Decarbonylation Products of Binuclear Methylphosphinidene Complexes of Cyclopentadienyliron Carbonyls: Triplet and Quintet Structures Are Favored Energetically over Singlet Structures with Iron-Iron Multiple Bonding.

The structures, energetics, and energetically preferred spin states of methylphosphinidene-bridged binuclear cyclopentadienyliron carbonyl complexes MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) related to the experimentally known (µ-RP)Fe2(µ-CO)(CO)2Cp2 (R = cyclohexyl, phenyl, mesityl, and 2,4,6-tBu3C6H2) complexes have been investigated by density functional theory. Singlet structures having a pyramidal pseudotetrahedral phosphorus environment with 18-electron iron configurations are energetically preferred in the tricarbonyl and tetracarbonyl systems MePFe2(CO)nCp2 (n = 4 and 3) with the lowest energy structures of the tricarbonyl very closely resembling the experimentally determined structures. For the more unsaturated dicarbonyl and monocarbonyl systems MePFe2(CO)nCp2 (n = 2 and 1), higher spin state triplet and quintet structures are energetically preferred over singlet structures. These more highly unsaturated structures can be derived from the lowest energy singlet MePFe2(CO)nCp2 (n = 4, 3) by the removal of carbonyl groups. The iron atoms giving up carbonyl groups in their 16- and 14-electron configurations bear the spin density of the unpaired electrons in the higher spin states. The lowest energy singlet structure of the monocarbonyl MePFe2(CO)Cp2, although a relatively high energy isomer, is unusual among the collection of MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) structures by having both the formal Fe=Fe double bond and the four-electron donor MeP unit with the planar phosphorus coordination required to allow each of its iron atoms to attain the favored 18-electron configuration.

Systematics of stable copper and silver clusters protected by small bite chelating bidentate sulfur and selenium ligands related to their polyhedral cavities: analogies to aliphatic compounds and three-dimensional spherical aromatic systems.

Silver and copper clusters capped by external chelating dithiolate ligands can be classified according to the cavities in their central coinage metal polyhedra. Silver clusters composed exclusively of fused tetrahedra are analogous to simple saturated organic compounds. The only interstitial atom that can be fit into an Ag4 tetrahedron is hydrogen. Silver polyhedra with larger trigonal prismatic or cubic cavities, including highly distorted cubic cavities, can accommodate halide and chalcogenide anions. The still larger 12-vertex icosahedral and cuboctahedral coinage metal cavities can accommodate oxoanions of the types SO32- and SO42- and their heavier congeners or alternatively interstitial coinage or platinum group metals leading to central M'@M12 units. Copper clusters with central cuboctahedra and silver clusters with central icosahedra possessing interstitial metal atoms approximate sphericality and provide examples of electron-rich metal superatoms with an average metal oxidation state of less than +1. Such copper cluster superatoms have two extra electrons corresponding to a filled 1S2 superatomic orbital. The silver cluster superatoms are electron richer with eight extra electrons corresponding to filled 1S2 + 1P6 superatomic orbitals. In these silver clusters seven or eight faces of the central Ag12 icosahedron are capped by additional silver atoms in order to provide these additional electrons.

Unraveling the Major Differences between the Trinuclear Cyclopentadienylmetal Carbonyl Chemistry of Cobalt and That of Nickel-A Theoretical Study.

The geometries and energetics of the trinuclear cyclopentadienylmetal carbonyls Cp3M3(CO)n (Cp = η5-C5H5); M = Co, Ni; n = 3, 2, 1, 0) have been investigated by density functional theory. The cobalt and nickel systems are found to be rather different owing to the different electronic configurations of the metal atoms. For cobalt, the small calculated energy separation of 5.0 kcal/mol between the two lowest-energy singlet Cp3Co3(µ3-CO)(µ-CO)2 and Cp3Co3(µ-CO)3 tricarbonyl structures accounts for the experimental results of both isomers as stable species that can be isolated and structurally characterized by X-ray crystallography. The corresponding Cp3Ni3(CO)3 species in the nickel system are predicted not to be viable owing to exothermic CO dissociation to give the experimentally observed very stable Cp3Ni3(µ-CO)2, which is found to be the lowest-energy isomer by a substantial margin of ∼25 kcal/mol. In all of the low-energy Cp3M3(CO)n (n = 2, 1) structures, including that of the experimentally known triplet spin state Cp3Co3(µ3-CO)2, all of the carbonyl groups are face-bridging or face-semi-bridging µ3-CO groups bonded to all three metal atoms of the M3 triangle. In the lowest-energy carbonyl-free Cp3M3 (M = Co, Ni) structures, agostic C-H-M interactions are found using hydrogens of the Cp rings. In addition, the lowest-energy Cp3Ni3 is the only structure among all of the low-energy Cp3M3(CO)n (M = Co, Ni; n = 3, 2, 1, 0) structures in which each Cp ring is a bridging rather than terminal ligand.

