Metal-metal Bond Research Articles

Dicarba-closo-dodecarborane-containing half-sandwich complexes of ruthenium, osmium, rhodium and iridium: biological relevance and synthetic strategies

This review describes how the incorporation of dicarba-closo-dodecarboranes into half-sandwich complexes of ruthenium, osmium, rhodium and iridium might lead to the development of a new class of compounds with applications in medicine. Such a combination not only has unexplored potential in traditional areas such as Boron Neutron Capture Therapy agents, but also as pharmacophores for the targeting of biologically important proteins and the development of targeted drugs. The synthetic pathways used for the syntheses of dicarba-closo-dodecarboranes-containing half-sandwich complexes of ruthenium, osmium, rhodium and iridium are also reviewed. Complexes with a wide variety of geometries and characteristics can be prepared. Examples of addition reactions on the metal centre, B-H activation, transmetalation reactions and/or direct formation of metal-metal bonds are discussed (103 references).

The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds

Although compounds that feature 3-centre 2-electron (3c-2e) bonds are well known, there has been no previous effort to classify the interactions according to the number of electrons that each atom contributes to the bond, in a manner analogous to the classification of 2-centre 2-electron (2c-2e) bonds as either normal covalent or dative covalent. This article provides an extension to the Covalent Bond Classification (CBC) method by categorizing 3c-2e interactions according to whether (i) the two electrons are provided by one or by two atoms and (ii) the central bridging atom provides two, one, or zero electrons. Class I 3c-2e bonds are defined as those in which two atoms each contribute one electron to the 3-centre orbital, while Class II 3c-2e bonds are defined as systems in which the pair of electrons are provided by a single atom. Class I and Class II 3c-2e interactions can be denoted by structure-bonding representations that employ the "half-arrow" notation, which also provides a convenient means to determine the electron count at a metal centre. In contrast to other methods of electron counting, this approach provides a means to predict metal-metal bond orders that are in accord with theory. For example, compounds that feature symmetrically bridging carbonyl ligands do not necessarily have to be described as "ketone derivatives" because carbon monoxide can also serve as an electron pair donor to two metal centres. This bonding description also provides a simple means to rationalize the theoretical predictions of the absence of M-M bonds in molecules such as Fe(2)(CO)(9) and [CpFe(CO)(2)](2), which are widely misrepresented in textbooks as possessing M-M bonds.

Electron transport through molecular wires based on a face-shared bioctahedral motif

Density functional theory in conjunction with non-equilibrium Green's functions is used to explore the electron transport properties of a series of molecules based on the face-shared bioctahedral (M2Cl9) motif. The metal-metal bond orders in the chosen molecules, [Rh2Cl9]3–, [Ru2Cl9]3– and [Mo2Cl9]3– vary from 0 (Rh) to 1 (Ru) and 3 (Mo), and the calculations indicate that there is a direct correlation between conductance and bond order. The [Mo2Cl9]3– case is particularly interesting as it is well known from crystallographic studies to be very flexible, the Mo–Mo bond length varying over a range of ∼0.35 A depending on cation. The upper limit of this range marks the point where homolytic cleavage of the δπ components of the triple bond is complete, and this has a marked impact on electron transport. The localization of the metal-based orbitals means that those on the left (source) and right (drain) sides respond very differently to applied bias, giving rise to resonance effects at particular bias voltages, and hence to negative differential resistance effects.

