Metal-ligand Bond Energies Research Articles

Dependence of the stability of lanthanide β-dicarbonyl complexes on the metal—ligand bond energy

Dependence of the stability of lanthanide β-dicarbonyl complexes on the metal—ligand bond energy

Does DFT+U mimic hybrid density functionals?

This work examines the question of how a Hubbard U correction to a local exchange–correlation functional compares with adding Hartree–Fock exchange to a local functional for both solid-state and molecular properties. We compute a solid-state property, namely the band gap, and thermochemical molecular properties, in particular, main-group bond energies, transition metal–ligand bond energies, and barrier heights, to elucidate whether the DFT+U method mimics hybrid DFT. We find that a calculation with a Hubbard U correction may or may not mimic a hybrid functional—depending on the atom, the subshell, and the property to which it is applied. For band gaps, we find that adding a Hubbard U correction to the valence d orbitals of transition metals increases the band gap, which thereby gets closer to the experimental value, while adding a Hubbard U correction to valence s or p orbitals of main-group elements need not always increase the band gap. For molecular thermochemistry, we find that adding a Hubbard U correction to a local density functional need not have the same effect as adding Hartree–Fock exchange to a local density functional. For example when compared to a DFT calculation with a local exchange-correlation functional, hybrid DFT increases the barrier height in all cases, but DFT+U does not always increase the barrier height. For the band gaps of transition metal monoxides, the Hubbard-corrected results lowered the mean errors significantly and were comparable to what could be achieved with a much more expensive hybrid functional, but for reaction barrier heights and bond energies of molecules, the Hubbard correction was found to lower the mean error by only approximately a kcal/mol. As part of the analysis, we also compare VASP and Gaussian 09 calculations for the same density functional.

Do Practical Standard Coupled Cluster Calculations Agree Better than Kohn-Sham Calculations with Currently Available Functionals When Compared to the Best Available Experimental Data for Dissociation Energies of Bonds to 3d Transition Metals?

Coupled-cluster (CC) methods have been extensively used as the high-level approach in quantum electronic structure theory to predict various properties of molecules when experimental results are unavailable. It is often assumed that CC methods, if they include at least up to connected-triple-excitation quasiperturbative corrections to a full treatment of single and double excitations (in particular, CCSD(T)), and a very large basis set, are more accurate than Kohn-Sham (KS) density functional theory (DFT). In the present work, we tested and compared the performance of standard CC and KS methods on bond energy calculations of 20 3d transition metal-containing diatomic molecules against the most reliable experimental data available, as collected in a database called 3dMLBE20. It is found that, although the CCSD(T) and higher levels CC methods have mean unsigned deviations from experiment that are smaller than most exchange-correlation functionals for metal-ligand bond energies of transition metals, the improvement is less than one standard deviation of the mean unsigned deviation. Furthermore, on average, almost half of the 42 exchange-correlation functionals that we tested are closer to experiment than CCSD(T) with the same extended basis set for the same molecule. The results show that, when both relativistic and core-valence correlation effects are considered, even the very high-level (expensive) CC method with single, double, triple, and perturbative quadruple cluster operators, namely, CCSDT(2)Q, averaged over 20 bond energies, gives a mean unsigned deviation (MUD(20) = 4.7 kcal/mol when one correlates only valence, 3p, and 3s electrons of transition metals and only valence electrons of ligands, or 4.6 kcal/mol when one correlates all core electrons except for 1s shells of transition metals, S, and Cl); and that is similar to some good xc functionals (e.g., B97-1 (MUD(20) = 4.5 kcal/mol) and PW6B95 (MUD(20) = 4.9 kcal/mol)) when the same basis set is used. We found that, for both coupled cluster calculations and KS calculations, the T1 diagnostics correlate the errors better than either the M diagnostics or the B1 DFT-based diagnostics. The potential use of practical standard CC methods as a benchmark theory is further confounded by the finding that CC and DFT methods usually have different signs of the error. We conclude that the available experimental data do not provide a justification for using conventional single-reference CC theory calculations to validate or test xc functionals for systems involving 3d transition metals.

