Structure and Bonding in Molecular Hydrogen Complexes of Osmium

2 Structure and Bonding in Molecular

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch002

Hydrogen Complexes of Osmium(II) Ian Bytheway , J. Simon Craw , George B. Bacskay and Noel S. Hush* 1

1,2

1

,1,3

Department of Physical and Theoretical Chemistry and Department of Biochemistry, University of Sydney, Sydney, New South Wales 2006, Australia 1

3

A quantum chemical study of geometries,H binding energies, and HD spin-spin coupling constants using self-consistent field theory, second -order Møller-Plesset theory (MP2), and density functional theory (DFT) techniques for a series of molecular hydrogen complexes [Os(NH ) L (η -H )] [L = (CH ) CO, H O, CH COO , Cl , H , C H N, CH CN , CN, NH OH, and NH ] is described. Electron correlation was found to be of crucial importance in the description of the H-H potential and the equilibrium H-H distance. The MP2 and DFT predictions of the geometries and energetics are in reasonable agreement, but there is noticeable divergence in the predicted H-H distances for weakly bound complexes containing trans ligands with strongπ-acceptorproperties. The calculated H-H distances range from 0.95 to 1.40 Åand are consis-tent with stretched molecular hydrogen acting as a ligand rather than dissociating into two atoms bound as hydrides. The H-H distance pre-dicted by both MP2 and DFT methods is in good agreement with that observed in the [Os(NH C H NH ) (CH COO )(η -H )] complex, the only one for which neutron diffraction data are available. 2

3 4

2

z

3

2

(z+2)+

z

-

3 2

2

2

-

3

-

-

5

5

3

2

2

4

2 2

-

3

THE FIRST IDENTIFICATION of an η - Η

2

2

+

complex, W(CO) (P(i-Pr )) (H ) (t-Pr = isopropyl), by Kubas and co-workers (J) in 1984 marked the beginning of a fascinating and rapidly expanding area of inorganic chemistry. Low-tem perature neutron diffraction studies of this complex showed that the hydrogen molecule is bound to the tungsten atom in a sideways manner, with an H - H separation of 0.82 Â (i.e., 10% longer than in H ) , indicating that the H - H 2

2

3

3

2

2

2

Current address: Department of Chemistry, University of Manchester, Oxford Rd., Manchester,

M13 9PL, United Kingdom. *Corresponding author.

© 1997 American Chemical Society

In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.

21

2

22

ELECTRON TRANSFER REACTIONS

bond is somewhat weakened upon complexation (2, 3). Since this discovery, the chemistry of dihydrogen complexes has blossomed, with over 150 com plexes of this type now known (4, 5). In 1971 Malin and Taube (6) synthesized the complex [Os(en) (H2)] (en = ethylenediamine), which has since been characterized as a molecular hydro gen complex (7). More recently, a series of complexes of the general type [ O s ( N H 3 ) L ^ - H 2 ) ] ^ ) , where U represents a wide variety of ligands, have been synthesized and studied by L i and Taube (7-9), and it is these complexes that we shall be concerned with in this chapter. The generic, pseudo-octa hedral geometry of these osmium complexes is depicted i n Figure 1, which shows clearly the trans relationship between the L and H ligands, with the ammonia ligands located i n the equatorial sites. A n interesting, well-characterized feature of these species is the marked dependence of hydrogen-deuterium nuclear spin-spin coupling constants, / , on the nature of the trans ligand L . The value of / varies from about 20 H z when L is acetonitrile, to 4 H z when L is acetone, suggesting that / correlates with the π-donor characteristics of the trans ligand. By way of com parison, the value of / for free H D is 43 H z (10). As the chemical shift of the η - Η signal appears i n the spectral window of -20 to 0 ppm, far from other 2

z


4

2

+2

+

z

z

H D

z

2

H

D

z

H

2

+

H

D

2

Figure 1. The structure

offOsfNH^L^rf-H^ ^. 2


D

2.

BYTHEWAY ET AL.

23

Mokcuhr Hydrogen Complexes of Os(II)

resonances, it is a useful diagnostic probe for U, which may have important applications in biochemical systems (II). The observed coupling constants in these osmium complexes are thought to be indicative of a substantial increase in H - H distance on complexation, considerably more so than in the aforementioned tungsten complex, i n which the observed value of / of 34 H z is consistent with the small 0.1 Â increase in H - H bond length (I). There are other complexes with longer H - H bond lengths, for example ReH (pTol ) (pTol = paratoluene), in which an H - H dis tance is 1.357 Â (12), although this finding could be a consequence of steric crowding due to the high coordination number, as in the ReH (dppe) [dppe = l,2-bis(diphenylphosphino)ethane] complex (13). The "four-legged piano stool" complexes (14,15) also contain hydrogen ligands separated by distances intermediate between that in free hydrogen and the values observed in "clas sical" hydrides, although chemically these complexes behave as polyhydrides (4 5). The aim of the work presented in this chapter has been the characteriza tion of a series of molecular hydrogen complexes of osmium: those synthesized by L i and Taube and some that have yet to be prepared, using quantum chemi cal methods. In particular, we are concerned with the nature of the Os-Η and H - H bonds, the geometries of the complexes, and the influence of the trans ligand on the H - H distance, the binding energy of H , and the H - D coupling constant. The role of electron correlation in the description of the properties of the complexes is also examined. These were performed mainly at the secondorder M0ller-Plesset (MP2) level of theory. However, given the recent devel opments in density functional theory (DFT) and its obvious computational advantages over conventional methods, we also embarked on a study of these dihydrogen complexes using DFT.


