水配合物

提示：此条目的主题不是水合物。

水配合物是指水作为配体和金属以配位键的方式结合，形成的配位化合物。许多金属盐（如硝酸盐、硫酸盐、高氯酸盐等）的溶液中，水合离子是金属离子存在的主要形式。水合离子具有[M(H2O)n]z+的通式，但通常会简写为Mz+。在这类配合物中，水分子可以作为唯一的配体，但很多情况也会同时存在其它配体。[1]

八面体的金属水配合物的结构

结构

许多水配合物都是八面体构型的，它们具有通式[M(H2O)6]2+或[M(H2O)6]3+，一些例子列入下表。少部分水配合物的配位数小于6。Pd(II)和Pt(II)形成的是平面四边形的[M(H2O)4]2+配离子。三价镧系金属的水配合物可以是八或者九配位的，这源于中心金属的大体积。

大约有三分之一的过渡金属（Zr、Hf、Nb、Ta、Mo、W、Tc、Re、Os和Au）的水合离子很少或几乎没有报道。四价及以上的水合离子的酸性会非常强，故无法在水溶液中存在而转化为含氧酸根或酰基阳离子，例如[Ti(H2O)6]4+是未知的，但是[Ti(H2O)6]3+却有着多方面的表征；又如Zr(IV)的水合离子水解，按其化学计量比生成[Zr4(OH)12(H2O)16]8+和对应数目的氢离子（参见氧氯化锆）而显酸性；[V(H2O)6]5+是未知的，而它的共轭碱[VO2(H2O)4]+很稳定。一价离子如Cu(I)和Rh(I)很难从水中分离出水配合物，但Ag(I)可以形成罕见的四面体水配合物[Ag(H2O)4]+。[2]

水溶液中的Cr(II)离子，显示出该离子的蓝色。

一些水合离子含有金属-金属键，如[Mo2(H2O)8]4+和[Rh2(H2O)10]4+。[2]

配合物 颜色 电子构型 M–O距离（Å）[3] 水交换速率（s−1，25℃）[4] M2+/M3+自交换速率（s−1，25℃）
[Ti(H2O)6]3+ 紫色 (t2g)1 2.025 1.8 x 105 ——
[V(H2O)6]2+ 紫色 (t2g)3 2.12 快
[V(H2O)6]3+ 黄色 (t2g)2 1.991 快
[Cr(H2O)6]2+ 蓝色 (t2g)3(eg)1 2.06, 2.33 1.2 x 108 慢
[Cr(H2O)6]3+ 紫色 (t2g)3 1.961 2.4 x 10−6 慢
[Mn(H2O)6]2+ 浅粉色 (t2g)3(eg)2 2.177 2.1 x 107 ——
[Fe(H2O)6]2+ 浅蓝色 (t2g)4(eg)2 2.095 4.4 x 106 快
[Fe(H2O)6]3+ 浅紫色* (t2g)3(eg)2 1.990 1.6 x 102 快
[Co(H2O)6]2+ 粉红色 (t2g)5(eg)2 2.08 3.2 x 106 ——
[Ni(H2O)6]2+ 绿色 (t2g)6(eg)2 2.05 3.2 x 104 ——
[Cu(H2O)6]2+ 蓝色 (t2g)6(eg)3 1.97, 2.30 5.7 x 109 ——

*注：Fe3+水解后显黄色，浅紫色仅在晶体或强酸抑制水解且无干扰配体的溶液中可见。

固体水配合物的结构

很多金属盐在固态或者溶液中都是水配合物，各种配位数的盐均是已知的。一些盐类结晶的时候会发生水合作用，在形成的水合物的结晶水中，一部分水不是配位水，而是通过非共价作用结合的，来稳定配合物的结构。[5]

反应

水配合物有三种基本的反应：配体交换、电荷转移以及O-H键的酸碱反应。

配体交换

配体交换即配合物的水配体（配位数）和溶液中的水的交换，在下例中，溶液中的水用H2O*表示：

[M(H2O)n]z+ + H2O* → [M(H2O)n-1(H2O*)]z+ + H2O

如果没有同位素标记，反应级数降级，即自由能变化为零。

不同离子的交换速率不同。影响交换速率的主要因素多种，如电荷，电荷多的离子交换速率慢于电荷少的离子，如[Na(H2O)6]+的交换速率比[Al(H2O)6]3+快109倍；电子构型也会影响交换速率，如[Al(H2O)6]3+和[Ir(H2O)6]3+也有109倍的差距。[4]水交换是游离-取代的过程，其速率常数表明其交换反应属于一级反应。