Accurate metal–ligand bond energies in the η2-C2H4 and η2-C60 complexes of Pt(PH3)2, with application to their Bis(triphenylphosphine) analogues

We have investigated the metal–ligand bond energies in C2H4–Pt(PH3)2 and C60–Pt(PH3)2 by means of ab initio correlated methods (MP2, MP4(SDQ), MP4, QCISD, CCSD and CCSD(T)) in conjunction with large basis sets. For D e (C2H4–Pt(PH3)2), an accurate value of 17.2 kcal/mol is established at the CCSD(T) level of theory. Due to the size of the system, the bond energy for the Buckminsterfullerene system was explored in terms of a sequence of model systems of increasing size, providing a D e estimate of 28.2 kcal/mol at the ONIOM(CCSD(T)/C14H8:MP2/C60) level of theory. The performance of a range of high-end density functionals (with and without dispersion) is evaluated for these systems by comparison to the best ab initio results. Among these, we find density functionals BLYP and B3LYP, augmented by Grimme’s D3 dispersion correction (to give the corresponding DFT-D methods), to provide good and consistent agreement with our best estimates. Next, DFT-optimised structures for C2H4–Pt(PPh3)2 and C60–Pt(PPh3)2 are presented for the first time. Ligand binding energies were computed for ethylene and C60 using DFT-D, thus allowing the first computational estimate of the driving force in the ligand substitution reaction (η2-C 2 H 4)Pt(PPh 3)2 + C 60 → (η2-C 60)Pt(PPh 3)2 + C 2 H 4.

ChemInform Abstract: Organometallic Reactivity: The Role of Metal—Ligand Bond Energies from a Computational Perspective

AbstractReview: 92 refs.

High-resolution electron spectroscopy, preferential metal-binding sites, and thermochemistry of lithium complexes of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons are model systems for studying the mechanisms of lithium storage in carbonaceous materials. In this work, Li complexes of naphthalene, pyrene, perylene, and coronene were synthesized in a supersonic metal-cluster beam source and studied by zero-electron-kinetic-energy (ZEKE) electron spectroscopy and density functional theory calculations. The adiabatic ionization energies of the neutral complexes and frequencies of up to nine vibrational modes in the singly charged cations were determined from the ZEKE spectra. The metal-ligand bond energies of the neutral complexes were obtained from a thermodynamic cycle. Preferred Li∕Li(+) binding sites with the aromatic molecules were determined by comparing the measured spectra with theoretical calculations. Li and Li(+) prefer the ring-over binding to the benzene ring with a higher π-electron content and aromaticity. Although the ionization energies of the Li complexes show no clear correlation with the size of the aromatic molecules, the metal-ligand bond energies increase with the extension of the π-electron network up to perylene, then decrease from perylene to coronene. The trends in the ionization and metal-ligand bond dissociation energies of the complexes are discussed in terms of the orbital energies, local quadrupole moments, and polarizabilities of the free ligands and the charge transfer between the metal atom and aromatic molecules.

Remote substituent effects on homolytic FeC bond energies of p‐G‐C6H4CH2Fe(CO)2(η5‐C5H5) and p‐G‐C6H4(H)(CN)CFe(CO)2(η5‐C5H5) studied by Hartree–Fock and density functional theory methods