Polyhedral Ferraboranes with Iron Carbonyl Vertices: Carbonyl Migration Processes in the Iron Tetracarbonyl Derivatives.

The structures and energetics of the neutral Bn-1Hn-1Fe(CO)x (x = 4, 3) and the dianions [Bn-1Hn-1Fe(CO)3]2- (n = 6-14) have been investigated by density functional theory. The low-energy structures of the tricarbonyl dianions [Bn-1Hn-1Fe(CO)3]2- are all found to have closo deltahedral structures in accordance with their 2n+2 skeletal electrons. The low-energy structures of the neutral tricarbonyls Bn-1Hn-1Fe(CO)3 (n = 6-14) with only 2n skeletal electrons are based on capped (n-1)-vertex closo deltahedra (n = 6, 7, 8) or isocloso deltahedra with a degree 6 vertex for the iron atom. The closo 8- and 9-vertex deltahedra are also found in low-energy Bn-1Hn-1Fe(CO)3 structures relating to the nondegeneracy of their frontier molecular orbitals. Carbonyl migration occurs in most of the low-energy structures of the tetracarbonyls Bn-1Hn-1Fe(CO)4. Thus, migration of a carbonyl group from an iron atom to a boron atom gives closo Bn-2Hn-2(BCO)(µ-H)Fe(CO)3 structures with a BCO vertex and a hydrogen atom bridging a B-B deltahedral edge. In other low-energy Bn-1Hn-1Fe(CO)4 structures, a carbonyl group is inserted into the central n-vertex FeBn-1 deltahedron to give a Bn-1Hn-1(CO)Fe(CO)3 structure with a central (n+1)-vertex FeCBn-1 deltahedron that can be an isocloso deltahedron or a µ3-BH face-capped n-vertex FeCBn-2 closo deltahedron. Other low-energy Bn-1Hn-1Fe(CO)4 structures include Bn-1Hn-1Fe(CO)2(µ-CO)2 structures with two of the carbonyl groups bridging FeB2 faces (n = 6, 7, 10) or Fe-B edges (n = 12) or structures in which a closo Bn-1Hn-1 ligand (n = 6, 7, 10, 12) is bonded to an Fe(CO)4 unit with exclusively terminal carbonyl groups through B-H-Fe bridges.

C3N2: the missing part of highly stable porous graphitic carbon nitride semiconductors.

Two-dimensional (2D) porous graphitic carbon nitrides (PGCNs) with semiconducting features have attracted wide attention because of built-in pores with various active sites, large surface area, and high physicochemical stability. However, only a few PGCNs have been synthesized, covering a 1.23-3.18 eV band gap. We systematically investigate two new 2D PGCN monolayers, T-C3N2 and H-C3N2, including possible pathways for their experimental synthesis. Based on first-principles calculations, the mechanical, electronic, and optical properties of T-C3N2 and H-C3N2 have been systematically investigated. These two architectural frameworks exhibit contrasting mechanical characteristics owing to their structural differences. Both T-C3N2 and H-C3N2 monolayers are predicted to be intrinsic semiconductors. Exceptionally, the stacking bilayers of T-C3N2 can transform into a rare 2D nodal-line semimetal structure. The narrow bandgap (0.35 eV) of the T-C3N2 monolayer and its extraordinary transformation in the bilayer electronic structure fill the vacancy of PGCNs as electronic devices in the middle/long wave infrared region. C3N2 structures possess ultrahigh anisotropic carrier mobilities (×104 cm2 V-1 s-1) and exceptional absorption coefficients (×105 cm-1) in the near-infrared and visible light regions, suggesting its possible optoelectronic applications. The findings expand the scope of 2D PGCNs and offer guides for their experimental realization.

