Dedicated to Professor Bernt Krebs on the occasion of his 75th birthdaySodium cyclopentadienide is a very important precursor for metallocene synthesis of s and related sandwich-and half sandwich-complexes of d-block elements by transmetallation or salt elimination routes. [1] Parent sodium cyclopentadienide (NaCp), which is easily prepared from dicyclopentadiene and sodium metal in laboratory scale, [2] is virtually insoluble in common organic solvents except tetrahydrofurane (THF), and its solid-state structure of infinite coordination polymer chains of the formula [h 5 -CpNa] 1 was determined [3] only a century after the initial discovery of the substance class. [4] There have been numerous reports of solid-state structures containing CpNa moieties. These smaller aggregates ultimately are nothing else than ligand-terminated fragments derived from the polymeric parent coordination polymer. However, few structural reports contain monomeric sodocene [6] [NaCp 2 ] À , inverse sandwich complexes [7] [L n NaCpNaL n ] + , or heterobimetallic sandwich complexes [8] (with n donating solvent molecules L), and only two different structures of piano stool-like complexes of the type [L n NaCp] have been published. [9] The latter have been restricted to bulky multidentate ligands L with resulting solid-state structures quite contrary to the small archetypical cyclopentadienyl carbonyl d-block metal complexes.The Cp-metal interaction in organo alkali metal species is dominated by the Coulomb attraction between anion and cation when compared to transition-metal complexes, where covalent contributions are more significant. [10] The donor ligand properties, including dipole moments, hapticity, and steric demand, play the most important role for the formation of the molecular assembly. [11] In the course of our work on the synthesis of reactive intermediates with organo alkali metal species, [12] it turned out that ammonia as coordinating solvent facilitates the synthesis of the missing link between the solvent-separated ion pair [L n Na] + [Cp] À and sandwich-or inverse sandwich complexes. The three-legged piano stool contact ion-pair complex [CpNa(NH 3 ) 3 ] (1), reported herein, was only feasible by crystallization from a saturated solution of ammonia in THF at À40 8C.We were able to obtain a high-resolution X-ray diffraction data set of a single crystal of 1 and conducted a XD2006 multipole refinement (Figure 1). A topological analysis according to the framework of Baders quantum theory of atoms in molecules (QTAIM) on the charge density map from an XD2006 multipolar refinement was carried out. [13] As the general structure of 1 is clear and not tainted by secondary (packing) effect of bulky donor bases (see Figure 1), it is a paradigmatic case to study intra-and intermolecular forces in the solid state.Most obviously, the sodium atom is shifted away from the geometric center of the Cp ring by 0.172 . A search in the Cambridge Structural Database (CSD) reveals that the observed shift of the sodium atom in 1 is unusual. ...