A Combined Spectroscopic and Computational Study of a High-SpinS= 7/2 Diiron Complex with a Short Iron–Iron Bond

The nature of the iron-iron bond in the mixed-valent diiron tris(diphenylforamidinate) complex Fe(2)(DPhF)(3), which was first reported by Cotton, Murillo et al. (Inorg. Chim. Acta 1994, 219, 7-10), has been examined using additional spectroscopic and theoretical methods. It is shown that the coupling between the two iron centers is strongly ferromagnetic, giving rise to an octet spin ground state. On the basis of Mössbauer spectroscopy, the two iron centers, formally mixed-valent Fe(II)Fe(I), are completely equivalent with an isomer shift δ = 0.65 mm s(-1) and quadrupole splitting ΔE(Q) = +0.32 mm s(-1). A large, positive zero-field splitting D(7/2) = 8.2 cm(-1) has been determined from magnetic susceptibility measurements. Multiconfigurational quantum studies of the complete molecule Fe(2)(DPhF)(3) found one dominant configuration (σ)(2)(π)(4)(π*)(2)(σ*)(1)(δ)(2)(δ*)(2), which accounts for 73% of the ground-state wave function. By considering all the configurations, an estimated metal-metal bond order of 1.15 has been calculated. Finally, Fe(2)(DPhF)(3) exhibits weak electronic absorptions in the visible and near-infrared regions, which are assigned as d-d transitions from the doubly occupied metal-metal π molecular orbital to half-occupied π*, δ, and δ* orbitals.

Theoretical Elucidation of a Classic Reaction: Protonation of the Quadruple Bond of the Octachlorodimolybdate(II,II) [Mo2Cl8]4– Anion

The protonation reaction of the unbridged quadruple metal-metal bond of [Mo(2)Cl(8)](4-) anion producing the triply bonded hydride [Mo(2)(μ-H)(μ-Cl)(2)Cl(6)](3-) is studied by accurate Density Functional Theory computations. The reactant, product, stable intermediates, and transition states are located on the potential energy surface. The water solvent is explicitly included in the calculations. Full reaction profiles are calculated and compared to experimental data. The mechanism of the reaction is fully elucidated. This involves two steps. The first is a proton transfer from an oxonium ion to the quadruple bond, being rate determining. The second, involves the internal rearrangement of chlorine atoms and is much faster. Activation energies with a mean value of 19 kcal/mol are calculated, in excellent agreement with experimental values.

ChemInform Abstract: Metal—Metal Multiple Bonds in Early/Late Heterobimetallic Complexes: Applications Toward Small Molecule Activation and Catalysis

AbstractReview: 67 refs.

Bis(μ2-η2:η2-2,4,6-trimethylbenzonitrile)bis[(N-isopropyl-3,5-dimethylanilido)molybdenum(III)](Mo—Mo)

The title compound, [Mo2(C11H16N)4(C10H11N)2], is a dinuclear molybdenum complex with a formal metal–metal bond [Mo⋯Mo separation = 2.5946 (8) Å], four anilide-type ligands and two bridging mesityl nitrile groups. There are two inversion symmetric mol­ecules in the unit cell (an inversion center is localized at the mid-point of the Mo—Mo bond), each with approximate non-crystallographic C 2h symmetry. The mol­ecules contain disordered isopropyl and 3,5-C6H3Me2 groups on different anilido ligands; the major component having an occupancy of 0.683 (7). The complex was obtained in low yield as the product from the reaction between the bridging pyrazine adduct of molybdenum tris­-anilide ([μ2-(C4H4N2){Mo(C11H16N)3}2]) and mesityl nitrile with a loss of one anilido ligand.

Carbon–Carbon Bond Formation from Azaallyl and Imine Couplings about Metal–Metal Bonds

Typical C-C bond-forming processes feature oxidative addition, insertion, and reductive elimination reactions. An alternative strategy toward C-C bond formation involves the generation of transient radicals that can couple at or around one or more metal centers. Generation of transient azaallyl ligands that reductively couple at CH positions possessing radical character is described. Two C-C bonds form, and the redox non-innocence of the resulting pyridine-imines may be critical to formation of a third C-C bond via dinuclear di-imine oxidative coupling. Unique metal-metal bonds are a consequence of the chelation.

