Reaction between cis-diaquabisbiguanideruthenium(III) and hexathiocyanatochromium(III) ions in aqueous media. A mechanistic survey on the formation of a dinuclear bridged complex

The reaction between two complex species [Ru(BigH)(2)(H2O)(2)](3+) and [Cr( SCN)(6)](3-) has been investigated in aqueous medium in the pH range 4.00-6 .10, and followed in the 21-40 degreesC range. Pseudo-first order rate cons tants were evaluated using the chromium complex in a ten-fold excess. The p lot of k(obs) versus pH has a 'bell' shaped profile. From this and other ev idence it was concluded that only the [Ru(BigH)(2)(OH)(H2O)](2+) form of th e ruthenium complex is reactive under the experimental conditions, and form s the dinuclear complex [Ru(BigH)(2)-mu-(SCN)(2)-Cr(NCS)(4)](0). The rate c onstant for the dinuclear complex formation at 31 degreesC is (2.57 +/- 0.0 5) x 10(-3) mol(-1) s(-1). DeltaH double dagger = 60 +/- 2 kJ mol(-1) and D eltaS double dagger = 116 +/- 10 J K-1 mol(-1).