Theoretical Chemistry Division, Institute of General, Inorganic and Theoretical Chemistry, Center for Chemistry and Biomedicine, University of Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria.
Dalton Trans. 2018 Oct 7;47(37):13032-13045. doi: 10.1039/c8dt02169d. Epub 2018 Aug 29.
The solvation structures of Pd(ii) and Pt(ii) are typically reduced to the well-known square-planar structural motif, although it has been shown, in both experimental and theoretical investigations, that these solutes have a tendency to bind ligand molecules at elongated distance in axial coordination sites. In aqueous solution both ions form a tetragonally elongated octahedral first hydration shell (coordination number 6). However, a recent QM/MM MD simulation of Pd(ii) in ammonia solution indicated that the properties of the axial ligands are highly sensitive with respect to the nature of the solvent. In the present study the solvation properties of these solutes in aqueous solution and pure ammonia are compared, presenting novel QM/MM MD data for Pt(ii) in ammonia solution. The simulation results indicate that in the latter case the coordination sites in axial position remain unoccupied, thus indeed forming the prototype four-fold, square planar solvation structure (coordination number 4). The respective solvation properties are thoroughly evaluated and compared to experimental enthalpy and entropy of activation for the first shell ligand exchange, which similarly point towards the unique solvation of Pt(ii) in ammonia solution. The observed solvation can be explained via the electrostatic potential of tetraammineplatinum(ii) and -palladium(ii), clearly highlighting a strongly negative potential perpendicular to the plane of solvation in the Pt(ii) case. This result can be well interpreted in the rather simple context of Pearson's electronegativity and hardness, which had to be re-evaluated for Pt(ii) employing novel data for the second and third ionisation potentials of elemental Pt. In contrast to earlier work these results indicate that the latter is less electronegative and softer than its Pd(ii) counterpart. Together with the lower electronegativity and hardness of NH ligands compared to HO the interaction between the solute and axial ligands is weakened up to a point at which the formation of a tetragonally elongated coordination is prevented.
Pd(ii) 和 Pt(ii) 的溶剂化结构通常简化为众所周知的平面正方形结构基序，尽管实验和理论研究都表明，这些溶质在轴向配位位置有拉长配位分子的趋势。在水溶液中，两种离子都形成了一个拉长的四方八面体的第一水合壳（配位数为 6）。然而，最近在氨溶液中对 Pd(ii)的QM/MM MD 模拟表明，轴向配体的性质对溶剂的性质非常敏感。在本研究中，比较了这些溶质在水溶液和纯氨中的溶剂化性质，为氨溶液中的 Pt(ii)提供了新的 QM/MM MD 数据。模拟结果表明，在后一种情况下，轴向配位位置仍然未被占据，因此确实形成了原型四配位、平面正方形的溶剂化结构（配位数为 4）。彻底评估了相应的溶剂化性质，并与第一壳层配体交换的实验焓和熵活化能进行了比较，这同样表明 Pt(ii)在氨溶液中的独特溶剂化作用。观察到的溶剂化可以通过四氨合铂(ii)和-钯(ii)的静电势来解释，这清楚地突出了 Pt(ii)情况下在溶剂化平面垂直方向上的强烈负电势。这一结果可以在 Pearson 的电负性和硬度的相当简单的背景下得到很好的解释，对于 Pt(ii)，必须使用元素 Pt 的第二和第三电离势的新数据来重新评估。与早期的工作相比，这些结果表明后者的电负性和硬度都比其 Pd(ii)对应物低。与 HO 相比，NH 配体的电负性和硬度较低，因此溶质与轴向配体之间的相互作用被削弱，直至形成四方拉长配位的过程被阻止。