Photochromic properties of perfluorosulfonic membranes modified by sodium molybdate

Transfer of a perfluorosulfonic membrane from the hydrogen form into the sodium form provides accessibility of pore space to sodium molybdate aqueous solution. Charge transfer spectra make it possible to judge the course of polymerization of capsulated molybdate ions and its intensification when the membranes are held in HCl vapor. The UV irradiation of the modified membranes causes a progressing absorption in the visible spectral region. The rate and depth of photoreduction increase as the concentration and degree of polymerization of molybdate ions increase. The scheme of the process was offered, which is based on taking into account a possibility of delocalization of 4d1 electrons of molybdenum(V) over the system of metal-metal bonds.

Effect of impurities on the mechanical and electronic properties of Au, Ag, and Cu monatomic chain nanowires

In this study, we have investigated the interaction of various different atomic and molecular species (H, C, O, H${}_{2}$, and O${}_{2}$) with the monatomic chains of Au, Ag, and Cu via total-energy calculations using the plane-wave pseudopotential method based on density functional theory. The stability, energetics, mechanical, and electronic properties of the clean and contaminated Au, Ag, and Cu nanowires have been presented. We have observed that the interaction of H, C, or O atoms with the monatomic chains are much stronger than the one of H${}_{2}$ or O${}_{2}$ molecules. The atomic impurities can easily be incorporated into these nanowires; they form stable and strong bonds with these one-dimensional structures when they are inserted in or placed close to the nanowires. Moreover, the metal-atomic impurity bond is much stronger than the metal-metal bond. Upon elongation, the nanowires contaminated with atomic impurities usually break from the remote metal-metal bond. We have observed both metallic and semiconducting contaminated nanowires depending on the type of impurity, whereas all clean monatomic chains of Au, Cu, and Ag exhibit metallic behavior. Our findings indicate that the stability and the electronic properties of these monatomic chains can be tuned by using appropriate molecular or atomic additives.

Relativistic-Consistent Electron Densities of the Coinage Metal Clusters M2, M4, M42–, and M4Na2(M = Cu, Ag, Au): A QTAIM Study

We employ second-order Møller-Plesset perturbation theory level in combination with recently developed pseudopotential-based correlation consistent basis sets to obtain accurate relativistic-consistent electron densities for small coinage metal clusters. Using calculated electron densities, we employ Bader's quantum theory of atoms in molecules (QTAIM) to gain insights into the nature of metal-metal bonding in the clusters M(2), M(4), M(4)(2-), and M(4)Na(2) (M = Cu, Ag, Au). For the simplest case of the metal dimer, M(2), we correlate the strength of the metal-metal bond with the value of the electron density at the bond critical point, the total energy density at the bond critical point, the sharing (delocalization) index, and the values of the two principle negative curvatures. We then consider changes to the metal-metal bonding and charge density distribution upon the addition of two metal atoms to form the metal tetramer, M(4), and then followed by the addition of an electron pair to form M(4)(2-) and finally followed by the addition of two alkali metal (sodium) ions to form M(4)Na(2). Using topological properties of the electron density, we present evidence for the existence of σ-aromaticity in Au(4)(2-). We also report the existence of two non-nuclear attractors in the molecular graph of Cu(4)(2-) and large negative charge accumulation in the nonbonded Cu basins of this cluster.

A Homometallic Tricapped Cubane Cluster: [(Cp*Mo)4B4H4(μ4-BH)3] (Cp* = η5-C5Me5)

The reaction of [Cp*MoCl(4)] with an excess of LiBH(4), followed by thermolysis with tellurium powder in toluene, afforded a tricapped cubane cluster, [(Cp*Mo)(4)B(4)H(4)(μ(4)-BH)(3)] (1), which represents an unprecedented metal-rich metallaborane cluster with a cubane core containing 58 cluster valence electrons (cve) and three metal-metal bonds.

