Quantum Chemical Study of the Properties of Molecular Hydrogen Complexes of Osmium(II):  A Comparison of Density Functional and Conventionalab InitioMethods

Abstract

The geometries of [Os(NH3)4L z (η2-H2)]( z +2)+ complexes, where molecular hydrogen is trans to the Lz ligand, have been calculated using density functional theory (DFT) and compared with the results of MP2 calculations. The quality of agreement between the DFT and MP2 geometries is found to be dependent on the trans ligand L z . When L z = acetone, water, acetate, and chloride, the agreement between the DFT and MP2 calculations is generally reasonable, and for the L z = acetate complex, the DFT and the MP2 predictions are in acceptable agreement with the experimental geometry. When L z = hydride, pyridine, acetonitrile, cyanide, hydroxylamine, and ammonia, the DFT calculations predict a much shorter H−H bond length and slightly longer Os−H distances when compared with the MP2 values. As the potential energy surfaces are very flat with respect to the H−H stretch, the differences between the DFT and MP2 geometries correspond to energy differences of approximately 3 kcal mol-1 when calculated at the same level of theory. The differences between the DFT and MP2 predictions appear to correlate with the properties of the trans ligand L z , in particular its σ-donor and π-acceptor/donor properties. The DFT predictions of the Os−H2 interaction energy are consistently smaller by ∼25% than the corresponding MP2 values, but the agreement is much better in the case of the Os−L z bond. Solvation by water, estimated by a self-consistent reaction field technique, is found to have little effect on the binding of H2, while significantly reducing the binding energies of the trans ligands, especially those that are charged. The integrated atomic charges for the dihydrogen ligand, obtained by the atoms in molecules method, are slightly negative, while Os varies between 1.01 and 1.81. The DFT calculations generally predict both species to be somewhat closer to neutral than those obtained at the MP2 level; this is consistent with the notion that DFT predicts stronger H−H but weaker Os−H bonding than MP2, i.e., less donation by Os into the antibonding σ* MO of H2.

Ligand Exchange Reactions in Molecular Hydrogen Complexes of Osmium(II): A Quantum Chemical Study Using Density Functional Theory

“…8,9 It was also demonstrated that other properties of these complexes such as the binding energies of H 2 and their dependence on the trans ligand L z and H-D spin-spin coupling constants (J HD ) could be correlated with the spectrochemical parameter 10 of the trans ligand. 8,9 In this paper we present further theoretical work on this series of complexes, focusing on the energetics of the trans ligand exchange reaction:…”

DFT Investigation of the Potential of [H‐M{(NHCH2CH2)3X}] Catalysts (M = Mo, Ru, Os; X = N, P) for the Reduction of N2 to NH3 by H2

“…Moreover, calculations of this kind are considered to predict reactivity trends correctly. A multitude of theoretical investigations of transition metal hydrides,11 among them ruthenium12 and osmium hydrides13 also show the B3LYP hybrid function to be appropriate for obtaining plausible structures and energy trends. For this reason we used the B3LYP hybrid function in this work 14.…”

Relationship between H−D Spin−Spin Coupling and Internuclear Distance in Molecular Hydrogen Complexes

“…It has recently been shown 1a that, at least for η 2 -H 2 complexes of Osmium of the type [Os(NH 3 ) 4 L z (η 2 -H 2 )] ( z +2)+ (type 1 ), where L z is a trans ligand of charge z , it is possible to calculate J HD to reasonable accuracy using density functional theory (DFT). The method which appears to account most successfully for correlation effects in these systems is use of the BLYP functional, which utilizes the Becke gradient corrected exchange functional and the Lee, Yang, and Parr correlation functional; this method is implemented in the GAUSSIAN 94 software package . These are the first DFT calculations of this quantity for molecular hydrogen complexes.…”

“…The bond length dependence of spin−spin coupling in free H 2 is calculated 1a,19 to be very different, increasing to a maximum for all bound levels. Calculations for a typical molecule of type 1 with L = Cl - (ref a) indicate that as hydrogen approaches the Os center, this positive gradient decreases to approximately zero at an Os−H separation of ca. 1 Å greater than the equilibrium value, and attains its final linear negative value at the equilibrium distance (1.63 Å).…”