In general, both stoichiometric and catalytic reactions of organometallic complexes involve breaking and forming metal–ligand bonds. Therefore, an evaluation of the thermodynamics of such reactions requires the knowledge of metal–ligand bond energies (BDEs). The homolytic FeC bond dissociation energies [i.e., ΔHhomo(FeC)s] of 12 para‐substituted benzyldicarbonyl(η5‐cyclopentadienyl)iron, p‐G‐C6H4CH2Fp [1,G = NO2, CN, COMe, CO2Me, CF3, Br, Cl, F, H, Me, MeO, NMe2; Fp = (η5‐C5H5)(CO)2Fe] and 12 para‐substituted α‐cyanobenzyldicarbonyl (η5‐cyclopentadienyl)iron, p‐G‐PANFp [2,PAN = C6H4CH(CN)] were studied using Hartree–Fock (HF) and density functional theory (DFT) methods with large basis sets. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of ΔHhomo(FeC)s. The B3LYP method satisfactorily predicts the α and remote substituent effects on ΔHhomo(FeC)s [ΔΔHhomo(FeC)s]. The fair correlations [r = 0.97 (g, 1), 0.99(g, 2)] of ΔΔHhomo(FeC)s of series 1 and 2 with the substituent σ constants imply that the para substituent effects on ΔHhomo(FeC)s originate mainly from polar effects, but those on radical stability originate from both spin delocalization and polar effects. The molecule stabilization effects (MEs) causes that not only the magnitude of ΔΔHhomo(FeC)s(1) varies significantly but also the direction changes from S‐pattern to O‐pattern. ΔΔHhomo(FeC)s(2) were found to conform to the Capto‐dative Principle. The detailed knowledge of the factors that determine the FpC bond strengths would greatly aid in understanding reactivity patterns in many processes. Copyright © 2010 John Wiley & Sons, Ltd.

Organometallic reactivity: the role of metal–ligand bond energies from a computational perspective

The association and dissociation of ligands plays a vital role in determining the reactivity of organometallic catalysts. Computational studies with density functional theory often fail to reproduce experimental metal-ligand bond energies, but recently functionals which better capture dispersion effects have been developed. Here we explore their application and discuss future challenges for computational studies of organometallic catalysis.

Bonding and structures of copper-aminopyridine complexes — High-resolution electron spectroscopy and ab initio calculations

Copper complexes of x-aminopyridine (x = 2, 3, 4) were prepared in a laser vaporization supersonic molecular beam source and identified using laser photoionization time-of-flight mass spectrometry. These complexes were studied by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and second-order Møller-Plesset perturbation theory. Three structures formed by Cu binding to the pyridine nitrogen (σα), the amino nitrogen (σβ), and the pyridine ring (π) were considered by the theoretical calculations, but only the σα structures with Cu binding to the pyridine nitrogen were confirmed by the spectroscopic measurements. Adiabatic ionization energies and metal-ligand and ligand-based vibrational frequencies of the σα complexes were measured from the ZEKE spectra, and the metal-ligand bond energies of the neutral and ionized complexes were predicted by the theory. The ionization energies of the Cu complexes are about 20 000 cm–1 lower than that of bare Cu atom. This ionization energy shift is the result of the stronger Cu+-ligand bonding because of the additional charge-dipole interaction in the ion. Although the three complexes are formed by Cu coordination to the pyridine nitrogen atom, the position of the amino group affects the metal-ligand bonding strengths in both neutral and ionized species. These effects include the structural resonance and hydrogen bonding in the neutral complexes and the electric dipole moment and bidentate bonding in the ions.Key words: photoelectron, PFI-ZEKE, ab initio, copper aminopyridine.

Reactivity of Pt- and Pd-bound nitriles towards nitrile oxides and nitrones: substitution vs. cycloaddition