Cyclopalladated 2-phenyldihydrooxazole complexes with ethylenediamine, 2,2′-bipyridine, and bridging acetate ligands

The 1H NMR, electronic absorption, and luminescence spectra, as well as voltammograms of the reduction and oxidation of the complexes [Pd(C∧N)(N∧N)]ClO4 and [Pd(C∧N)(μ-OOCCH3)]2 [where (C∧N)− is deprotonated 2-phenyl-4,5-dihydro-1,3-oxazole, and N∧N is ethylenediamine or 2,2′-bipyridine (bpy)] were compared. Magnetic nonequivalence of protons in the dihydrooxazole ring and upfield shift of the corresponding signals were observed as a result of anisotropic effect of the ring current in palladated phenyl substituents in the [Pd(C∧N)(μ-OOCCH3)]2 complex having a C 2 symmetry. One-electron reduction wave of [Pd(C∧N)bpy]+ was assigned to ligand-centered electron transfer to the π* orbital of 2,2′-bipyridine, and two oxidation waves of [Pd(C∧N)(μ-OOCCH3)]2 were attributed to successive one-electron oxidations of the palladium centers. Low-temperature (77 K) phosphorescence of [Pd(C∧N)En]+ and [Pd(C∧N)bpy]+ was ascribed to optical transition localized on the metal-complex fragment {Pd(C∧N)} and to interligand charge transfer between the chelating and cyclopalladated ligands. The formation of metal-metal bond in the complex [Pd(C∧N)(μ-OOCCH3)]2 gives rise to radiative decay of photoexcitation energy from two electronically excited states, one of which is localized on the {Pd(C∧N)} fragment, and the second corresponds to the charge transfer metal-metal-cyclopalladated ligand.

METAL-BASED MOLECULAR CHAINS: DESIGN BY COORDINATION CHEMISTRY

Molecular extended metal atom chains (EMACs) or “molecular chains” are discrete molecules exhibiting arrays of closely spaced metals in a nearly straight fashion. Such species are gaining momentum as possible crucial components of molecular functional devices. The synthetic strategies for the preparation of such species have diversified over the years. The existing strategies rely either on the formation of “unsupported” metal-metal bonds or depend entirely on the ability of specifically designed ligands to promote the assembly of metals in desired topologies. In this review, a summary of the methods employed for the preparation of molecular EMACs is presented, and several important examples are described.

Bond Characterization on a Cr–Cr Quintuple Bond: A Combined Experimental and Theoretical Study

A combined experimental and theoretical charge density study on a quintuply bonded dichromium complex, Cr(2)(dipp)(2) (dipp = (Ar)NC(H)N(Ar) and Ar = 2,6-i-Pr(2)-C(6)H(3)), is performed. Two dipp ligands are bridged between two Cr ions; each Cr atom is coordinated to two N atoms of the ligands in a linear fashion. The Cr atom is in a low oxidation state, Cr(I), and in low coordination number condition, which stabilizes a metal-metal multiple bond, in this case, a quintuple bond. Indeed, it gives an ultrashort Cr-Cr bond distance of 1.7492(1) Å in the complex. The bond characterization of such a quintuple bond is undertaken both experimentally by high-resolution single-crystal X-ray diffraction and theoretically by density functional calculation (DFT). Electron densities are depicted via deformation density and Laplacian distributions. Bond characterizations of the complex are presented in terms of topological properties, Fermi hole function, source function (SF), and natural bonding orbital (NBO) analysis. The electron density at the Cr-Cr bond critical point (BCP) is 1.70 e/Å(3), quite a high value for metal-metal bonding and mainly contributed from the metal ion itself. The quintuple bond is confirmed with one σ, two π, and two δ interactions by NBO analysis and Fermi hole function. The molecular orbitals (MOs) illustrate that five bonding orbitals are predominantly contributed from the 3d orbitals of the Cr(I) ion. The effective bond order from NBO analysis is 4.60. The detail comparison between experiment and theory will be given. Additionally, three closely related complexes are calculated for systematic comparison.

The Remarkable Nb2(CO)12 with Seven-Coordinate Niobium: Decarbonylation to Nb2(CO)11 and Nb2(CO)10.

