Formal Double Bond Research Articles

An unexpected tautomer: synthesis and crystal structure of N-[6-amino-4-(methylsulfanyl)-1,2-dihydro-1,3,5-triazin-2-ylidene]benzenesulfonamide

The title compound, C10H11N5O2S2, consists of an unexpected tautomer with a protonated nitrogen atom in the triazine ring and a formal exocyclic double bond C=N to the sulfonamide moiety. The ring angles at the unsubstituted nitrogen atoms are narrow, at 115.57 (12) and 115.19 (12)°, respectively, whereas the angle at the carbon atom between these N atoms is very wide, 127.97 (13)°. The interplanar angle between the two rings is 79.56 (5)°. The molecules are linked by three classical hydrogen bonds, forming a ribbon structure. There are also unusual linkages involving three short contacts (< 3 Å) from a sulfonamide oxygen atom to the C—NH—C part of a triazine ring.

Binuclear methylphosphinidine complexes of cyclopentadienylruthenium carbonyls: effects of the higher ligand field strength of ruthenium derivatives relative to iron derivatives.

The structures and energetics of the binuclear methylphosphinidene complexes of cyclopentadienylruthenium carbonyls of the type [MePRu2(CO)nCp2] (n = 4, 3, 2, 1) have been investigated for comparison with their previously studied iron analogues. For the tetracarbonyls and tricarbonyls [MePM2(CO)nCp2] (n = 4, 3) substituting ruthenium for iron has relatively little effect on the energetically preferred structures. Thus such structures have two-electron donor bridging MeP groups with no metal-metal bond for the tetracarbonyls and a metal-metal single bond for the tricarbonyls. This leads to favored 18-electron configurations for both ruthenium atoms in all cases. However, the higher ligand field strengths of ruthenium complexes relative to analogous iron complexes have major effects on the energetically preferred structures for the dicarbonyls and monocarbonyls [MePM2(CO)nCp2] (M = Fe, Ru; n = 2, 1). Thus the 11 lowest energy structures for the dicarbonyl [MePFe2(CO)2Cp2] are triplet or quintet spin state structures whereas the 6 lowest energy structures for the ruthenium analogue [MePRu2(CO)2Cp2] are all singlet structures. These low-energy singlet [MePRu2(CO)2Cp2] structures include species in which both ruthenium atoms attain the favored 18-electron configurations in different ways: either by a Ru-Ru single bond and an agostic C-H-Ru interaction from the methyl group, a Ru-Ru single bond and a four-electron donor bridging MeP ligand with PRu double bonds, or a formal RuRu double bond with a two-electron donor bridging MeP ligand. The 8 lowest energy structures for the diiron monocarbonyl [MePFe2(CO)Cp2] are all triplet or quintet spin structures whereas the lowest energy structure for the diruthenium monocarbonyl [MePRu2(CO)Cp2] by more than 20 kcal mol-1 is a singlet structure with a formal RuRu double bond and bridging CO and four-electron donor MeP groups. Thermochemical information predicts such monocarbonyl derivatives to be the dominant binuclear decarbonylation products of the tricarbonyls [RPRu2(CO)3Cp2] by thermal or photochemical methods.

Changing aromatic properties through stacking: the face-to-face dimer of Ni(II) bis(pentafluorophenyl)norcorrole.

Nuclear magnetic resonance (NMR) shielding constants have been calculated for Ni(II) bis(pentafluorophenyl)norcorrole and its face-to-face stacked dimer at the Hartree-Fock (HF), second-order Møller-Plesset perturbation theory (MP2), complete-active-space self-consistent-field (CASSCF) levels as well as at density functional theory (DFT) levels using several functionals. The calculated 1H NMR shielding constants agree rather well with the experimental ones. The shielding constants of N and Ni calculated at DFT, HF, and MP2 levels differ from those obtained in the CASSCF calculations due to near-degeneracy effects at the Ni atom. The calculated magnetically induced current densities show that the monomer is antiaromatic, sustaining a strong global paratropic ring current, and the dimer is aromatic, sustaining a strong diatropic ring current. Qualitatively the same current density is obtained at the employed levels of theory. The most accurate ring-current strengths are probably obtained at the MP2 level. The aromatic dimer has a short intermolecular distance of less than 3 Å. The intermolecular interaction changes the nature of the frontier orbitals leading to a formal double bond between the norcorrole macrocycles.

