Both reaction steps observed for the substitution of water
∆V # = Ϫ5 cm 3 mol Ϫ1 . The temperature and pressure dependence of all the processes studied suggest an associative sub-by thiourea in the complexes [Pt(en)(OH 2 ) 2 ] 2+ and [Pt(phen)(OH 2 ) 2 ] 2+ (en = ethylenediamine, phen = 1,10-phe-stitution mechanism. The hydroxo-bridged dinuclear complex [{Pt(phen)(µ-OH)} 2 ] 2+ is formed from [Pt(phen)(OH 2 ) 2 ] 2+ nanthroline) were investigated under pseudo-first-order conditions using the stopped-flow technique. The substitution of in aqueous solution unless the solution is very dilute and highly acidic. The X-ray structure of [{Pt(phen)(µ-the second water molecule in each complex was also studied under high pressure. The observed pseudo-first-order rate OH)} 2 ](F 3 CSO 3 ) 2 • 2 H 2 O was determined. It belongs to the triclinic space group P1 ¯and has one formula unit in the unit constants k obs (s Ϫ1 ) obeyed the equation k 1,2 obs = k 1,2 [tu] (tu = thiourea), where "1" and "2" refer to the first and the second cell. The unit cell dimensions are a = 7.126( ), b = 9.665(5), c = 12.774(7) A ˚; α = 71.85(5) , β = 85.52(5), γ = 73.12(5) deg; substitution reactions, respectively. Kinetic parameters associated with the substitution process are: k 1 en (25.0°C, pH = V = 799.9(8) A ˚3. The structure was solved with the Patterson method and refined to R = 0.061. A square planar coordina-3.0,
en (same conditions) = 12.1 M Ϫ1 s Ϫ1 , ∆H # = 30 tion of the platinum centers is observed, with no deviations from planarity but distortions due to the small bite angle of kJ mol Ϫ1 , ∆S # = Ϫ124 J K Ϫ1 mol Ϫ1 , ∆V # = Ϫ7 cm 3 mol Ϫ1 ; k 1 phen (25.0°C, pH = 1.0, I = 0.1 M) = 2900 M Ϫ1 s Ϫ1 , ∆H # = 41 phen and the four-membered ring. No significant lengthening of the PtϪO bond [mean value: 2.03(1) A ˚] is observed in com-kJ mol Ϫ1 , ∆S # = Ϫ41 J K Ϫ1 mol Ϫ1 ; k 2 phen (same conditions) = 1170 M Ϫ1 s Ϫ1 , ∆H # = 37 kJ mol Ϫ1 , ∆S # = Ϫ61 J K Ϫ1 mol Ϫ1 , parison with [{Pt(NH 3 ) 2 (µ-OH)} 2 ] 2+ .