Reactions of the nitrone CH3CH=N(CH3)O and the nitrile oxide CH3C[triple bond]NO with the nitrile complexes trans-[MCl2(N[triple bond]CCH3)2] (M = Pt, 1; Pd, 2) were investigated by theoretical methods at B3LYP and, for some processes, CCSD(T) levels of theory. The mechanisms of substitutions and cycloadditions were studied in detail. The former occur via a concerted asynchronous mechanism of dissociative type. The calculations of the metal-ligand bond energies in the starting complexes and substitution products and the analysis of structural features of the transition states indicate that the M-N bond dissociation (rather than M-O bond formation) is the step, which controls the reactivity of and in substitutions. The different chemical behaviours of the Pt and Pd complexes towards the 1,3-dipoles were investigated. The exclusive isolation of cycloaddition rather than substitution products in any solvents in the case of is both kinetically and thermodynamically controlled. The switch of the reaction mode from cycloaddition to substitution for 2 in CH2Cl2 solution is caused by the significantly lower Pd-N bond energy in comparison with the Pt-N bond energy, consistent with the higher lability of the Pd complexes. The different chemical behaviour of 2 in CH3CN and CH2Cl2 solvents is accounted for by the great excess of acetonitrile in the CH3CN solution rather than a different solvation character. The relative variation of Wiberg bond indices along the reaction path is proposed as a quantitative criterion for the classification of the reaction mechanism.

Design of Density Functionals by Combining the Method of Constraint Satisfaction with Parametrization for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions.

We present a new hybrid meta exchange-correlation functional, called M05-2X, for thermochemistry, thermochemical kinetics, and noncovalent interactions. We also provide a full discussion of the new M05 functional, previously presented in a short communication. The M05 functional was parametrized including both metals and nonmetals, whereas M05-2X is a high-nonlocality functional with double the amount of nonlocal exchange (2X) that is parametrized only for nonmetals. In particular, M05 was parametrized against 35 data values, and M05-2X is parametrized against 34 data values. Both functionals, along with 28 other functionals, have been comparatively assessed against 234 data values: the MGAE109/3 main-group atomization energy database, the IP13/3 ionization potential database, the EA13/3 electron affinity database, the HTBH38/4 database of barrier height for hydrogen-transfer reactions, five noncovalent databases, two databases involving metal-metal and metal-ligand bond energies, a dipole moment database, a database of four alkyl bond dissociation energies of alkanes and ethers, and three total energies of one-electron systems. We also tested the new functionals and 12 others for eight hydrogen-bonding and stacking interaction energies in nucleobase pairs, and we tested M05 and M05-2X and 19 other functionals for the geometry, dipole moment, and binding energy of HCN-BF3, which has recently been shown to be a very difficult case for density functional theory. We tested eight functionals for four more alkyl bond dissociation energies, and we tested 12 functionals for several additional bond energies with varying amounts of multireference character. On the basis of all the results for 256 data values in 18 databases in the present study, we recommend M05-2X, M05, PW6B95, PWB6K, and MPWB1K for general-purpose applications in thermochemistry, kinetics, and noncovalent interactions involving nonmetals and we recommend M05 for studies involving both metallic and nonmetallic elements. The M05 functional, essentially uniquely among the functionals with broad applicability to chemistry, also performs well not only for main-group thermochemistry and radical reaction barrier heights but also for transition-metal-transition-metal interactions. The M05-2X functional has the best performance for thermochemical kinetics, noncovalent interactions (especially weak interaction, hydrogen bonding, π···π stacking, and interactions energies of nucleobases), and alkyl bond dissociation energies and the best composite results for energetics, excluding metals.

