Novoa Juan J, Ribas-Ariño Jordi, Shum William W, Miller Joel S
Department of Physical Chemistry, CERQT, and Institute of Nanotechnology and Nanoscience, University of Barcelona, E-08028 Barcelona, Spain.
Inorg Chem. 2007 Jan 8;46(1):103-7. doi: 10.1021/ic0611037.
Cu(PPh3)3(TCNE) (TCNE = tetracyanoethylene) and 14 other examples form [TCNE]22- dimers possessing a long 2.89 +/- 0.05 A two-electron four-center (2e-/4c) C-C bond in the solid state. This bond arises from the overlap of the b2g pi* singly occupied molecular orbital (SOMO) on each [TCNE]- fragment, forming a filled bonding orbital of b2u symmetry, and the stabilizing effect of the cation...anion interactions in the crystal that exceed the anionic repulsion. In contrast, Mn(C5H5)(CO)2(TCNE) exhibits a related, but different, [TCNE]-...TCNE]- motif in the solid state that lacks the 2e-/4c C-C bonding. To better understand the unusual nature of 2e-/4c C-C bonding, the genesis of the differences between their respective pi-[TCNE]-...TCNE]- interactions was sought. The lack of 2e-/4c C-C bond formation is attributed to the weaker radical character of the [TCNE]- ligand, which has a total spin population of only 0.5 electron, half of that required for two S = 1/2 [TCNE]- moieties to form a [TCNE]22- dimer. Hence, the antiferromagnetic MnII-[TCNE]- intramolecular interaction (between the formally S = 1/2 Mn-bound [TCNE]- and the paramagnetic Mn(II)) dominates over the intermolecular pi-[TCNE]--[TCNE]- spin coupling (between two S = 1/2 [TCNE]- needed to form [TCNE]22-). Therefore, by selecting specific metal ions that can interact with sigma-[TCNE]*-, dimerization forming [TCNE]22- can be favored or disfavored.
三（三苯基膦）铜（TCNE）（TCNE = 四氰基乙烯）以及其他14个例子在固态下形成了具有2.89±0.05 Å长的双电子四中心（2e-/4c）C-C键的[TCNE]22-二聚体。该键源于每个[TCNE]-片段上b2gπ单占据分子轨道（SOMO）的重叠，形成了具有b2u对称性的填充成键轨道，以及晶体中阳离子 - 阴离子相互作用的稳定作用……这种相互作用超过了阴离子间的排斥作用。相比之下，二茂锰羰基（TCNE）在固态下呈现出一种相关但不同的[TCNE]-……[TCNE]-基序，其缺乏2e-/4c C-C键合。为了更好地理解2e-/4c C-C键合的异常性质，人们探寻了它们各自π-[TCNE]-……[TCNE]-相互作用之间差异的根源。2e-/4c C-C键形成的缺失归因于[TCNE]-配体较弱的自由基特性，其总自旋布居仅为0.5个电子，这是两个S = 1/2的[TCNE]-部分形成[TCNE]22-二聚体所需电子数的一半。因此，反铁磁性的MnII-[TCNE]-分子内相互作用（在形式上S = 1/2的与锰结合的[TCNE]-和顺磁性的Mn(II)之间）比分子间π-[TCNE]--[TCNE]-自旋耦合（形成[TCNE]22-所需的两个S = 1/2的[TCNE]-之间）占主导地位。所以，通过选择能够与σ-[TCNE]-相互作用的特定金属离子，可以促进或抑制形成[TCNE]22-的二聚化。
