Metal-ligand Binding Energies Research Articles

Effects of ligand coverage on properties of palladium clusters. A density functional theory study

Ligand stabilized clusters of palladium are treated at density functional theory level to investigate effects of ligand coverage on properties, such as preferred coordination sites, metal–ligand binding energies, and structure. Systems investigated include cores of Pdn, for n = 1,2,13,16,19,23,35,39, stabilized by CO, PH3, and P(CH3)3, which are either known experimentally or have been proposed as intermediates in syntheses. Some of these systems, e.g. Pd13(CO)i, served as models to study aspects of ligand metal binding. The main results are the following. CO binds preferably as μ3 such that coordination sites share at most one vertex atom. The ligand coverage weakens Pd–Pd bonding and quenches the high spin of pure metal clusters. Ligands tend to stabilize Pd cores showing icosahedral Pd13 structural units such as biicosahedra in Pd19 and Pd23, whereas the corresponding bare metal clusters are most stable as octahedron and decahedron, respectively. The BP86 density functional yields reliable structure parameters and ligand metal binding energies in cases where comparison with experiment or accurate calculations is possible, only Pd(CO)i, i = 1–4, appears to be an exception.

Zero electron kinetic energy photoelectron spectroscopy and density functional theory calculations of gallium–methylamine complexes

Gallium–dimethylamine and –trimethylamine were produced in pulsed laser vaporization molecular beams and studied by threshold photoionization and pulsed-field ionization zero electron kinetic energy (ZEKE) photoelectron spectroscopies and density functional theory calculations. Analyses of the ZEKE spectra yield molecular adiabatic ionization potentials and metal–ligand and ligand-based vibrational frequencies. Comparisons of the experimental and theoretical results establish the ground electronic states of the neutral and ionic complexes. The ionization potentials, Ga+/Ga–N stretching, and Ga+/Ga–N–C bending frequencies are 38 790, 206/177, and 132/128 cm−1 for the dimethylamine complex and 38 081 cm−1, 188/133, and 111/92 cm−1 for the trimethylamine species. The electronic ground states are A′1/2A′ for Ga+/Ga–NH(CH3)2 and A11/2A″ for Ga+/Ga–N(CH3)3. The calculated metal–ligand binding energies of Ga+/Ga–NH(CH2)2 and –N(CH3)3 are 36.2/9.7 and 37.1/8.5 kcal mol−1, respectively.

Activation of Hydrocarbons by W+ in the Gas Phase

The gas-phase reactivity of W+ with hydrocarbons containing up to six carbon atoms has been studied by Fourier-transform ion cyclotron resonance mass spectrometry. W+ is found to be one of the most reactive bare metal cations studied to date with all hydrocarbons except acetylene being activated at thermal energies. The observed reactivity is dominated by dehydrogenation, which is often multiple. Thus, reaction with propene leads to loss of two dihydrogen molecules (yielding WC3H2) and reaction with n-hexane leads to loss of 4 H2 (yielding WC6H6). In all cases, the product ions are themselves reactive with the same neutral species, leading to reaction sequences of various lengths. Isotope labeling, collisioninduced dissociation, and ligand exchange experiments have been extensively carried out in order to try to establish the structure of some of the intermediate and final product ions. It turned out that deriving structures from such experiments is much more difficult than for first-row transition metal ions. This is due to the higher reactivity and metal-ligand binding energies of tungsten.

Density functional calculations of the structures and bond energies of Cr(CO)6 and (?6-C6H6) Cr(CO)2(CX) (X=O, S) complexes

Quantum chemical calculations based on density functional theory have been performed on Cr(CO)6, (η6-C6H6)Cr(CO)3 and (η6-C6H6)Cr(CO)2(CS) at the local and nonlocal level of theory using different functionals. Good agreement is obtained with experiment for both optimized geometries and metal-ligand binding energies. In particular, a comparison of metal-arene bond energies calculated for the (η6-C6H6)Cr(CO)3 and (η6-C6H6)Cr(CO)2(CS) complexes correlates well with kinetic data demonstrating that substitution of one CO group by CS leads to an important labilizing effect of this bond, which may be primarily attributed to a larger π-backbonding charge transfer to the CS ligand as compared with CO.