R. Bruce King

A Stable Silicon(0) Compound with a Si=Si Double Bond

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is:C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si–Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.2294 ± 0.0011 (standard deviation) angstroms in L:Si=Si:L is consistent with a Si=Si double bond. Complementary computational studies confirm the nature of the bonding in L:(Cl)Si–Si(Cl):L and L:Si=Si:L.

Carbene-stabilized diphosphorus.

The potassium graphite reduction of L:PCl3 leads to the formation of carbene-stabilized diphosphorus molecules, L:P-P:L, 1 (L: = :C{N(2,6-Pri2C6H3)CH}2) and 2 (L: = :C{N(2,4,6-Me3C6H2)CH}2), respectively. The nature of the bonding in 1 and 2 was delineated by DFT computations.

Planar, twisted, and trans-bent: conformational flexibility of neutral diborenes.

The potassium graphite reduction of R‘BBr3 (R‘ = :C{N(2,4,6-Me3C6H2)CH}2) in Et2O led to the isolation of 3 (R‘(H)BB(H)R‘) and 4 (R‘(H)2B−B(H)2R‘), with BB double and B−B single bonds, respectively. These compounds were characterized by single-crystal X-ray diffraction, 1H and 11B NMR, and elemental analyses. Neutral diborene 3 exhibits polymorphic planar (3a), twisted (3b), and trans-bent (3c) geometries in the solid state.

Chemical applications of topology and group theory. 11. Degenerate edges as a source of inherent fluxionality in deltahedra

Abstract The 5, 8, 9, and 11 vertex deltahedra containing the minimum number of tetrahedral chambers are inherently fluxional since certain possible diamond-square-diamond rearrangements can lead directly to a polyhedron identical to the original one except for interchanges of some vertices. By this criterion the 6, 10, and 12 vertex deltahedra are inherently rigid. The 7 vertex pentagonal bipyramid is also inherently rigid but can stereochemically non-rigid in cases where a capped octahedron is an accessible intermediate. These topological observations are consistent with known information on the stereochemical non-rigidity and hydrolytic stability stability of the delta-hedral borane anions BnH2−n and the stereochemical non-rigidity of MLn coordination complexes. Thus the stereochemical non-rigidity of all ML7 complexes investigated as contrasted with the stereochemical rigidity of B7H2−7 can be related to the energetic accessibility of capped octahedral species in ML7 complexes but not in B7H2−7.

Rhodium phosphine complexes as homogeneous catalysts: 14. Asymmetric hydrogenation of a Schiff base of acetophenone — effect of phosphine and catalyst structure on enantioselectivity

Abstract Using catalysts prepared in situ from [Rh(NBD)Cl] 2 and chiral diphosphines of the type Ph 2 PCHRCH 2 PPh 2 (R = Ph, i-Pr, PhCH 2 ) optical yields above 60% were achieved in the hydrogenation of PhMeCNCH 2 Ph. Although reproducibility of the results was poor, it can be concluded that the chiral diphosphines DIOP and diPAMP are much less effective, and that the halide ligand is necessary for good enantioselectivity.

Chemical applications of topology and group theory. 12. Post-transition element clusters with particular emphasis on nine vertex

Abstract The previously reported graph theoretical model of aromaticity in two and three dimensions is used to treat homoatomic post-transition element ions containing germanium, tin, lead, antimony, bismuth, selenium, and tellurium. Localized bonding models are sufficient to treat three and seven vertex systems as well as 12 skeletal electron trigonal bipyramidal five vertex systems such as Sn2−5, Pb2−5, and Bi3+5. The squares Bi2−4, Se2+4, and Te2+4 are two-dimensional aromatic systems completely analogous to C4H2−4. Most interesting are the nine vertex systems which can be classified into the following three types: (1) The 20 skeletal electron Ge2−9 system which adopts the tricapped trigonal prismatic configuration expected for a closo 2n + 2 skeletal electron system; (2) The 22 skeletal electron Ge4−9, Sn4−9, and Pb4−9 systems which adopt the capped square antiprismatic configuration with one non-triangular face expected for a nido 2n + 4 skeletal electron system; (3) The 22 skeletal electron Bi5+9 system which is ‘anomalous’ since it adopts the closo deltahedral tricapped trigonal prismatic configuration rather than the nido capped square antiprismatic configuration expected for a 2n + 4 skeletal electron system. However, the tricapped trigonal prism in the 22 electron Bi5+9 system is more ‘elongated’ than the tricapped trigonal prisms in 20 skeletal electron nine vertex clusters such as B9H2−9, B7H7C2(CH3)2, and Ge2−9. Detailed graph theoretical calculations show that such elongation of the tricapped trigonal prism can lead to incomplete overlap of the nine unique internal orbitals which generates the two core bonding orbitals required to accommodate the four core bonding electrons in a 2n + 4 skeletal electron system. These calculations also provide an illustration of a graph splitting algorithm for the symmetry factoring of graph characteristic polynomials.