Chemical Bonding Topology of Metal-Centered Polygonal Wheels: Two-Dimensional Analogues of Metallaboranes Related to Benzene and Cyclopentadienide.

The anion [Au@Ru5(CO)15(µ-CO)4]- has a pentagonal wheel structure that can be derived from a hypothetical pentagonal ruthenium carbonyl cluster Ru5(CO)20 by insertion of a gold atom in the center, thereby splitting the original Ru5 pentagon in Ru5(CO)20 into five AuRu2 triangles. The six electrons used to form 3c-2e bonds in three of the five AuRu2 triangles suggest a relationship to the aromatic sextet of the likewise pentagonal cyclopentadienide anion. Furthermore, the pentagonal wheel framework of [Au@Ru5(CO)15(µ-CO)4]- can be derived from a pentagonal bipyramid, such as that found in the deltahedral borane anion B7H72-, by bringing the two C5 axial vertices together at the center of the equatorial pentagon. Similarly, the hexagonal wheel complexes Ni@P6R6 and Pd@Pd6(µ-N═CtBu2)6 with six triangular faces can be derived from a hexagonal bipyramid, such as that found in the dirhenaborane (η5-Me5C5)2Re2B6H4Cl2, by bringing the two C6 axial vertices together at the center of the equatorial hexagon. A reasonable chemical bonding model for the hexagonal wheel complexes has three-fold symmetry with 3c-2e bonds in three of these six triangular faces analogous to the C═C double bonds in a Kekulé structure of benzene.

Aromaticity in P8 allotropes and (CH)8 analogues: significance of their 40 valence electrons?

The currently unknown phosphorus allotrope P8 is of interest since its 40 total valence electrons is a "magic number" corresponding to a filled 1S21P61D101S21F142P6 shell such as found in the relatively stable main group element clusters Al13- and Ge94-. However, P8 still remains as an elusive structure not realized experimentally. The lowest energy P8 structure by a margin of ∼9 kcal mol-1 is shown by density functional theory to be a cuneane analogue with no PP double bonds and two each of P5, P4, and P3 rings. Higher energy P8 structures are polycyclic systems having at most a single PP double bond. These P8 systems are not "carbon copies" of the corresponding (CH)8 hydrocarbons with exactly one hydrogen atom bonded to each carbon atom. Thus the lowest energy (CH)8 structure is cyclooctatetraene with four CC bonds followed by benzocyclobutene with three CC bonds. The cuneane (CH)8 structure is a relatively high energy isomer lying ∼36 kcal mol-1 above cyclooctatetraene. The cubane P8 and (CH)8 structures are even higher energy structures, lying ∼37 and ∼74 kcal mol-1 in energy above the corresponding global minima. Our results demonstrate differences in medium sized aggregates of elemental phosphorus and isolobal hydrocarbon species.

Trivalent Polyhedra as Duals of Borane Deltahedra: From Molecular Endohedral Germanium Clusters to the Smallest Fullerenes.

The duals of the most spherical closo borane deltahedra having from 6 to 16 vertices form a series of homologous spherical trivalent polyhedra with even numbers of vertices from 8 to 28. This series of homologous polyhedra is found in endohedral clusters of the group 14 atoms such as the endohedral germanium cluster anions [M@Ge10]3- (M = Co, Fe) and [Ru@Ge12]3- The next members of this series have been predicted to be the lowest energy structures of the endohedral silicon clusters Cr@Si14 and M@Si16 (M = Zr, Hf). The largest members of this series correspond to the smallest fullerene polyhedra found in the endohedral fullerenes M@C28 (M = Zr, Hf, Th, U). The duals of the oblate (flattened) ellipsoidal deltahedra found in the dirhenaboranes Cp*2Re2Bn-2Hn-2 (Cp* = η5-Me5C5; 8 ≤ n ≤ 12) are prolate (elongated) trivalent polyhedra as exemplified experimentally by the germanium cluster [Co2@Ge16]4- containing an endohedral Co2 unit.

Triplet Spin-State Capped Deltahedral Structures Rather than Singlet Spin-State Oblatocloso Structures as Energetically Favored Dimanganaborane Structures.

