Helicoid Shiftamers for the Transport of π‐Clumps and Charges

Hypothetical helical organic polymers with localized electrons, charges, and/or atoms are described, in which these localized substructures can move along the backbones of the polymers via sigmatropic H shifts.     

Electron-deficient bonding in rhomboid rings

The bonding environment of boron is usually thought about in terms of localized 2c-2e/3c-2e bonding (as in diborane) or completely delocalized polyhedral bonding (as in B(12)H(12)(2)(-)). Recently, a number of boron compounds having a rhomboidal B(4) framework have been synthesized; these show an amazing variation in their skeletal electron count, one that cannot be interpreted in familiar ways. In this report, we systematically explore the origin of the range of electron counts in these compounds. We find that four skeletal MOs are primarily responsible for keeping the B(4) skeleton together. As a subunit in a macropolyhedral environment, termed rhombo-B(4), such an arrangement of B atoms deviates from Wade's rule by three electron pairs (if treated as a distorted arachno system derived from B(6)H(6)(2)(-)). Aided by this analysis, we examine the nature of bonding in Na(3)B(20), where the rhombo-B(4) unit forms linear chains fusing closo-B(7) units. Theory suggests that this structure requires one more electron per formula unit for optimal bonding. Finally, we study the nature of bonding in beta-SiB(3), where silicon atoms also adopt the rhomboid framework.     

Orbital Theory of Heterolytic Fragmentation and Remote Effects on Nitrogen Inversion Equilibria

From a molecular orbital study of model systems we derive the electronic requirements for the Grob fragmentation. The necessary condition for an allowed fragmentation in an X-C₁-C₂-C₃-N system, or the amino cation ⁺C₁-C₂-C₃-N is the level ordering A below S . This in turn is set by maximal through-bond coupling of the empty cation orbital and the nitrogen lone pair. The conformational dependence of through-bond coupling is exactly that derived by Grob, namely parallel orientation of the cation orbital (or the C-X bond), the C₂-C₃-σ-bond, and the N-lone-pair. When the C₁-C₂-C₃ and C₂-C₃-N angles are small, the through-space interaction dominates, reversing the level ordering to S below A , and consequently makes the fragmentation forbidden even though the conformational requirements for it are met. Ring closure becomes allowed. Some examples exploiting this result are presented, as well as procedures for enhancing through-bond coupling and thus fragmentation. The through-bond-effect has also kinetic consequences, allowing the definition of a new type of remote neighbouring group participation operative through bonds and not by direct overlap. The position of equilibria in nitrogen inversion processes should also be influenced by remote substituents which are π-acceptors or donors.     

Hypothetical C(60) Metal-Cluster Fullerides and General Aspects of Tetrahedral Cluster Bonding

It is geometrically feasible to insert metal-metal-bonded M(4) tetrahedra and M(6) octahedra into the tetrahedral and octahedral holes, respectively, of the fcc C(60) lattice. The electronic structure of the hypothetical tetrahedral variants C(60)(M(4))(2), M = Rh, Co, is analyzed with approximate molecular orbital methods and band structure calculations. These compounds feature M-M and M-C(60) bonding and a variable degree of electron transfer to or from C(60). The C(60)(M(4))(2) phases should be metallic, but we have no way of predicting if they will be superconducting. A number of discrete molecular tetrahedral cluster compounds which serve as models for the solid state materials are analyzed. There is a clear indication that tetranuclear and even mononuclear Rh, Ir, and Co arene complexes should be relatively unstable.     

Electronic Structure of [Ta(5)(NH)(4)Cl(17)](6-): A Cluster with a Distorted Square Pyramidal Ta(5) Core

We report approximate molecular orbital calculations on the [Ta(5)(NH)(4)Cl(17)](6-) cluster synthesized by Simon and Meyer. The cluster is based on a "flattened" square pyramid of tantalum atoms, basal bridging imides, and terminal chlorides. This cluster was of interest to us due to the unusual presence of imide ligands, the distorted nature of the metal core, and the possible resemblance to B(5)H(9). Our calculations indicate that metal-metal bonding is limited to Ta(apical)-Ta(basal) bonding, with no significant bonding between the basal metal atoms. The imide ligands, which bridge the base of the pyramid, were found to have a significant amount of capping character. The metal-metal bonding orbitals have some unusual features due to the pyramid's distortion. Additionally, the flattened nature of the pyramid leads to an interesting energy ordering of the metal-metal bonding orbitals.     

The Four-Connected Net in the CeCu(2) Structure and Its Ternary Derivatives. Its Electronic and Structural Properties

The crystallochemistry of and the bonding in the orthorhombic four-connected nets of BaIn(2) (CeCu(2) structure) and of CaPtSn (TiNiSi structure, a derivative of the CeCu(2) structure) are analyzed with approximate molecular orbital calculations. Following the Zintl concept, in BaIn(2) the In(-) ions are isoelectronic with group IV tin and should adopt a four-connected structure. In contrast to alpha-tin, which has a cubic diamond structure, the indium ions in BaIn(2) build up an orthorhombic three-dimensional four-connected net containing distorted tetrahedra and ladder polymers of four-membered rings. In the CeCu(2) structure (space group Imma) two bond angles in these distorted tetrahedra are fixed at 90 degrees. The four-connected net in the CeCu(2) structure is topologically related to the layers in black phosphorus (space group Cmca). In CaPtSn (TiNiSi structure) the orthorhombic four-connected net is formed by (PtSn)(2)(-) ions in an ordered arrangement. Calculations on BaIn(2) and CaPtSn show that the four-connected nets are increasingly stabilized as the valence electron count is increased from 16 to 30 valence electrons per 4 formula units. For more than 30e, the nets are destabilized due to filling of M-E antibonding states. Structural data obtained by precise single crystal investigations for the TiNiSi series CaPdIn (20e), CaPdSn (24e), CaPdSb (28e), and CaAgSb (32e), confirm the results of the extended Hückel calculations. We find an interesting and understandable angular asymmetry of the tetrahedral sites in these ternary compounds.