Polaritonic Chemistry: Hindering and Easing Ground State Polyenic Isomerization via Breakdown of σ-π Separation.

The ground state conformational isomerization in polyenes is a symmetry allowed process. Its low energy barrier is governed by electron density transfer from the formal single bond that is rotated to the nearby formal double bonds. Along the reaction pathway, the transition state is therefore destabilized. The rules of polaritonic chemistry, i.e., chemistry in a nanocavity with reflecting windows, are barely beginning to be laid out. The standing electric field of the nanocavity couples strongly with the molecular wave function and modifies the potential energy curve in unexpected ways. A quantum electrodynamics approach, applied to the torsional degree of freedom of the central bond of butadiene, shows that formation of the polariton mixes the σ-π frameworks thereby stabilizing/destabilizing the planar, reactant-like conformations. The values of the fundamental mode of the cavity field used in the absence of the cavity do not trigger this mechanism.

Conformations of 4‐tert‐Butyloxy‐, 4‐(Trimethylsilyl)oxy‐ and 4‐(Trimethylstannyl)oxy‐6‐methyl‐2H‐pyran‐2‐ones in the Crystalline State and in Solution

AbstractThe conformational studies in solution and in the solid state of substituted 6‐methyl‐4‐hydroxy‐2‐pyrones indicated that the substituents on the 4‐hydroxy group clearly favored the s‐cis arrangement towards the formal double bond. Steady state kinetic NOE measurements of the tert‐butyl, trimethylsilyl and the trimethylstannyl derivatives showed a marked preference for this conformation in solution. The structures determined by X‐ray diffraction demonstrated the exclusive presence of the same conformation in the solid state. In these three derivatives, the pertinent dihedral angle deviates only slightly from the value for coplanarity with the formal double bond. The predictions of the τ‐bond model are in accordance with the observations. The X‐ray structure of the trimethylstannyl derivative reveals pentacoordinated tin with short contacts to the oxygens at the 2 and 4 position of neighboring molecules. The tin center plays both the role of a protecting group for the 4‐hydroxy moiety and of a Lewis acid towards the 2‐carbonyl oxygen. The X‐ray structure of the stannyl derivative shows unique features which can be compared to the extended transition state of aldol reactions.

A new synthetic route for the preparation of 2,2′,5′‐trimethyl‐7‐oxo‐4,7‐dihydro‐[6,7′‐bipyrazolo[1,5‐a]pyrimidine]‐3,3′‐dicarbonitrile, structural elucidation, Hirshfeld surface analysis, energy framework, density functional theory and molecular docking investigations

AbstractA new bipyrazolo [1,5‐a]pyrimidine derivative, 2,2′,5′‐trimethyl‐7‐oxo‐4,7‐dihydro‐[6,7′‐bipyrazolo[1,5‐a]pyrimidine]‐3,3′‐dicarbonitrile dihydrate acetic acid solvate (3) has been synthesized via two different methods and characterized by single‐crystal X‐ray diffraction (XRD) and spectroscopic techniques. A plausible reaction mechanism in the synthesis of 3 through the two synthetic routes used has been proposed. The XRD analysis reveals that the two bicyclic moieties in 3 are close to perpendicular to one another. Some formal double bonds appear localized while others appear to involve some delocalization. In the crystal structure, a layer structure is formed by OH⋯O, OH⋯N and NH⋯O hydrogen bonds, along with weak CH⋯O and π‐stacking interactions. The energy framework indicates that the packing of 3 is mainly determined by dispersion interactions between molecules. The frontier molecular orbital analysis shows that 3 may act more as a nucleophile than an electrophile. The analysis of the Hirshfeld surface maps and 2D fingerprint plots of 3 reveal that intermolecular hydrogen bonding and H⋯H, N⋯H and O⋯H contacts are the major ones between the units of 3. The antibacterial activity of 3 is explored by its molecular docking into the binding site of tyrosyl‐tRNA synthetase, where it formed a stable complex with the amino acids of tyrosyl‐tRNA synthetase mainly through hydrogen bonding.