The dissociation of Nb2(CO)12 into two Nb(CO)6 units is predicted to require ∼13 kcal/mol so that Nb2(CO)12 rather than Nb(CO)6 is the anticipated initial oxidation product of the known Nb(CO)6(-) anion. This differs from the corresponding vanadium carbonyl chemistry where V(CO)6 rather than V2(CO)12 is found experimentally to be the oxidation product of V(CO)6(-). The lowest energy Nb2(CO)12 structure consists of two Nb(CO)6 fragments joined by a Nb-Nb bond of ∼3.4 Å length so that each niobium atom is heptacoordinate, counting the metal-metal bond. These niobium coordination polyhedra can be approximated by capped octahedra. Among unsaturated binuclear niobium carbonyls the lowest energy Nb2(CO)11 structure has a formal four-electron donor bridging η(2)-μ-CO group and a formal Nb-Nb single bond rather than only two-electron donor carbonyl groups and a formal Nb═Nb double bond. The Nb2(CO)11 structures with formal Nb═Nb double bonds and exclusively two-electron donor carbonyl groups lie more than 13 kcal/mol above this low-energy Nb2(CO)10(η(2)-μ-CO) structure. However, Nb2(CO)11 is predicted to be thermodynamically disfavored, owing to disproportionation into Nb2(CO)12 + Nb2(CO)10, a slightly exothermic process by ∼4 kcal/mol. The Nb2(CO)10 structures with formal Nb≡Nb triple bonds and all two-electron donor carbonyl groups appear to be particularly favorable, as suggested by high CO dissociation energies and viability toward disproportionation. Such structures are isolobal with Cp2Mo2(CO)4, which was the first stable metal carbonyl to be discovered with a short metal-metal distance, corresponding to a formal triple bond. Considerably higher energy Nb2(CO)10 structures (by more than 20 kcal/mol) have two four-electron donor bridging carbonyl groups and long niobium-niobium distances. Such structures can be considered to consist of a bidentate Nb(CO)6 "ligand" coordinating to a Nb(CO)4 unit through the two η(2)-μ-CO groups.

Crystallographic and structural characterisation of heterometallic platinum compounds: Part I. Heterobinuclear Pt compounds

Abstract This review covers almost 290 heterobinuclear Pt derivatives. When the heterometals (M) are non transition and the binuclear are found both with and without a metal to metal bond. Where M is a transition metal or actinide, only those with a metal-metal bond have been included here. There are thirteen non-transition metals (Sn, Hg, Ge, Sb, Tl, Zn, Pb, Cd, Na, K, Ga, Ca and In). The shortest Pt-M bond distance is 235.2(1) (Pt-Ge). There are eighteen transition metals (Fe, W, Rh, Re, Pd, Ag, Ir, Mo, Mn, Re, Co, Cu, Cr, Au, Ni, Ti, Ta and V). The shortest Pt-M bond distance is 249.5(2) pm (Pt-Cr). There is one example of an actinide, Pt-Th at 298.4(1) pm. The Pt atom has oxidation numbers 0, +2 and +4. The Pt coordination geometries include square planar (most common), trigonal bipyramidal, pseudo octahedral (Pt(IV)) and a few prevalently capped trigonal prismatic seven coordinate species. There are at least two types of isomerism distortion and polymerisation. Factors affecting bond lengths and angles are discussed and some ambiguities in coordination polyhedra are outlined.

