Gagliardi Laura, Roos Björn O
Dipartimento di Chimica Fisica F. Accascina, Universita' degli Studi di Palermo, Viale delle Scienze- Parco d'Orleans II, I-90128 Palermo, Italy.
Nature. 2005 Feb 24;433(7028):848-51. doi: 10.1038/nature03249.
Covalent bonding is commonly described by Lewis's theory, with an electron pair shared between two atoms constituting one full bond. Beginning with the valence bond description for the hydrogen molecule, quantum chemists have further explored the fundamental nature of the chemical bond for atoms throughout the periodic table, confirming that most molecules are indeed held together by one electron pair for each bond. But more complex binding may occur when large numbers of atomic orbitals can participate in bond formation. Such behaviour is common with transition metals. When involving heavy actinide elements, metal-metal bonds might prove particularly complicated. To date, evidence for actinide-actinide bonds is restricted to the matrix-isolation of uranium hydrides, including H2U-UH2, and the gas-phase detection and preliminary theoretical study of the uranium molecule, U2. Here we report quantum chemical calculations on U2, showing that, although the strength of the U2 bond is comparable to that of other multiple bonds between transition metals, the bonding pattern is unique. We find that the molecule contains three electron-pair bonds and four one-electron bonds (that is, 10 bonding electrons, corresponding to a quintuple bond), and two ferromagnetically coupled electrons localized on one U atom each-so all known covalent bonding types are contributing.
共价键通常由路易斯理论描述，两个原子之间共享的一对电子构成一个完整的键。从氢分子的价键描述开始，量子化学家进一步探索了整个元素周期表中原子化学键的基本性质，证实大多数分子确实是通过每个键一对电子结合在一起的。但是当大量原子轨道可以参与键的形成时，可能会出现更复杂的键合情况。这种行为在过渡金属中很常见。当涉及重锕系元素时，金属-金属键可能会特别复杂。到目前为止，锕系元素-锕系元素键的证据仅限于氢化铀的基质隔离，包括H2U-UH2，以及铀分子U2的气相检测和初步理论研究。在这里，我们报告了对U2的量子化学计算，结果表明，虽然U2键的强度与过渡金属之间的其他多重键相当，但键合模式是独特的。我们发现该分子包含三个电子对键和四个单电子键（即10个成键电子，相当于一个五重键），以及两个分别位于一个U原子上的铁磁耦合电子——所有已知的共价键类型都有贡献。