Prolate octahedral Ti2C4 clusters in binuclear cyclobutadiene titanium carbonyls

The structures and energetics of the binuclear cyclobutadiene titanium carbonyl derivatives (C4H4)2Ti2(CO)n (n = 8, 7, 6, 5, 4, 3) have been investigated by density functional theory. The lowest energy (C4H4)2Ti2(CO)8 structure is found to have a central prolate Ti2C4 octahedron with Ti(CO)5 and (η4-C4H4)Ti(CO)3 moieties in polar positions. Similar central Ti2C4 prolate octahedra are found in an only slightly higher energy triplet (C4H4)2Ti2(CO)8 structure as well as in the lowest energy triplet (C4H4)2Ti2(CO)7 structure. An only slightly higher energy (C4H4)2Ti2(CO)8 isomer has a totally different type of structure which can be dissected into the complex (η4-C4H4)Ti(CO)5 with an 18-electron titanium configuration functioning as a Lewis base to a coordinatively unsaturated (η4-C4H4)Ti(CO)3 fragment through an O → Ti bond from one of its carbonyl groups. The low-energy structures of the more highly unsaturated (C4H4)2Ti2(CO)n (n = 6, 5, 4, 3) species have various combinations of four-electron donor bridging η2-µ-CO groups and six-electron donor bridging η4,η1-C4H4 cyclobutadiene ligands with agostic CHTi interactions bridging a central formal TiTi double bond.

CO2 Fixation and Catalytic Reduction by a Neutral Aluminum Double Bond.

CO2 fixation and reduction to value-added products is of utmost importance in the battle against rising CO2 levels in the Earth's atmosphere. An organoaluminum complex containing a formal aluminum double bond (dialumene), and thus an alkene equivalent, was used for the fixation and reduction of CO2 . The CO2 fixation complex undergoes further reactivity in either the absence or presence of additional CO2 , resulting in the first dialuminum carbonyl and carbonate complexes, respectively. Dialumene (1) can also be used in the catalytic reduction of CO2 , providing selective formation of a formic acid equivalent via the dialuminum carbonate complex rather than a conventional aluminum-hydride-based cycle. Not only are the CO2 reduction products of interest for C1 added value products, but the organoaluminum complexes isolated represent a significant step forward in the isolation of reactive intermediates proposed in many industrially relevant catalytic processes.

CO2Fixation and Catalytic Reduction by a Neutral Aluminum Double Bond

AbstractCO2fixation and reduction to value‐added products is of utmost importance in the battle against rising CO2levels in the Earth's atmosphere. An organoaluminum complex containing a formal aluminum double bond (dialumene), and thus an alkene equivalent, was used for the fixation and reduction of CO2. The CO2fixation complex undergoes further reactivity in either the absence or presence of additional CO2, resulting in the first dialuminum carbonyl and carbonate complexes, respectively. Dialumene (1) can also be used in the catalytic reduction of CO2, providing selective formation of a formic acid equivalent via the dialuminum carbonate complex rather than a conventional aluminum–hydride‐based cycle. Not only are the CO2reduction products of interest for C1added value products, but the organoaluminum complexes isolated represent a significant step forward in the isolation of reactive intermediates proposed in many industrially relevant catalytic processes.

A Tetrasilicon Analogue of Bicyclo[1.1.0]but‐1(3)‐ene Containing a Si=Si Double Bond with an Inverted Geometry

AbstractBicyclo[1.1.0]tetrasil‐1(3)‐ene 1, a tetrasilicon analogue of bicyclo[1.1.0]but‐1(3)‐ene that contains a formal double bond between bridgehead silicon atoms in an inverted geometry, was synthesized and isolated in the form of thermally stable orange crystals. The distance between the bridgehead Si atoms in 1 is much longer than those in typical Si=Si bonds, but still shorter than that of a previously reported pentasila[1.1.1]propellane. DFT calculations suggest that the bridgehead bond in 1 comprises a σ bond with an inverted geometry and a π bond. This notion is supported by the UV/Vis spectrum of 1, which exhibits several absorption bands in the UV/Vis region. While 1 is stable toward typical trapping agents for Si=Si double bonds, 1 reacts with carbon tetrachloride to furnish a hexachlorotetrasilane.

