Dodecahedron interactions

One interesting aspect of the structure of [Pr(TTA)4] is the short intramolecular sulphur-oxygen distances of 2.8—2.9 A. This value is about 0.4 A shorter than the sum of the van der Waals radii indicating some interaction between these atoms. The structure of NH4[Pr(TTA)4] HaO is stabilized by the combination of water of hydration and the small cation. The line defined by HaO—Pr—NH4 corresponds to one of the twofold axes of the dodecahedron. However, the large distances between Pr and OH 3 and Pr and NH4 (3.7—3.9 A) indicate no appreciable interactions. 

Although the topology of the hydrogen-bonded water host structure is closely related in the gas hydrates, the alkylamine hydrates, and the alkylammonium salt hydrates, with the pentagonal dodecahedron shown in Fig. 21.3 playing a prominent role, the interactions between the host and guest spedes are different. 

Contours of the dodecahedron characteristic for the dimethyl derivative may still be discerned, but agostic Li H(Si) interactions are responsible for a strong distortion of the molecular skeleton and especially for tetramerization. A detailed discussion of the structure leads to the conclusion that the newly formed Li H interactions are at least as strong as or even stronger than the broken Li-N bonds. 

The structure of Np(Np02)2(Se03)3 is extraordinarily complex as it contains three crystallographically unique Np centers with three different coordination environments in two different oxidation states. Two of the Np centers are +V and one is +IV. The Np(V) centers are found as both NpOy pentagonal bipyramids and NpOs hexagonal bipyramids. The Np(IV) center is found as a distorted NpOs dodecahedron. Cation-cation interactions (CCTs), exist between both the neptunyl units, and these units also coordinate the Np(IV) center creating a two-dimensional substructure. There are three crystallographically unique selenite anions. 

A) a 2 1 derivative can also be isolated in which cadmium is tetrahedrally coordinated, although four perchlorate interactions yield a polyhedral co-ordination intermediate between a dodecahedron and a square antiprism. ... 

The bipolar (e.g. bipyramids and bicapped antiprisms) and non-polar (e.g. D2d-dodecahedron and tricapped trigonal prism) deltahedral custers are best analysed in terms of the interactions between the two sets (polar and non-polar or equatorial and non equatorial) of symmetry-equivalent atoms which make up the cluster1573. In this way it has been shown that, although by symmetry there are no degenerate L /L" pairs and therefore no symmetry-induced departures from the (n + 1) rule, the frontier orbitals of bipolar deltahedral clusters consist of two parity matched U1 and L e pairs, giving rise to possible SEP counts of (n - 1), (n + 1) or (n + 3). In the case of the non-polar deltahedra the frontier orbitals are non-degenerate 1/ and L" orbitals, giving rise to possible SEP counts of n, (n + 1) or (n + 2). 

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