Density functional studies show that the singlet spin-state flattened oblatocloso deltahedral structures found experimentally in the dimetallaboranes Cp*2Re2Bn-2Hn-2 (Cp* = Me5C5; n = 8-12) of the third row group 7 element rhenium are not favored for analogous dimetallaboranes Cp2Mn2Bn-2Hn-2 (n = 8-14) of its first row congener manganese. Instead, the energetically preferred structures for the dimanganaboranes are higher spin-state triplet and quintet spin-state structures. This appears to be related to the lower ligand field splittings in complexes of the first row transition-metal manganese relative to analogous complexes of the third row transition-metal rhenium. The lowest-energy Cp2Mn2Bn-2Hn-2 (n = 8-13) structures typically have a central MnBn-2 closo deltahedron with one face capped by the second CpMn unit. However, for the 14-vertex Cp2Mn2B12H12 system the lowest-energy structures consist of B12 icosahedra with faces capped by both CpMn units. The thermochemistry of cluster buildup reactions of the type Cp2Mn2Bn-2Hn-2 + BH → Cp2Mn2Bn-1Hn-1 suggests that the 11- and 13-vertex structures are likely to be favored products in reactions of cyclopentadienylmanganese derivatives with borane sources. The paramagnetism of the predicted triplet and quintet spin states for the lowest-energy dimanganaboranes Cp2Mn2Bn-2Hn-2 (n = 8-14) suggests possible applications in novel magnetic materials.

Carbon-hydrogen bond activation in bridging cyclobutadiene ligands in unsaturated binuclear vanadium carbonyl derivatives.

The structures and energetics of the binuclear cyclobutadiene vanadium carbonyls (C4H4)2V2(CO)n (n = 8, 7, 6, 5, 4, 3, 2) have been investigated by density functional theory (DFT). The lowest energy (C4H4)2V2(CO)8 structure consists of two C4H4V(CO)4 units linked by a V-V single bond of length 3.4 Å. The two lowest energy (C4H4)2V2(CO)7 structures also have formal V-V single bonds. The "extra" two electrons to give each vanadium atom in these heptacarbonyls the favored 18-electron configuration can come from either an agostic C-H-V interaction activating a hydrogen atom from one of the cyclobutadiene rings or from a four-electron donor bridging η2-µ-CO group with a short V-O distance. The lowest energy (C4H4)2V2(CO)6 structure has a formal V≡V triple bond of length 2.52 Å similar to the V≡V triple bond of length 2.46 Å found in the experimentally known cyclopentadienyl derivative (η5-C5H5)2V2(CO)5. The lowest energy structures for the more highly unsaturated (C4H4)2V2(CO)n (n = 5, 4, 3, 2) have at least two four-electron donor bridging η2-µ-CO groups and a vanadium-vanadium bond order sufficient to give each vanadium atom at least a 16-electron configuration. The structures and energetics of the binuclear cyclobutadiene vanadium carbonyls (C4H4)2V2(CO)n (n = 8, 7, 6, 5, 4, 3, 2) have been investigated by density functional theory. The two lowest energy (C4H4)2V2(CO)7 structures include one with an agostic C-H-V interaction activating a hydrogen atom from one of the cyclobutadiene rings and another with a four-electron donor bridging η2-µ-CO group with a short V-O bonding distance.

Substituent, Solvent, and Dispersion Effects on the Zwitterionic Character and Dimerization Thermochemistry of the Group 6 Fulvene Metal Tricarbonyl Complexes.

Dimerizations of fulvene metal tricarbonyl complexes of the type (C5H4CRR')M(CO)3 (R, R' = MeO, Me, H; M = Cr, Mo, W) to form a metal-metal bond and a new carbon-carbon bond, thereby giving binuclear cyclopentadienyl metal carbonyl derivatives, are predicted to be thermochemically favored but to have significant activation energies ranging from ΔE = 19 to 42 kcal/mol. However, the introduction of dimethylamino but not methoxy substituents onto the exocyclic carbon atom changes the situation drastically so that the monomers [C5H4CH(NMe2)]M(CO)3 and [C5H4C(NMe2)2]M(CO)3 become strongly thermochemically favored, lying ΔE = 43 kcal/mol (M = W) to 63 kcal/mol (M = Cr) below their corresponding dimers. In such dimethylamino-substituted (fulvene)M(CO)3 derivatives, the M-C distance to the exocyclic fulvene carbon is lengthened beyond the bonding distance to give a zwitterionic structure with a pentahapto fulvene ligand. Such M-C distances in (fulvene)M(CO)3 complexes, which have preferred zwitterionic structures, increase with increasing solvent polarity (i.e., dielectric constant) until a saturation point is reached.

