Goesten Maarten G, Hoffmann Roald, Bickelhaupt F Matthias, Hensen Emiel J M
Inorganic Materials Chemistry, Schuit Institute of Catalysis, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands;
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301;
Proc Natl Acad Sci U S A. 2017 Jan 31;114(5):828-833. doi: 10.1073/pnas.1615742114. Epub 2017 Jan 17.
Fluoride, nature's smallest anion, is capable of covalently coordinating to eight silicon atoms. The setting is a simple and common motif in zeolite chemistry: the box-shaped silicate double-four-ring (D4R). Fluoride seeks its center. It is the strain of box deformation that keeps fluoride in the middle of the box, and freezes what would be a transition state in its absence. Hypervalent bonding ensues. Fluoride's compactness works to its advantage in stabilizing the cage; chloride, bromide, and iodide do not bring about stabilization due to greater steric repulsion with the box frame. The combination of strain and hypervalent bonding, and the way they work in concert to yield this unusual case of multiple hypervalence, has potential for extension to a broader range of solid-state compounds.
氟化物，自然界最小的阴离子，能够与八个硅原子形成共价配位。这种结构是沸石化学中一个简单且常见的结构单元：盒状硅酸盐双四元环（D4R）。氟化物位于其中心。正是盒状结构变形产生的张力将氟化物保持在盒子中间，并冻结了在没有这种张力时本应是过渡态的状态。由此产生了超价键。氟化物的紧凑性有利于稳定笼状结构；而氯、溴和碘由于与盒状框架存在更大的空间排斥力，不会产生稳定作用。张力和超价键的结合，以及它们协同作用产生这种多个超价态的特殊情况，有可能推广到更广泛的固态化合物中。