Density Functionals for Inorganometallic and Organometallic Chemistry

We present a database of 21 bond dissociation energies for breaking metal-ligand bonds. The molecules in the metal-ligand bond energy database are AgH, CoH, CoO+, CoOH+, CrCH3+, CuOH2+, FeH, Fe(CO)5, FeO, FeS, LiCl, LiO, MgO, MnCH3NiCH2+, Ni(CO)4, RhC, VCO+, VO, and VS. We have also created databases of metal-ligand bond lengths and atomic ionization potentials. The molecules used for bond lengths are AgH, BeO, CoH, CoO+, FeH, FeO, FeS, LiCl, LiO, MgO, RhC, VO, and VS and the ionization potentials are for the following atoms: C, Co, Cr, Cu, Ni, O, and V. The data were chosen based on their diversity and expected reliability, and they are used along with three previously developed databases (transition metal dimer bond energies and bond lengths and main-group molecular atomization energies) for assessing the accuracy of several kinds of density functionals. In particular, we report tests for 42 previously defined functionals: 2 local spin density approximation (LSDA) functionals, 14 generalized gradient approximation (GGA) methods, 13 hybrid GGA methods, 7 meta GGA methods, and 8 hybrid meta GGA methods. In addition to these functionals, we also examine the effectiveness of scaling the correlation energy by testing 13 functionals with scaled or no gradient-corrected correlation energy, and we find that functionals of this kind are more accurate for metal-metal and metal-ligand bonds than any of the functionals already in the literature. We also present a readjusted GGA and a hybrid GGA with parameters adjusted for metals. When we consider these 57 functionals for metal-ligand and metal-metal bond energies simultaneously with main-group atomization energies, atomic ionization potentials, and bond lengths we find that the most accurate functional is G96LYP, followed closely by MPWLYP1M (new in this article), XLYP, BLYP, and MOHLYP (also new in this article). Four of these five functionals have no Hartree-Fock exchange, and the other has only 5%. As a byproduct of this work we introduce a convenient diagnostic, called the B1 diagnostic, for ascertaining the multireference character in a bond.

Copper-substituted ethanes as a model for copper-acetylene interactions on the metal surface Quantum mechanical study of the structure and bonding of copper-acetylene and copper-ethylene compounds Cu n(C 2H 2) ( n = 1, 2, 4), Cu(C 2H 2) +, Cu n(C 2H 4 ( n = 1, 2) and Cu(C 2H 4) +1,2

Quantum mechanical ab initio calculations are reported for the Cu and Cu + complexes with acetylene and ethylene and for the dicopper-substituted ethylenes C 2H 2Cu 2 and di- and tetracopper-substituted ethanes C 2H 4Cu 2 and C 2H 2Cu 4. The geometries were optimized at the MP2 level of theory while the bond energies are predicted at CCSD(T) using relativistic effective core potentials with valence basis sets of TZ + P quality for copper. The calculated copper-acetylene and copper-ethylene complexes have C 2v symmetry, which is in agreement with experimental evidence. The metal-ligand bond energies of the charged species Cu + (C 2H 2) ( D e = 40.6 kcal mol −1) and Cu +(C 2H 4) ( D e = 43.9 kcal mol −1) are significantly higher than predicted in most previous studies. The neutral complexes Cu(C 2H 2) and Cu(C 2H 4) are much more weakly bonded. There are four energy minima with C 2v symmetry on the C 2H 2Cu 2 potential energy surface which have rather strong CuC bonds. The geminal isomer of dicopper-substituted ethylene is 2.8 kcal mol −1 lower in energy than the trans form and 6.4 kcal mol −1 more stable than the cis form. The global minimum structure is an end-on bonded Cu 2 complex with acetylene, which is 5.9 kcal mol −1 lower in energy than 1,1-dicopperethylene. Strong CuC bonds are also predicted for the dicopper- and tetracopper-substituted ethanes. Two energy minima are found on the C 2H 4Cu 2 potential energy surface. The anti form of C 2H 4Cu 2 is 4.5 kcal mol −1 lower in energy than the gauche form. There are also two energy minima on the C 2H 2Cu 4 potential energy surface. The gauche form of tetracopperethane is calculated as only 0.3 kcal mol −1 more stable than the anti form. It is suggested that the deformed structure 5d of C 2H 2Cu 4 with two copper atoms bridging the carbon atoms may be used as a model for the interactions between acetylene adsorbed on a copper(III) surface. The nature of the CuC bonding was analysed using the NBO partitioning scheme and the topological analysis of the electronic charge distribution. The charged complexes Cu +(C 2H 2) and Cu + (C 2H 4) have a T-shaped rather than a cyclic electronic structure. They are held together mainly by Coulombic interactions. The neutral complexes Cu(C 2H 2) and Cu(C 2H 4) show cyclic electronic structures. The weak CuC bonds of the neutral complexes Cu(C 2H 2) and Cu(C 2H 4) are caused by dispersion forces. The rather strong CuC bonds of the copper-substituted ethylenes and ethanes are strongly polarized towards the carbon end. Relativistic effects yield clearly shorter CuC and CuCu bonds.