Tetrahedral Cyclopentadienylmetal Carbonyl Clusters of Manganese and Chromium: A Theoretical Study.

Tetranuclear Cp4M4(CO)4 clusters have been synthesized for iron and vanadium but not for the intermediate first-row transition metals manganese and chromium. All of the low-energy structures of these "missing" Cp4M4(CO)4 (M = Mn, Cr) species are shown by density functional theory to consist of a central M4 tetrahedron with each of the four faces capped by a µ3-CO group. The individual low-energy structures differ in their spin states and in their formal metal-metal bond orders along the six edges of their central M4 tetrahedra. The two low-energy Cp4Mn4(µ3-CO)4 structures are a triplet structure with all Mn-Mn single bonds and a singlet structure with one Mn≡Mn triple bond and five Mn-Mn single bonds along the six tetrahedral edges. Related low-energy Cp4Cr4(µ3-CO)4 structures include a quintet structure with all Cr-Cr single bonds and a singlet structure with two Cr≡Cr triple bonds and four Cr-Cr single bonds. However, the potential energy surface of the Cp4Cr4(CO)4 system is complicated by three other structures of comparable energies including two triplet structures and one quintet structure with various combinations of single, double, and triple chromium-chromium bonds in the central Cr4 tetrahedron.

Lantern-Type Divanadium Complexes with Bridging Ligands: Short Metal-Metal Bonds with High Multiple Bond Orders.

Vanadium forms binuclear complexes with a variety of ligands often containing V≡V triple bonds. Many tetragonal divanadium paddlewheel complexes with bridging bidentate ligands have been experimentally characterized. This research exhaustively treats model tetragonal, trigonal, and digonal paddlewheel-type divanadium complexes V2 Lx (L=formamidinate, guanidinate, and carboxylate; x=2, 3, 4), each in the three lowest-energy spin states. The V-V formal bond orders are obtained from metal-metal MO diagrams for representative structures. A number of short V-V multiple bonds of order 3, 3.5, and 4 are found in these model complexes. The short V≡V triple bonds and singlet ground state predicted here for the model tetragonal complexes correspond well with the limited experimental results for the series of known tetragonal paddlewheels. Digonal divanadium lanterns with very short V-V quadruple bonds are predicted as interesting synthetic targets. The V-V bond distances are categorized into distinct ranges according to the formal bond order values from 0.5 to 4. These bond length ranges are compared with the ranges compiled for other divanadium complexes including carbonyl complexes.

Mechanism for the Reaction of White Phosphorus with Cp2Cr2(CO)6 Leading Ultimately to the Triple-Decker Sandwich Cp2Cr2(µ-η5,η5-P5): A Theoretical Study.

The experimentally known reaction of Cp2Cr2(CO)6 with white phosphorus (P4) to give CpCr(CO)2(η3-P3), Cp2Cr2(CO)4(µ-η,2η2-P2), and the triple-decker sandwich Cp2Cr2(µ-η,5η5-P5) is of interest since the P4 reactant having a tetrahedral cluster of four phosphorus atoms is converted to products having P2, P3, and P5 ligands. The mechanism of this obviously complicated reaction can be dissected into three stages using a coupled cluster theoretical method that has been benchmarked with the P2, Mn(CO)5, and CpCr(CO)3 dimerization processes. The first stage of the Cp2Cr2(CO)6/P4 reaction mechanism generates the unsaturated singlet intermediate Cp2Cr2(CO)5 that combines with the P4 reactant. Decarbonylation of the resulting Cp2Cr2(CO)5(P4) complex provides a singlet tetracarbonyl readily fragmenting into the stable triphosphacyclopropenyl complex CpCr(CO)2(η3-P3) and the chromium phosphide CpCr(CO)2(P). The isomeric triplet tetracarbonyl Cp2Cr2(CO)4(P4), readily fragments into CpCr(CO)2(η2-P2), which can generate the stable diphosphaacetylene complex Cp2Cr2(CO)4(η,2η2-P2) as well as the pentamer [CpCr(CO)2]5(P10). Combination of the coordinately unsaturated CpCr(CO)(η3-P3) with CpCr(CO)2(η2-P2) can lead to a ring expansion. This generates the P5 pentagonal ligand in a Cp2Cr2(CO)3(P5) precursor to the experimentally observed carbonyl-free triple-decker sandwich Cp2Cr2(µ-η,5η5-P5) after three successive decarbonylations.

