Solution equilibria of tetrakis(ethyisocyanide)platinum(II) with tetracyanoplatinate(II): Equilibria and thermodynamics of the formation of di-, tri-, and tetraplatinum species

In aqueous solution, the tetrakis(ethylisocyanide)platinum(II) cation, Pt(C NC2H5)(4)(2+), reacts with tetracyanoplatinate(II), Pt(CN)(4)(2-), to form four multinuclear species. These species are the diplatinum(II) compound [P t(CNC2H5)(4)(2+)][Pt(CN)(4)(2-)], two triplatinum compounds, [Pt(CNC2H5)(4) (2+)](2)[Pt(CN)(4)(2-)](2+) and [Pt(CNC2H5)(4)(2+)][Pt(CN)(4)(2-)](2)(2-), and the tetraplatinum species [Pt(CNC2H5)(4)(2+)](2)[Pt(CN)(4)(2-)](2). The interactions leading to the stability of these species are not simply ioni c. The interactions result in major differences in the electronic absorptio n spectra, and the oligomers are not disrupted by the addition of other sal ts. The formation constant for each of the oligomers was determined at seve ral temperatures, and the enthalpies and entropies of formation were measur ed. Species with even numbers of platinum ions have more negative stepwise enthalpies of formation than do the species with odd numbers of platinum io ns. Entropies of formation are negative as well; however, the values are si gnificantly more positive for the species with odd numbers of platinum ions than for those with an even number. These thermodynamic trends can be rati onalized by recognizing that the species with even numbers of platinum ions are dipolar and organize the solvent better than the species with odd numb ers of platinum ions, which are not dipolar. Thus, the more negative enthal pies of formation of the species with even numbers of platinum ions are a c onsequence of better solvation than species with odd numbers of platinum io ns. Accordingly, the entropies of formation of the species with even number s of platinum ions are more negative because they organize solvent to a gre ater extent. Exclusive of solvent effects, the anion-cation interaction is about 30 kJ/mol.