Co-ordination of amines to [Fe(CN)5(OH2)]3?

Equilibrium constants K for the substitution of co-ordinated H2O in the complex [Fe(CN)5(OH2)]3– by amines have been determined in 0.1 mol dm–3 aqueous acetonitrile solution with I= 0.1 mol dm3–(NaClO4) at 25 °C by UV/VIS spectrophotometry. The species previously assigned to the NH2OH complex is shown to be the product of some oxidative side-reaction. In the absence of complicating factors, values of log K obey the linear free-energy relation log K=a·p K+b with a= 0.05 and b= 3.75 for over 11 pK units from NH2NH3+(pK–0.9) to NH2Me (pK 10.6), i.e. log K is virtually independent of pK. It is suggested that log K may become independent of pK where the increase in intrinsic metal–ligand bond energy with pK is balanced by the increase in the overall loss in solvation (including hydrogen-bonding) energy on co-ordination with pK. In contrast to the pattern previously found with complexes of FeIII and CoIII, a decrease in log K(below that expected from basicity alone) is observed for glycine (attributed to coulombic repulsion between the negative charges on the ligand and Complex), NH3 and NH2OH (both attributed to repulsion from the additional strongly held solvent molecules).

Electron transfer in the electrochemistry of metals, metal compounds and metal complexes

Because electron-transfer occurs at the electrode/solution interface, the oxidation of metal electrodes involves electron removal (within the interface) from a solvent molecule or basic constituent (ligand) rather than from the valence-electron shell of the metal ▪. The difference in oxidation potential for the free ligand in the absence of the metal electrode and in its presence is a measure of the metal-ligand differential bond energy (e.g. for Ag-Cl, -Δ G BF-Δ E° × 23.1 kcal (eV) −150.6 kcal mol −1). Likewise, oxidation of reduced transition-metal complexes and metalloporphyrins are ligand-centered (or solvent-centered) ▪, as is the reduction of the oxidized state ▪. In each case the potential for the oxidation of free ligand (or solvent) is decreased in the presence of metal or reduced-metal complex by an amount that is proportional to the metal-ligand bond energy (-Δ G BF).

Application of molecular mechanics for calculations of metallocarborane molecules

Molecular mechanics was applied for the first time to calculations of metallocarboranes. For selection of the force field parameters, the results of HF/6-31G*ab initio calculations of (C2B4H6)M model compounds were used, where M = Mg and SiH2. The Hill potential rather than the conventional force field was used for description of deformation of the metal-ligand bond energies. Calculations of [RR′C2B4H4]2M, where R and R′= Me3Si or Me, M = Si, Ge, Sn, Pb, similar to dicyclopentadienyl sandwich metal complexes, showed that these compounds have parallel-sandwich conformation for relatively short metal-ligand distances (M = Si, Ge), and bent-sandwich conformation for the longer distances (M = Sn, Pb). As in the case of bent-sandwich metallocenes, this effect can be accounted for by interligand attraction (dispersion interaction) appearing with the increase of interligand distances.