Binuclear Cobalt Paddlewheel-Type Complexes: Relating Metal-Metal Bond Lengths to Formal Bond Orders.

Paddlewheel-type complexes are prominent among experimentally known binuclear cobalt complexes and incorporate substituted formamidinate, guanidinate, and carboxylate ligands in digonal, trigonal, and tetragonal arrays around the bimetallic core. Such complexes are modeled here by density functional theory using unsubstituted ligands, extending the whole set to incorporate a variety of metal oxidation states and spin multiplicities. The DFT results for ground state cobalt-cobalt bond lengths and ground state spin multiplicity of the model complexes are often quite close to the experimental results for the corresponding substituted complexes. The three series of complexes often exhibit parallel trends with regard to effects of change in the metal oxidation state and spin multiplicity. The formamidinate and guanidinate series show marked resemblances. The lowered symmetry in many model trigonal complexes implies that such deviations in the experimentally known congeners arise from the inherent electronic structure. For ground state species, the DFT results provide Co-Co bond orders (BOs) from MO occupancy considerations. Further, using a revised electron bookkeeping method, Co-Co formal bond order (fBO) values from 0.0 to 2.0 are assigned to all of the 85 complexes studied. The computed Co-Co bond lengths fall into distinct ranges according to the formal bond order values (from 0.5 to 2).

Th@C86, Th@C82, Th@C80, and Th@C76: role of thorium encapsulation in determining spherical aromatic and bonding properties on medium-sized endohedral metallofullerenes.

Thorium encapsulated metallofullerenes (Th-EMFs) with external C76, C80, C82, and C86 cages have been synthesized, with the 13C-NMR spectrum recorded for Th@C82. Here, we explore computationally the chemical bonding, NMR and spherical aromaticity of Th@C82 and related thorium-encapsulated metallofullerenes. Our results show that these Th-EMFs are new examples of spherical aromatic structures, representing interesting low-symmetry exceptions to the Hirsch 2(N + 1)2 rule of spherical aromaticity. Their electronic structures are based on π-electron counts of 80, 84, 86, and 90, respectively, with a shell structure ranging from S2P6D10F14G18H22I8 to S2P6D10F14G18H22I18, where the partially filled I-shell remains as a frontier orbital. Their behavior is comparable to that of the spherical aromatic alkali-C606- phases, which in addition to the favorable endohedral Th-fullerene bonding account for their particular abundance exhibiting the ability to sustain a long-range shielding cone as a result of the favorable metal-cage bonding. This rationalization of such species as neutral spherical aromatic EMFs suggests the possibility of an extensive series of aromatic fullerenes with nuclearity larger than C60 buckminsterfullerene as stable building blocks towards nanostructured metal-organic materials.

Comparative Study of the Thermal Stabilities of the Experimentally Known High-Valent Fe(IV) Compounds Fe(1-norbornyl)4 and Fe(cyclohexyl)4.

The high stability of the experimentally known homoleptic 1-norbornyl derivative (nor)4Fe of iron in the unusual +4 oxidation state is a consequence of the high reaction barriers of the singlet or triplet potential surfaces constrained by the global dispersion attraction and the great steric demands of the norbornyl groups. The much more limited stability of the corresponding cyclohexyl derivative (cx)4Fe may result from the conical intersection between the singlet potential surface and the quintet spin potential surface arising from the weaker dispersion attraction and the reduced steric effect of the cyclohexyl groups relative to the 1-norbornyl groups. In contrast, the high stability of the likewise experimentally known (cx)4M (M = Ru or Os) structures results from the larger ligand field splitting (Δ) of the d-orbital energies for the second and third-row transition metals ruthenium and osmium relative to that of the first-row transition metal iron. The cyclohexyl derivative (cx)4Fe is predicted to be reactive toward carbon monoxide to insert CO into up to two Fe-C bonds. However, the dispersion effect as well as the much larger size of the 1-norbornyl substituents prevents similar reactivity of (nor)4Fe with carbon monoxide.

