma* - (0.081 +/- 0.006)theta and log k(298) = (4.6 +/- 0.6) - 0.72 Sigma sigma* - (0.080 +/- 0.006)theta, respectively. Hence, the reactivity trends of thioethers toward square-planar complexes can be given a general interpretation in terms of intrinsic, electronic, and steric parameters, by use of log k = gamma + alpha Sigma sigma* + beta theta. Large variations in both electronic and steric properties of the entering ligands indicate that there is no "duality behavior" in the reactions of thioethers with square-planar metal centers, as claimed in previous literature. No steric threshold is observed for these sterically unhindered systems.