.pi.-Acidity of thioethers and selenoethers: truth or fiction? A comparative density functional study

The relevance of thioether-transition metal complexes as possible intermediates in the hydrodesulfurization of fossil fuels has prompted numerous investigations into the structure and reactivity of transition metal-thioether complexes, as well as its heavier congener selenoether. Using density functional theory (DFT) within the local density approximation (LDA), the electronic and geometrical structure of octahedral (OC)[sub 5]Cr-L with L = CO, NR[sub 3], PR[sub 3], AsR[sub 3], SR[sub 2], SeR[sub 2] (N, P, As, R = F, H; S, Se, R = F, H, Me), and OR[sub 2] (R = H, Me) were studied. All structures have been optimized in C[sub s] symmetry. The resulting geometries are rationalized in terms of orbital interactions. The metal-ligand bond energy has been determined for all systems in calculations where nonlocal corrections were added to the LDA energy expression. It was possible to break down the total metal-ligand bond energy into contributions stemming from [sigma]-donation and [pi]-back-bonding; hence, it is possible to rank thioethers and selenoethers in terms of [sigma]-donor and [pi]-acceptor ability. In general, thio- and selenoethers are rather poor [pi]-acceptors and weak [sigma]-donors. The methyl-substituted chalcogenoethers are better [sigma]-donors than the chalcogenhydrides. It is found that the fluoro-substituted chalcogen systems exhibit good [pi]-acceptor abilities,more » comparable to those found for PF[sub 3]. Surprisingly, the fluorine substitution does not significantly decrease the [sigma]donor ability. The Cr-E bond energies increase in the order Cr-EH[sub 2] < Cr-EMe[sub 2] < Cr-EF[sub 2] (E = O, S, Se). 21 refs., 3 figs., 4 tabs.« less

Gas-phase reactions of lanthanum dication with small alkanes and the photodissociation of LaC2H4n+ and LaC3H6n+ (n = 1 and 2)

The gas-phase reactivity of La{sup 2+} with C{sub 1}-C{sub 4} linear alkanes is investigated and compared to the analogous reactions of La{sup +}. La{sup 2+} is unreactive with methane and ethane but reacts with propane and butane to give dehydrogenation, alkane loss, and for butane charge-splitting reaction products. Interestingly, charge transfer is not observed in agreement with a simple curve-crossing model. Four ligated species, namely, LaC{sub 2}H{sub 4}{sup +}, LaC{sub 2}H{sub 4}{sup 2+}, LaC{sub 3}H{sub 6}{sup +}, and LaC{sub 3}H{sub 6}{sup 2+}, are photodissociated and the photodissociation thresholds are used directly, or together with information from ligand displacement reactions, to assign thermochemical values for these processes and metal ligand bond energies. The bonding properties of LaC{sub 2}H{sub 2}{sup +} and LaC{sub 2}H{sub 2}{sup 2+} are also investigated. A surprising outcome of this study is that the monocation is in some cases more strongly bound to these ligands than the dication. Theoretical calculations by Bauschlicher and co-workers are in good agreement with these results. 28 refs., 6 figs., 5 tabs.

Metal-ligand bond energies and solvation energies for gas-phase transition-metal tris(acetylacetonate) complexes and their negative ions

Metal-ligand bond energies and solvation energies for gas-phase transition-metal tris(acetylacetonate) complexes and their negative ions

Heteronuclear diatomic metal cluster ions in the gas phase: theoretical treatment of magnesium-iron cation (MgFe+) and study of its reactions with hydrocarbons

The unique reactivity of MgFe + is described. This cluster reacts with linear alkenes (C 2 -C 6 ) by displacement of magnesium to form an Fe + -olefin product. In contrast, MgFe + reacts with alcohols by displacement of the iron to form a Mg + -alcohol product. This is the first example of a changeover in the relative metal-ligand bond energies with both metals in a dimer being selectively displaced. MgFe + also reacts with cyclic alkenes. For example, it dehydrogenates cyclohexene to form MgFe + -benzene, which reacts further to displace the magnesium and form benzene-Fe + -cyclohexene. MgFe + is unreactive with small alkanes (C 2 -C 4 ) but reacts with larger alkanes (C 5 -C 7 ) to form a MgFe + -alkene product. An uppere limit of 34±5 kcal/mol for D°(Mg + -Fe) has been established by use of ion-molecule reactions