Systematics of Atomic Orbital Hybridization of Coordination Polyhedra: Role of f Orbitals.

The combination of atomic orbitals to form hybrid orbitals of special symmetries can be related to the individual orbital polynomials. Using this approach, 8-orbital cubic hybridization can be shown to be sp3d3f requiring an f orbital, and 12-orbital hexagonal prismatic hybridization can be shown to be sp3d5f2g requiring a g orbital. The twists to convert a cube to a square antiprism and a hexagonal prism to a hexagonal antiprism eliminate the need for the highest nodality orbitals in the resulting hybrids. A trigonal twist of an Oh octahedron into a D3h trigonal prism can involve a gradual change of the pair of d orbitals in the corresponding sp3d2 hybrids. A similar trigonal twist of an Oh cuboctahedron into a D3h anticuboctahedron can likewise involve a gradual change in the three f orbitals in the corresponding sp3d5f3 hybrids.

On the formation of spherical aromatic endohedral buckminsterfullerene. Evaluation of M@C60 (M = Cr, Mo, W) from relativistic DFT calculations.

Endohedral metallofullerenes are key species for expanding the range of viable fullerenes, their versatility, and applications. Here we report our computational evaluation on the formation of spherical aromatic counterparts of the C60 fullerene from relativistic DFT calculations, based on the inclusion of Cr, Mo and W endohedral atoms. The resulting M@C60 endohedral fullerenes are 66-π electron neutral species exhibiting bonding properties and electronic structure mimicking the aromaticity and diamagnetic insulator behavior of alkali-C606- phases. The resulting structures are interesting candidates for further experimental realization as novel neutral building blocks for more flexible nanostructured organic materials, highlighting truly spherical aromatic neutral species retaining the truncated icosahedral structure of the seminal Buckminster fullerene.

Perfluoroolefin complexes versus perfluorometallacycles and perfluorocarbene complexes in cyclopentadienylcobalt chemistry.

Fluorocarbons have been shown experimentally by Baker and coworkers to combine with the cyclopentadienylcobalt (CpCo) moiety to form fluoroolefin and fluorocarbene complexes as well as fluorinated cobaltacyclic rings. In this connection density functional theory (DFT) studies on the cyclopentadienylcobalt fluorocarbon complexes CpCo(L)(CnF2n) (L = CO, PMe3; n = 3 and 4) indicate structures with perfluoroolefin ligands to be the lowest energy structures followed by perfluorometallacycle structures and finally by structures with perfluorocarbene ligands. Thus, for the CpCo(L)(C3F6) (L = CO, PMe3) complexes, the perfluoropropene structure has the lowest energy, followed by the perfluorocobaltacyclobutane structure and the perfluoroisopropylidene structure less stable by 8 to 11 kcal mol-1, and the highest energy perfluoropropylidene structure less stable by more than 12 kcal mol-1. For the two metal carbene structures Cp(L)Co[double bond, length as m-dash]C(CF3)2 and Cp(L)Co[double bond, length as m-dash]CF(C2F5), the former is more stable than the latter, even though the latter has Fischer carbene character. For the CpCo(L)(C4F8) (L = CO, PMe3) complexes, the perfluoroolefin complex structures have the lowest energies, followed by the perfluorometallacycle structures at 10 to 20 kcal mol-1, and the structures with perfluorocarbene ligands at yet higher energies more than 20 kcal mol-1 above the lowest energy structure. This is consistent with the experimentally observed isomerization of the perfluorinated cobaltacyclobutane complexes CpCo(PPh2Me)(-CFR-CF2-CF2-) (R = F, CF3) to the perfluoroolefin complexes CpCo(PPh2Me)(RCF[double bond, length as m-dash]CF2) in the presence of catalytic quantities of HN(SO2CF3)2. Further refinement of the relative energies by the state-of-the-art DLPNO-CCSD(T) method gives results essentially consistent with the DFT results summarized above.