Metal cluster topology. 1. Osmium carbonyl clusters

Abstract Important theoretical approaches to metal cluster bonding including the Wade-Mingos skeletal electron pair method, the Teo topological electron count, the King-Rouvray graph theory derived method, and Lauhers extended Huckel calculations are shown to agree in their apparent skeletal electron counts for the most prevalent metal cluster polyhedra including the tetrahedron, the trigonal bipyramid (both ordinary and elongated), square pyramid, octahedron, bicapped tetrahedron, pentagonal bipyramid, and capped octahedron. The graph theory derived method is used to treat osmium carbonyl clusters containing from five to eleven osmium atoms. In this connection most osmium carbonyl clusters can be classified into the following types: (1) Clusters exhibiting edge- localized bonding containing multiple tetrahedral chambers (e.g., Os 5 (CO) 16 , Os 6 (CO) 18 , H 2 Os 7 (CO) 20 and HOs 8 (CO) 22 − ); (2) Capped octahedral clusters derived from osmium carbonyl fragments of the type Os 6+ p (CO) 19+2 p ( p = 0, 1, 2, and 4) (e.g., Os 6 - (CO) 18 2− , Os 7 (CO) 21 , Os 8 (CO) 22 2− , and H 4 Os 10 - (CO) 24 2− ). Other more unusual osmium carbonyl clusters such as the planar Os 6 (CO) 17 [P(OCH 3 ) 3 ] 4 , the Os 9 cluster [Os 9 (CO) 21 C 3 H 2 R] − , and the Os 11 cluster Os 11 C(CO) 27 2− can also be treated satisfactorily by these methods. The importance of the number of ligands around isoelectronic Os n systems in determining the cluster polyhedron is illustrated by the different cluster polyhedra found for each member of the following isoelectronic pairs: HOs 6 - (CO) 18 − /H 2 Os 6 (CO) 18 . Os 7 (CO) 21 /H 2 Os 7 (CO) 20 , Os 8 (CO) 22 2− /HOs 8 (CO) 22 − . The tendency for osmium carbonyl clusters frequently to form polyhedra exhibiting edge-localized rather than globally delocalized bonding relates to the facility for osmium carbonyl vertices to contribute more than three internal orbitals to the cluster bonding. In this way Wades well-known analogy between boron hydride clusters and metal clusters, which assumes exactly three internal orbitals for each vertex atom, is frequently no longer followed in the case of osmium carbonyl clusters.

Chemical applications of group theory and topology

The following procedure is described for investigating the qualitative dynamics of simple chemical systems: 1) A so-called influence diagram is generated representing the relationships between the reference reactants (phase-determining intermediates); 2) This influence diagram is used to generate a “truth table” indicating possible transitions between state vectors representing the signs of the time derivatives of of the reference reactant concentrations; 3) The truth table is used to determine a state transition diagram representing the flow topology around unstable equilibrium points; 4) The characteristic equation of the adjacency matrix of the influence diagram is solved in order to determine the presence of such unstable equilibrium points. The two types of qualitative dynamics possible for chemical systems containing two reference reactants and one feedback circuit are bifurcation between two attracting regions (bistability) and limit cycle oscillation. However, in two reference reactant systems oscillation requires an additional self-activating loop to generate the unstable equilibrium point required for its realization. Bistability and limit cycle oscillation are also two of the possible types of qualitative dynamics for chemical systems containing three reference reactants. However, chemical systems with three reference reactants and two or more feedback circuits can also contain interlocking limit cycles, which can lead to toroidal oscillations or chaos. The influence diagrams are given for the systems exhibiting these various types of dynamic behavior along with a summary of the important properties of all 729 possible influences for simple chemical systems containing three reference reactants.

Metal cluster topology. 3. Platinum carbonyl clusters

Previously discussed topological models of metal cluster bonding are now extended to the treatment of stacked platinum carbonyl clusters, whose structute and bonding exhibit a variety of new features. The stacked triangle cluster dianions Pt3k(CO)6k2− (k = 2,3,4,5) are best regarded as built from edge- localized bonds with additional Mobius delocalization on both the top and bottom triangles of the stack. These stacked triangles thus appear to be the first examples of stable chemical species having planar rings of atoms exhibiting twisted Mobius rather than untwisted Huckel delocalization. Such Mobius delocalization can naturally arise from the phase changes of appropriate d orbitals of each of the platinum atoms in the top and bottom triangles of the stack. The more complicated tetraanion Pt19- (CO)224− can be regarded structurally as a threaded tube in which a Pt15 stack of three pentagons is the tube and a Pt4 chain is the thread. Edge-localized bonding is then seen to occur within both the Pt15 stack (25 edges) and the Pt4 thread (3 edges) with additional delocalized bonding within the pentagonal pyramidal chambers at each end of the stack. These seemingly rather exotic topological bonding models are consistent with the general principles of metal cluster bonding and reproduce exactly the observed electron counts for the stacked platinum carbonyl clusters.

Hypoelectronic dirhenaboranes having eight to twelve vertices: internal versus surface rhenium-rhenium bonding.

Fehlner, Ghosh, and their co-workers have synthesized a series of dirhenaboranes Cp(2)Re(2)B(n-2)H(n-2) (n = 8, 9, 10, 11, 12) exhibiting unprecedented oblate (flattened) deltahedral structures. These structures have degree 6 and/or 7 rhenium vertices at the flattest regions on opposite sides of an axially compressed deltahedron thereby leading to Re═Re distances in the range 2.69 to 2.94 Å suggesting internal formal double bonds. These experimental oblate (flattened) deltahedral structures are shown by density functional theory to be the lowest energy structures for these dirhenaboranes. In some cases the energy differences between such oblate deltahedral structures and the next higher energy structures are quite considerable, that is, up to 25 kcal/mol for the nine-vertex Cp(2)Re(2)B(7)H(7) structures. The higher energy Cp(2)Re(2)B(n-2)H(n-2) structures are of the two types: (1) Most spherical (closo) deltahedra having unusually short 2.28 to 2.39 Å Re-Re edges with unusually high Wiberg bond indices suggesting formal multiple bonds on the deltahedral surface; (2) Deltahedra having one or two degree 3 vertices and 2.6 to 2.9 Å Re-Re edges. The latter deltahedra are derived from smaller deltahedra by capping Re(2)B faces with the degree 3 vertices.