Ring Expansion of Alkylidenecarbenes Derived from Lactams, Lactones, and Thiolactones into Strained Heterocyclic Alkynes: A Theoretical Study.

Strained cycloalkynes are of considerable interest to theoreticians and experimentalists, and possess much synthetic value as well. Herein, a series of cyclic alkylidenecarbenes—formally obtained by replacing the carbonyl oxygen of four-, five-, and six-membered lactams, lactones, and thiolactones with a divalent carbon—were modeled at the CCSD(T)/cc-pVTZ//B3LYP/6-311+G** and CCSD(T)/cc-pVTZ//CCSD/6-311+G** levels of theory. The singlet carbenes were found to be more stable than the triplets. The strained heterocyclic alkynes formed by ring expansion of these singlet carbenes were also modeled. Interestingly, the C≡C bonds in the five-membered heterocycles, obtained from the rearrangement of β-lactam- and β-lactone-derived alkylidenecarbenes, displayed lengths intermediate between formal double and triple bonds. Furthermore, 2-(1-azacyclobutylidene)carbene was found to be nearly isoenergetic with its ring-expanded isomer, and 1-oxacyclopent-2-yne was notably higher in energy than its precursor carbene. In all other cases, the cycloalkynes were lower in energy than the corresponding carbenes. The transition states for ring-expansion were always lower for the 1,2-carbon shifts than for 1,2-nitrogen or oxygen shifts, but higher than for the 1,2-sulfur shifts. These predictions should be verifiable using carbenes bearing appropriate isotopic labels. Computed vibrational spectra for the carbenes, and their ring-expanded isomers, are presented and could be of value to matrix isolation experiments.

Alumazene Reactivity with Quinolinols – Addition and Substitution Reactions on the Al3N3 Ring

Molecular structures of two compounds obtained in reactions of alumazene [DippNAlMe]3 (1, Dipp = 2,6-i-Pr2C6H3) with substituted quinolinols have been elucidated by the single-crystal X-ray diffraction analysis. Quinolin-8-ol (Hq) provides a dinuclear complex [(DippNH)2Al2Me2(q)2] (2) with a central Al2O2 ring and five-coordinate Al atoms. The compound 2×THF crystallizes in the orthorhombic Pbca space group. The molecular structure of a mononuclear complex [(DippNH)Al(Meq)2] (3) obtained in the reaction of 1 with 2-methylquinolin-8-ol (HMeq) possesses a five-coordinate Al center. The structure was solved in a triclinic cell P-1. The dinuclear complex 2 can be considered as a model product of alcohol addition on the formal Al-N double bond, while the mononuclear complex 3 represents subsequent reaction of remaining Al-Me moiety with proton. Both complexes 2 and 3 are highly luminescent showing emission around 500 nm in solid state.

Binuclear vanadium dimethylphosphino carbonyls: vanadium-vanadium multiple bonds and four-electron donor carbonyl groups as structural features in unsaturated systems

