Institute of Chemistry and The Lise Meitner-Minerva Center for Computational Quantum Chemistry, Hebrew University of Jerusalem, 91904, Jerusalem, Israel.
Nat Chem. 2012 Jan 29;4(3):195-200. doi: 10.1038/nchem.1263.
Triple bonding is conventionally considered to be the limit for multiply bonded main group elements, despite higher metal-metal bond orders being frequently observed for transition metals and lanthanides/actinides. Here, using high-level theoretical methods, we show that C(2) and its isoelectronic molecules CN(+), BN and CB(-) (each having eight valence electrons) are bound by a quadruple bond. The bonding comprises not only one σ- and two π-bonds, but also one weak 'inverted' bond, which can be characterized by the interaction of electrons in two outwardly pointing sp hybrid orbitals. A simple way of assessing the energy of the fourth bond is proposed and is found to be ~12-17 kcal mol(-1) for the isoelectronic species studied, and thus stronger than a hydrogen bond. In contrast, the analogues of C(2) that contain higher-row elements, such as Si(2) and Ge(2), exhibit only double bonding.
三重键通常被认为是多键主族元素的极限，尽管过渡金属和镧系/锕系元素经常观察到更高的金属-金属键序。在这里，我们使用高级理论方法表明，C(2)及其等电子分子 CN(+)、BN 和 CB(-)（每个分子都有八个价电子）通过四重键结合。该键合不仅包含一个 σ-键和两个 π-键，还包含一个较弱的“反向”键，该键可以通过两个指向外部的 sp 杂化轨道中电子的相互作用来表征。提出了一种评估第四键能量的简单方法，对于所研究的等电子物种，发现其约为 12-17 kcal mol(-1)，因此比氢键更强。相比之下，含有更高主族元素的 C(2)类似物，如 Si(2)和 Ge(2)，仅表现出双键。
