High pressure ¹⁷O NMR study of solvent exchange on square planar complexes: Water exchange on [Pd(dien)H₂O]²⁺

Comparative Rates of Ligand Substitution Reactions of Pt−C-Bonded Complexes in Aqueous Solution and the X-ray Crystal Structure of [Pt{C6H3(CH2NMe2)2-2,6}(OH2)][OSO2CF3]

“…The literature data − summarized in Table , including the results of this and our earlier studies, nicely demonstrate that the lability of square-planar d 8 complexes can be systematically tuned through electronic and steric effects. For example, the metal ion itself plays a very important role, as is illustrated by the fact that solvent exchange and aquation reactions of the Pd(II) complexes [Pd(H 2 O) 4 ] 2+ and [Pd(dien)Cl] + are 6 orders of magnitude faster than those of the corresponding Pt(II) complexes [Pt(H 2 O) 4 ] 2+ and [Pt(dien)Cl] + .…”

“…In this context it is worth remarking that, although we have not succeeded in isolating trigonal-bipyramidal Pt(II) species based on the C 6 H 3 (CH 2 NMe 2 ) 2 -2,6 -ligand, the neutral square-planar species [Pt{C 6 H 3 (CH 2 NMe 2 ) 2 - ∆H q , kJ mol -1 ∆S q , J mol -1 K -1 ∆V q , cm 3 mol ∆H q , kJ mol -1 ∆S q , J mol -1 K -1 ∆V q , cm 3 mol The literature data [41][42][43][44][45][46][47][48][49] summarized in Table 7, including the results of this and our earlier studies, nicely demonstrate that the lability of square-planar d 8 complexes can be systematically tuned through electronic and steric effects. For example, the metal ion itself plays a very important role, as is illustrated by the fact that solvent exchange and aquation reactions of the Pd(II) complexes [Pd(H 2 O) 4 ] 2+ and [Pd(dien)Cl] + are 6 orders of magnitude faster than those of the corresponding Pt(II) complexes [Pt(H 2 O) 4 ] 2+ and [Pt-(dien)Cl] + .…”

Activation and Reaction Volumes in Solution. 3

“…For a wide variety of nonexchanging ligands on the central palladium(II) ion (entries 63-66) the solvent exchange is associative. [92][93][94] In cases where the solvents used are common the exchange is often more markedly associative in the case of Pt complexes than for Pd complexes (entries 67-73 and 96-99), 94,95 undoubtedly resulting from a radius increase down the group. Complexes of gadolinium(III) are prominent in the search for improved magnetic resonance imaging materials.…”

Theoretical Modeling of Water Exchange on [Pd(H2O)4]2+, [Pt(H2O)4]2+, and trans-[PtCl2(H2O)2]

“…A number of exchange processes at square-planar palladium(II) and platinum(II) complexes with oxygen-, nitrogen-, sulfur-, and carbon-bonded ligands have been studied. − Except for cases where there is extensive transfer of electron density from strongly σ-bonding carbon-donor ligands to the metal center, , these processes, like square-planar substitution reactions in general, seem to take place via associative mechanisms.…”

“…For instance, the activation volumes Δ V ⧧ for water exchange at [Pd(H 2 O) 4 ] 2+ and [Pt(H 2 O) 4 ] 2+ are −2.2 and −4.6 cm 3 mol -1 , respectively, whereas for the exchange of dimethyl sulfide (Me 2 S) at the corresponding tetrasolvated cations, the values are −9.4 and −22.0 cm 3 mol -1 . Attempts have been made to correlate these differences with the size and steric requirements of the ligands and/or with the steric encumbrance of the inner coordination sphere of the complex. ,, However, later high-pressure studies of complex formation reactions of the nonsterically hindered tetraaqua complexes of palladium(II) and platinum(II) indicate that there is no such obvious correlation , other factors that might influence the magnitude of the activation volumes are bond stretching in the transition state, changes in electrostriction of solvent molecules due to changes of ionic radii in the activation process, and (for associative square-planar reactions in particular) the possibility of loss of axially bound solvent molecules when a five-coordinate transition state is formed …”