Abstract The structures and energetics of the binuclear dimethylphosphido vanadium carbonyls (Me2P)2V2(CO)n (n = 10, 9, 8, 7, 6) have been examined using density functional theory. The experimentally known (Me2P)2V2(CO)8 with two bridging Me2P groups and all terminal CO groups is found to be the lowest energy structure by a substantial margin. The predicted V V distance in this structure of 2.696 A is close to the experimental distance of 2.733 A as determined by X-ray crystallography. The Wiberg bond index (WBI) of 0.94 supports the formulation of this V V interaction as a formal double bond, consistent with the 18-electron rule. Decarbonylation of (Me2P)2V2(CO)8 is predicted to give a singly CO-bridged (Me2P)2V2(CO)6(µ-CO) structure with a shorter V V distance of 2.374 A corresponding to a WBI of 1.30 and suggesting a formal triple bond. Further decarbonylation to (Me2P)2V2(CO)6 is predicted to lead to structures with four-electron donor bridging η2-µ-CO groups rather than structures with formal V V quadruple bonds. For the carbonyl-richer system (Me2P)2V2(CO)9 the lowest energy structure retains two bridging Me2P ligands and all terminal CO groups. Its V V distance of 3.138 A coupled with a WBI value of 0.23 suggests the formal single bond required by the 18-electron rule. The lowest energy (Me2P)2V2(CO)10 structure has a similar V V single bond. However, only one of its Me2P ligands bridges the central V V bond; the other Me2P group is a terminal ligand bonded to a single vanadium atom. Structures with intact Me2P-PMe2 ligands having direct P P bonds are very high energy structures relative to their isomers with two separate Me2P units lacking a direct P P bond.

Ligands’ σ-donation and π-backdonation effects on metal-metal bonding in trinuclear [M3(Tr)2(L)3]2+ (M = Fe, Ni, Pd, Pt, Tr = tropylium and L = CO, HCN and C2H4) sandwich compounds: Theoretical investigation

Abstract DFT calculations with full geometry optimization using GGA BP86 and meta-GGA M06L functionals have been performed on the [M3(Tr)2L3]2+ (M = Fe, Ni, Pd, Pt, Tr+ = C7H7+ and L = CO, HCN and η2-C2H4) sandwich complexes. The M3 triangular core is encapsulated between two tropylium cations which tends to establish M–L bonding with regards to the metal nature and the spin state. It turned out that each tropylium cation is connected to the M3 triangular core through an η2, η2, η2 coordination mode engaging six carbon atoms amongst seven. For the Fe singlet structures, the calculated HOMO–LUMO gaps by both BP86 and M06L are small, thus favouring the triplet one. The [Fe3(Tr)L3]2+ singlet structures are predicted to have two formal Fe Fe single bonds and one formal Fe Fe triple one within the Fe3 triangle based on the bond distances and the Wiberg bond indices (WBIs), while their analogues of triplet state exhibit two formal Fe Fe single bonds and one formal Fe Fe double bond. For the nickel, palladium and platinum d10 metal structures, they display large HOMO–LUMO gaps and adopt three formal metal–metal single bonds within the M3 core, in accordance with the small WBIs. The different binding capabilities of the isoelectronic CO, HCN and C2H4 auxiliary ligands are highlighted by the corresponding MOs’ diagrams and their donation and backdonation amounts.

Aluminum‐poor hexacarbalane structures: The transition from localized organoaluminum structures to delocalized polyhedra

AbstractThe series of hexacarbalanes C6Aln–6Men (n = 7–11) represent a progression from localized organoaluminum structures to delocalized polyhedral structures en route to experimentally known 13‐ and 14‐vertex hexacarbalanes such as (AlMe)8(CCH2Ph)5(µ4H), (AlMe)8(CCH2Ph)5(CCPh), [R4N+]2[(AlH)8(CR)6], and (AlNMe3)2(AlR)6(CR)6. In this connection, the lowest energy seven‐vertex C6AlMe7 structure has a tetrahapto benzene ring with the four AlC(cage) bonding interactions required to give the aluminum the favored octet configuration. Related eight‐vertex C6Al2Me8 structures are found with a benzene ring bound to an Al2 unit with a short AlAl distance of ∼2.55 Å suggesting a formal double bond. However, the lowest energy C6Al2Me8 structure has a dialuminacyclobutene unit fused to a tricyclohexane unit through an Al2 edge. Other relatively low‐energy C6AlMe7 and C6Al2Me8 structures consist of a six‐carbon hexatriene chain either forming a seven‐membered C6Al ring in the seven‐vertex structure or acting as a “flyover” between an Al2 unit. The lowest energy nine‐vertex hexacarbalane C6Al3Me9 has two separate C3 units bridged by both an Al2 pair and a single aluminum atom. Higher energy C6Al3Me9 hexacarbalanes contain a pentadienyl chain and an isolated carbon atom with an imbedded bonded Al3 triangle. The low‐energy 10‐vertex C6Al4Me10 structures have a central Al4 butterfly with nonbonding distances between the wingtips ranging from 3.35 to 3.91 Å. The lowest energy 11‐vertex C6Al5Me11 structure has a central Al4 quadrilateral with a diagonal bridged by the fifth aluminum atom. Higher energy C6Al5Me11 structures have an edge rather than a diagonal of the central Al4 quadrilateral bridged by the fifth aluminum atom.