Cyclopalladated complexes based on 2-phenylbenzothiazole and 1,4-(benzothiazol-2-yl)benzene with acetate ligands and ethylenediamine

Complexes [Pd(bt)En]ClO4, [Pd(bt)(μ-OOCCH3)]2, [(PdEn)2(μ-dbt)](CH3COO)2, and [Pd2(μ-dbt)· (μ-OOCCH3)2]2 (bt− and dbt2− are the mono- and bisdeprotonated forms of 2-phenylbenzothiazole and 1,4-bis-(benzthiazol-2-yl)benzene, En is ethylenediamine) are characterized by 1H NMR, electron absorption spectroscopy, and voltammetry. The upfield shift of the signal of protons of heterocyclic ligands in complexes with acetate ligands is assigned to anisotropic effect of the ring current of the two fragments {Pd(bt)} and {Pd(dbt)} in the complexes. The red shift of the optical transition of the metal-ligand charge transfer as well as the anodic shift of the ligand-centered reduction of [(PdEn)2(μ-dbt)](CH3COO)2 relative to [Pd(bt)En]ClO4 is due to the decrease in the LUMO energy of the complexes. The cathodic shift of the oxidation potential and the long wavelength absorption in complexes with acetate ligands is assigned to variation in the HOMO nature due to the metal-metal bond formation.

Characterizing the Deformational Isomers of Bimetallic Ir2(dimen)42+ (dimen = 1,8-diisocyano-p-menthane) with Vibrational Wavepacket Dynamics

We studied the Ir(2)(dimen)(4)(2+) complex with ultrafast transient absorption spectroscopy and density functional theory and concluded that it possesses two singlet ground state isomers in room temperature solution. The molecule can adopt either a paddle wheel or a propeller conformation in solution, where the paddle wheel structure possesses a metal-metal bond of 4.4 Å and a dihedral angle between the quasi-C(4v) planes of 0° and the propeller structure has a metal-metal bond of 3.6 Å and a dihedral angle of 17° when crystallized. Each conformation has a distinct absorption in the visible attributed to a (1)(dσ(z)* → pσ(z)) excitation, with the long eclipsed structure absorbing at 475 nm and the short twisted structure absorbing at 585 nm. We independently pumped at each of these visible transitions to form vibrational wavepackets on the ground and excited state potential energy surfaces, which modulated the ground state bleach and stimulated emission signals, respectively. We found that the ground state wavepacket oscillates with a frequency of 48 cm(-1) when pumping the red peak and 11 cm(-1) when pumping the blue peak. We assign these frequencies to the Ir-Ir symmetric stretch, with the variation in frequency reflecting the variation in metal-metal bond strength in support of our assignment of the blue peak to the longer Ir-Ir bond length conformer and the red peak to the shorter Ir-Ir bond length conformer. When pumping the red peak, we found two modes with frequencies of 80 and 119 cm(-1) in the stimulated emission and only one mode at 75 cm(-1) when pumping the blue peak. We assign the 75-80 cm(-1) frequency to the Ir-Ir stretch and the 119 cm(-1) vibration to the dihedral angle twist in the excited state. The variation in the excited state dynamics does not result from the excitation of different electronic states, but rather from excitation to different Franck-Condon regions of the same electronic excited state potential energy surface. This occurs because of the large difference in ground state molecular structure. DFT calculations support the existence of a single electronic excited state being accessed from two distinct structural isomers with conformations similar to those observed with X-ray crystallography.

Elastic properties of MgxTi1−xB2(0 ⩽x ⩽1)studied by first-principles calculations

The elastic properties, including elastic constants, bulk, and shear moduli, and anisotropic index of hexagonal Mg${}_{x}$Ti${}_{1\ensuremath{-}x}$B${}_{2}$ (0 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 1) are obtained using a first-principles density-functional-theory method. A difference in chemical bonding occurs between Ti-rich and Mg-rich diborides, leading to significantly different elastic properties: a small elastic anisotropy with relatively large bulk and shear moduli for Ti-rich diborides with $x$ 0.25, and a large elastic anisotropy and relatively small bulk and shear moduli for Mg-rich diborides with $x$ > 0.25. The calculated results reveal a dominant role of the interplanar metal-metal bonds in predicting the extent of shear elastic anisotropy in Mg${}_{x}$Ti${}_{1\ensuremath{-}x}$B${}_{2}$ (0 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 1).