电荷转移

该反应通常适用于二价和三价金属离子的相互转换，其仅涉及交换一个电子，该过程称为自交换，即表面上平衡反应中电子仅和自己交换。平衡反应的标准电极电位如下：

[M(H2O)6]2+ + [M(H2O)6]3+ ⇌ [M(H2O)6]3+ + [M(H2O)6]2+
M2+/M3+电对的氧化还原反应标准电势（V）
V Cr Mn Fe Co
-0.26 -0.41 +1.51 +0.77 +1.82

这表明随着原子序数增加，较低氧化态的稳定性增加。锰的电对的值非常大，这是因为八面体Mn(II)的晶体场稳定化能（CFSE）为0，而Mn(III)的CFSE却有3。[6]

如果用星号（*）标记出金属离子，那么这个自交换过程可以写为：

[M(H2O)6]2+ + [M*(H2O)6]3+ → [M*(H2O)6]2+ + [M(H2O)6]3+

电荷转移（交换）的速率有大有小，这归因于不同的重组能量：当+2价的离子和+3价的离子在结构上差异很大时，速率通常很慢。[7]电荷转移通过球外电荷转移（英语：outer sphere electron transfer）来完成。大多数重组能量和eg的变化有关，这对八面体配合物尤其适用。

酸碱反应

水配体和质子结合，会使金属的水配合物溶液显酸性。在稀溶液中，Cr(III)的水合离子的pKa约为4.3：

[Cr(H2O)6]3+ ⇌ [Cr(H2O)5(OH)]2+ + H+

因此，这个水合离子是弱酸，酸性和醋酸（pKa约为4.8）相当。Cr(III)是三价离子的典型例子。电子构型在对离子酸性的影响上有所表现，如Rh(III)比Ru(III)的电负性更小，但[Ru(H2O)6]3+（pKa = 2.7）的酸性却强于[Rh(H2O)6]3+（pKa =4）。这和(t2g)5Ru(III)中心稳定的π-供体羟基化合物有关。[2]

在更浓的溶液中，一些羟合的金属离子会发生缩合，即羟桥化作用（英语：olation），缩合会产生多聚物种。很多矿物就会经过这一羟桥化作用形成多聚物种。二价金属的水合离子的酸性比三价金属的要弱。

对六水合的离子来说，其水解的产物的性质会有很大不同，如[Al(H2O)5OH]2+的水交换反应比[Al(H2O)6]3+快20000 倍。

参考文献

  1. ^ Mark I. Ogden and Paul D. Beer "Water & O-Donor Ligands" in Encyclopedia of Inorganic Chemistry, Wiley-VCH, 2006, Weinheim. doi:10.1002/0470862106.ia255
  2. ^ 2.0 2.1 2.2 S. F. Lincoln, D. T. Richens, A. G. Sykes "Metal Aqua Ions" Comprehensive Coordination Chemistry II Volume 1, Pages 515-555. doi:10.1016/B0-08-043748-6/01055-0
  3. ^ For Mn(II), Fe(II), Fe(III): T. K. Sham, J. B. Hastings, M. L. Perlman "Structure and dynamic behavior of transition-metal ions in aqueous solution: an EXAFS study of electron-exchange reactions" J. Am. Chem. Soc., 1980, 102 (18), pp 5904–5906. doi:10.1021/ja00538a033. For Ti(III), V(III), Cr(III): B. Kallies, R. Meier "Electronic Structure of 3d [M(H2O)6]3+ Ions from ScIII to FeIII: A Quantum Mechanical Study Based on DFT Computations and Natural Bond Orbital Analyses" Inorg. Chem., 2001, 40 (13), pp 3101–3112. doi:10.1021/ic001258t
  4. ^ 4.0 4.1 Lothar Helm, André E. Merbach "Inorganic and Bioinorganic Solvent Exchange Mechanisms" Chemical Reviews 2005, volume 105, 1923-1959. doi:10.1021/cr030726o
  5. ^ K. Waizumi, H. Masuda, H. Ohtaki, "X-ray structural studies of FeBr24H2O, CoBr24H2O, NiCl2 4H2O, and CuBr24H2O. cis/trans Selectivity in transition metal(I1) dihalide Tetrahydrate" Inorganica Chimica Acta, 1992 volume 192, pages 173–181.
  6. ^ Burgess, John. Metal Ions in Solution. Chichester: Ellis Horwood. 1978. ISBN 0-85312-027-7.  p. 236.
  7. ^ R. G. Wilkins Kinetics and Mechanism of Reactions of Transition Metal Complexes, 2nd Edition, VCH, Weinheim, 1991. ISBN 1-56081-125-0

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