N-Benzylnicotinamide and N-benzyl-1,4-dihydronicotinamide: useful models for NAD+ and NADH.

3-Aminocarbonyl-1-benzylpyridinium bromide (N-benzylnicotinamide, BNA), C13H13N2O+·Br-, (I), and 1-benzyl-1,4-dihydropyridine-3-carboxamide (N-benzyl-1,4-dihydronicotinamide, rBNA), C13H14N2O, (II), are valuable model compounds used to study the enzymatic cofactors NAD(P)+ and NAD(P)H. BNA was crystallized successfully and its structure determined for the first time, while a low-temperature high-resolution structure of rBNA was obtained. Together, these structures provide the most detailed view of the reactive portions of NAD(P)+ and NAD(P)H. The amide group in BNA is rotated 8.4 (4)° out of the plane of the pyridine ring, while the two rings display a dihedral angle of 70.48 (17)°. In the rBNA structure, the dihydropyridine ring is essentially planar, indicating significant delocalization of the formal double bonds, and the amide group is coplanar with the ring [dihedral angle = 4.35 (9)°]. This rBNA conformation may lower the transition-state energy of an ene reaction between a substrate double bond and the dihydropyridine ring. The transition state would involve one atom of the double bond binding to the carbon ortho to both the ring N atom and the amide substituent of the dihydropyridine ring, while the other end of the double bond accepts an H atom from the methylene group para to the N atom.

Metal–metal bonding in biscycloheptatrienyl dimetal compounds of the second‐row transition metals

AbstractDensity functional theory has been used to examine the dimetallocene‐like dicycloheptatrienyl dimetal compounds of the second‐row transition metals (C7H7)2M2 (M = Ru, Tc, Mo, Nb, Zr). The lowest energy (C7H7)2Mo2 structure is a coaxial structure with terminal η7C7H7 rings, whereas the lowest energy (C7H7)2M2 structures (M = Ru, Tc, Nb, Zr) are perpendicular structures with bridging η4,η4C7H7 rings except for the perpendicular (η4,η3C7H7)2Ru2 structure. The metal–metal bond orders in the (C7H7)2M2 structures (M = Ru, Tc, Mo, Nb), as determined by analysis of their frontier molecular orbitals, suggest preferred 16‐ rather than 18‐electron configurations for the central metal atoms. Thus, in the coaxial (η7C7H7)2M2 structures the formal bond orders are two for M = Tc and three for M = Mo. For the perpendicular structures both (η4,η3C7H7)2Ru2 and (η4,η4C7H7)2Tc2 have 16‐electron configurations with metal–metal single bonds owing to the different modes of bonding of the bridging C7H7 rings in the two structures. For the (C7H7)2Zr2 system the perpendicular structure has a formal ZrZr double bond and the coaxial structure has a very long (∼3.5 Å) ZrZr bond indicating only 12‐ to 14‐electron configurations for the zirconium atoms.

