Department of Chemistry, Faculty of Natural and Agricultural Sciences, University of Pretoria, Lynnwood Road, Hillcrest, Pretoria, 0002 South Africa.
J Phys Chem A. 2011 Jun 23;115(24):6629-40. doi: 10.1021/jp201922w. Epub 2011 May 31.
The structure of the complexes of the type Ni(L)(H(2)O)(2), where L is an amino alcohol ligand, L = N,N'-bis(2-hydroxyethyl)-ethane-1,2-diamine (BHEEN), N,N'-bis(2-hydroxycyclohexyl)-ethane-1,2-diamine (Cy(2)EN), and N,N'-bis(2-hydroxycyclopentyl)-ethane-1,2-diamine, (Cyp(2)EN) were investigated at the X3LYP/6-31+G(d,p) level of theory both in the gas phase and in solvent (CPCM model) to gain insight into factors that control the experimental log K(1) values. We find that (i) analyses based on Bader's quantum theory of atoms in molecules (QTAIM) are useful in providing significant insight into the nature of metal-ligand bonding and in clarifying the nature of weak "nonbonded" interactions in these complexes and (ii) the conventional explanation of complex stability in these sorts of complexes (based on considerations of bond lengths, bite angles and H-clashes) could be inadequate and indeed might be misleading. The strength of metal-ligand bonds follows the order Ni-N > Ni-OH ≥ Ni-OH(2); the bonds are predominantly ionic with some covalent character decreasing in the order Ni-N > Ni-OH > Ni-OH(2), with Ni-OH(2) being close to purely ionic. We predict that the cis complexes are preferred over the trans complexes because of (i) stronger bonding to the alcoholic O-donor atoms and (ii) more favorable intramolecular interactions, which appear to be important in determining the conformation of a metal-ligand complex. We show that (i) the flexibility of the ligand, which controls the Ni-OH bond length, and (ii) the ability of the ligand to donate electron density to the metal are likely to be important factors in determining values of log K(1). We find that the electron density at the ring critical point of the cyclopentyl moieties in Cyp(2)EN is much higher than that in the cyclohexyl moieties of Cy(2)EN and interpret this to mean that Cyp(2)EN is a poorer donor of electron density to a Lewis acid than Cy(2)EN.
研究了配合物的结构Ni(L)(H(2)O)(2)，其中 L 是一种氨基酸醇配体，L = N，N'-双（2-羟乙基）-乙二胺（BHEEN），N，N'-双（2-羟基环己基）-乙二胺（Cy(2)EN）和 N，N'-双（2-羟基环戊基）-乙二胺（Cyp(2)EN），在气相和溶剂（CPCM 模型）中，在 X3LYP/6-31+G(d,p)理论水平上进行了研究，以深入了解控制实验 log K(1)值的因素。我们发现：（i）基于 Bader 的原子在分子中的量子理论（QTAIM）的分析在提供对金属-配体键合性质的深入了解以及阐明这些配合物中弱“非键”相互作用的性质方面非常有用；（ii）在这些类型的配合物中，基于考虑键长、咬合角和 H-冲突的常规配合物稳定性解释可能是不充分的，事实上可能是具有误导性的。金属-配体键的强度遵循 Ni-N > Ni-OH ≥ Ni-OH(2)的顺序；这些键主要是离子键，具有一定的共价性质，Ni-N > Ni-OH > Ni-OH(2)依次降低，Ni-OH(2)接近于纯离子键。我们预测顺式配合物优于反式配合物，因为：（i）与醇性 O-供体原子的更强键合；（ii）更有利的分子内相互作用，这似乎是决定金属-配体配合物构象的重要因素。我们表明：（i）配体的灵活性，它控制 Ni-OH 键长；（ii）配体向金属提供电子密度的能力，可能是决定 log K(1)值的重要因素。我们发现 Cyp(2)EN 中环戊基部分的环临界点的电子密度远高于 Cy(2)EN 中环己基部分的电子密度，并解释说 Cyp(2)EN 向路易斯酸提供电子密度的能力比 Cy(2)EN 差。
