Tzeli Demeter, Karapetsas Ioannis
Laboratory of Physical Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, Athens 157 84, Greece.
Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 11635, Greece.
J Phys Chem A. 2020 Aug 20;124(33):6667-6681. doi: 10.1021/acs.jpca.0c03208. Epub 2020 Aug 5.
Multiple bonds between atoms are one of the most fundamental aspects of chemistry. Double and triple bonds are quite common, while quadruple bonds are a true oddity and very rare for the main group elements. Identifying molecules containing quadruple bonds is very important and, even more so, determining the necessary requirements for the existence of such bonds. Here we present high-level theoretical calculations on the isoelectronic MX molecules, , TcN, RuC, RhB, and PdBe, showing that such a quadruple bond with main group elements is not that uncommon. We found that quadruple bonds are formed in their ground states XΔ (TcN) and ΧΣ (RuC, RhB, and PdBe) and in the two lowest excited states of TcN (Σ, Δ), RuC (Δ), and RhB (Δ). The quadruple bonds consist of two π and two σ bonds: (4d-2p), (4d-2p), (4d-2p), and 5s ← 2s (Σ) or 5p←2s (Δ). Bond lengths, dissociation energies, dipole moments, spectroscopic parameters, and relative energy ordering of the states were calculated multireference and coupled cluster methodology using the aug-cc-pV5Z(-PP) basis sets. We study how the atomic states involved and how the gradual transition from covalent to dative bond, from TcN to PdBe, influence all of the calculated data, such as bond dissociation energies, bond lengths, and relative energy ordering of the states. Finally, we report the requirements for the occurrence of such bonds in molecular systems. All Be, B, C, and N atoms combining with the appropriate second-row transition metal can form quadruple bonds, while they cannot form such bonds with the first-row transition metals.
原子间的多重键是化学中最基本的方面之一。双键和三键很常见，而四键则非常奇特，对于主族元素来说极为罕见。识别含有四键的分子非常重要，更重要的是确定此类键存在的必要条件。在此，我们展示了对等电子体MX分子（、TcN、RuC、RhB和PdBe）的高水平理论计算，结果表明这种与主族元素形成的四键并非那么罕见。我们发现四键在它们的基态XΔ（TcN）和ΧΣ（RuC、RhB和PdBe）以及TcN（Σ、Δ）、RuC（Δ）和RhB（Δ）的两个最低激发态中形成。四键由两个π键和两个σ键组成：(4d - 2p)、(4d - 2p)、(4d - 2p)，以及5s←2s（Σ）或5p←2s（Δ）。使用aug - cc - pV5Z(-PP)基组，通过多参考和耦合簇方法计算了键长、解离能、偶极矩、光谱参数以及各状态的相对能量顺序。我们研究了所涉及的原子态以及从TcN到PdBe从共价键到配位键的逐渐转变如何影响所有计算数据，如键解离能、键长和各状态的相对能量顺序。最后，我们报告了分子体系中此类键出现的条件。所有与适当的第二行过渡金属结合的Be、B、C和N原子都能形成四键，而它们与第一行过渡金属不能形成此类键。