Computational analysis of polyazoles and their N-oxides

The polyazoles have five-membered heterocyclic rings with N/C ratios of at least 2/3. Their high nitrogen contents result in relatively high crystal densities and large positive heats of formation, features that make them attractive as frameworks for energetic materials. However the presence of linked nitrogens (catenation) is accompanied by reduced stability (reflected in the large heats of formation). Several related factors may be involved in this, including the weakness of N-N bonds, the possibility of decomposing through release of the very stable N2(g) and the repulsion between lone pairs of neighboring nitrogens. We show that in the polyazoles a particularly important source of reduced stability is the presence of adjacent doubly-coordinated nitrogens, especially if connected by a formal double bond. Our computed polyazole electrostatic potentials are consistent with significant repulsion between the lone pairs of these nitrogens. Introducing an N-oxide linkage on one of them leads to some stabilization; the oxygen withdraws electronic charge from the heterocyclic ring, reducing the electronic repulsion within it. However the N-oxide derivatives having more nitrogen catenation are still the less stable ones. Comparing the polyazoles to the polyazines (six-membered N/C heterocyclic rings), the π electrons in the former are less delocalized and the computed bond lengths in the polyazole rings are consistent with the formal single and double bonds in their Lewis structures. An interesting consequence is that if a second N-oxide is introduced next to the original one, the bond between the two nitrogens remains intact in polyazoles if it is formally double but it is considerably weakened or broken in polyazines, and in polyazoles if formally single.

Competition between phosphorus lone pairs and ring π-systems in binding to transition metals: Binuclear diphosphacyclobutadiene cobalt carbonyl derivatives

Metal complexes of the 2,4-di-t-butyl-1,3-diphosphacyclobutadiene ligand have been synthesized by head-to-tail dimerization of t-BuCP: on transition metal sites. In this connection the geometries and energetics of the complete series of binuclear cobalt carbonyl complexes (Me2C2P2)2Co2(CO)n (n=5, 4, 3, 2, 1) of the simpler 2,4-dimethyl-1,3-diphosphacyclobutadiene ligand have been investigated by density functional theory. The lowest energy structures of these complexes have bridging η4,η1-Me2C2P2 ligands donating six electrons to the central Co2 unit through a tetrahapto ring-metal linkage to one cobalt atom and a phosphorus lone pair to the other cobalt atom. For the tetracarbonyl (Me2C2P2)2Co2(CO)4 doubly CO-bridged structures with terminal η4-Me2C2P2 ligands similar to known (η5C5H5)2Fe2(μ-CO)2(CO)2 and (η4-Me4C4)2Co2(μ-CO)2(CO)2 structures lie only ∼5kcal/mol in energy above the η4,η1-Me2C2P2 bridged structures. For the unsaturated (Me2C2P2)2Co2(CO)3 and (Me2C2P2)2Co2(CO)2 systems, structures with one or two bridging η4,η1-Me2C2P2 ligands, respectively, are preferred energetically over isomeric structures with formal CoCo double bonds and CoCo triple bonds, respectively. The lowest energy structure for the monocarbonyl (Me2C2P2)2Co2(CO) is a triplet structure in which a (Me2C2P2)2Co sandwich unit functions as a tridentate ligand to a CoCO unit through a phosphorus atom on each ring as well as the central cobalt atom.

Binuclear cyclopentadienylmetal carbonyl hydrides of iridium, osmium, and rhenium: The effect of electron count on the preferred structures

The Cp2M2H2(CO)n (M=Ir, Os, Re; n=3, 2, 1) systems, related to the experimentally known and structurally characterized permethylated derivatives Cp∗2M2H2(CO)2 (Cp∗=η5-Me5C5; M=Ir, Os), provide examples of binuclear structures having metal–metal bonds of formal orders ranging from zero to four as determined by the 18-electron rule. The low-energy Cp2Ir2H2(CO)3 structures lacking a direct iridium–iridium bond necessarily have a bridging CO group holding together the two iridium atoms. For Cp2Ir2H2(CO)2, low-energy unbridged structures are found corresponding to the experimental unbridged Cp∗2Ir2H2(CO)2 structure. However, for Cp2Os2H2(CO)2 the lowest energy structure has two hydrogen atoms bridging the formal OsOs double bond similar to the experimental doubly bridged Cp∗2Os2H2(CO)2 structure. The lowest energy structures of the monocarbonyls Cp2M2H2(CO) have all three ligands (2H+CO) bridging a central metal–metal bond. The Cp2Re2H2(CO) structures provide examples of both bridged and unbridged structures containing formal ReRe quadruple bonds with highly bridged structures being preferred energetically.