H D

7

3

7

2

Computational Details A detailed description of the self-consistent field (SCF)/MP2 computational approaches used has been given elsewhere (16); consequently only a brief dis cussion of the methodology is given here. Effective core potentials (ECP), parameterized so as to account for relativistic corrections, were used in con junction with double- and triple-ζ quality basis sets. The E C P s and basis sets used are those of Stoll and co-workers (17, 18). The Os basis set consists of a [5s4p3d] Gaussian basis set in order to describe the valence 5s, 5p, and 5d electrons, whereas for the C, N , O, and C l atoms, nonrelativistic E C P s were used along with [2s2p] ([3s3p] for Cl) basis sets to describe the valence elec trons. For the hydrogen atoms bound to the osmium, a double-ζ basis set has been used (19) extended with a set of 2p polarization functions (ζ = 0.80). The geometries were optimized at the S C F level with respect to the parameters not involving the ligated H moiety, while the Os-Η and H - H distances were opti mized by pointwise energy calculations using M P 2 perturbation theory. In 2


24


some cases {U = C N ~ , N H O H , or NH3) all geometrical parameters involving the osmium atom were fully optimized at the M P 2 level. In addition to the S C F and M P 2 methods, we have also used techniques based on DFT. These include exchange and correlation effects via a functional, thereby avoiding the lengthy configuration interaction (CI) type expansions of conventional M P and C I methods. The computational advantages of D F T approaches over the standard methods are especially significant when studying large molecules such as transition-metal complexes, making it an attractive alternative. In this work we report results obtained using the B L Y P functional (20), that is, a hybrid of Slater's exchange functional and the Lee, Yang, Parr (LYP) correlation functional with Becke's gradient correction. The H - D coupling constants, / , were calculated by the finite perturba tion technique of Kowalewski et al. (21), using the unrestricted Hartree-Fock (UHF) and U H F + M P 2 (UMP2) method. It was assumed that the Fermi con tact term represents the dominant contribution to / (22), allowing the spin dipolar and orbital effects to be neglected. The calculations were performed using a variety of software packages: H O N D O (23), M O L E C U L E (24-26), T U R B O M O L E (27, 28) and G A U S S I A N 9 2 / D F T (20).


2

H D

H D

Geometries and Vibrational Frequencies The optimized key geometrical parameters are given in Table I, along with the available experimental values. Somewhat surprisingly, the M P 2 H - H , O s - H , and O s - N distances are quite uniform across the range of complexes studied,

Table I. Calculated Bond Lengths (A) for the [0$(ΝΗ )^(η -Η )]( ) 3

2

r(H-H) (CH^CO H 0 2

CH3OO-

ciHC H N CH CN CNNH OH NH 5

5

3

2

3

2

ζ+2

+

Complexes

r(Os-H)

MP2

DFT

MP2

DFT

1.380 1.350 1.389 (1.34) 1.400 1.330 1.300 1.330 1.293 1.256 1.252

1.249 1.250 1.316

1.596 1.590 1.580 (1.60) 1.600 1.630 1.616 1.580 1.614 1.582 1.581

1.613 1.635 1.635

1.314 0.978 0.998 0.985 0.953 1.031 1.057

1.630 1.750 1.689 1.691 1.746 1.670 1.659

N O T E : L means ligand with charge z; r means bond length; MP2 means second-order M0ller-Plesset perturbation theory; D F T means density functional theory. Values in parentheses are neutron difiraction distances at 165 Κ (29). z


2.

BYTHEWAY ET AL.

25

Molecular Hydrogen Complexes of Os(II)

suggesting that the electronic structures of these molecules are very similar. The qualitative trend in the M P 2 distances, for the range of trans ligands con sidered, is reproduced by D F T , although i n same cases, for example, L = C H N and C H C N , the predicted distances differ by as much as 0.3 Â. The variation in Os-U distances is similarly quite small (ca. 0.1 Â) for the first-row ligands, with an increase of about 0.3 A when U = C l " . Comparison with experiment is possible for the acetate complex because the crystal structure for the related complex [Os(en) (CH COO")(r| -H )] has been determined by both X-ray and neutron diffraction techniques (29). The M P 2 and D F T predic tions of the H - H separation are 1.39 and 1.32 Â, respectively, which are in good agreement with the observed value of 1.34 Â. The level of consistency between theory and experiment with respect to the other geometrical parame ters is similarly quite good. An unusual feature of these complexes is the crucial role of electron corre lation in the description of the geometries, especially the H - H distance. S C F theory predicts a H - H separation of 0.8 Â in the acetate complex, compared with the M P 2 value of 1.39 Â (16, 30). Such a large, qualitatively important dif ference necessitated the examination of the validity of single-reference S C F and M P 2 techniques in these calculations. The coefficient of the S C F reference configuration in the M P 2 wavefunction was found to be on the order of 0.85 for all of the complexes examined (16). Although this value may seem low at first, it was found to be due to the large number of double excitations that individually make only small contributions ( fragment and H . 4

z

z+2

+

2


2.

BYTHEWAY ET AL.

29

Molecular Hydrogen Comphxes of Os(H)

charge, whereas the reverse holds when U is an anion. The Mulliken charges are qualitatively similar to those obtained in the Roby-Davidson analyses. The main difference is in the osmium charges, which are predicted to be less posi tive when calculated by the Mulliken method, while the N H and H ligands are slightly more positive. To quantify the degree of σ donation and π back donation, we calculated the difference in gross Mulliken populations between the various dihydrogen complexes and the fragments that result by removing H . The resulting differ ences were partitioned into M O contributions involving the σ and σ* orbitals of H and the d and d orbitals of osmium and are displayed in Figure 4, which shows the average amount of charge gained and lost by the osmium atom and the H ligand, respectively, as a result of dihydrogen complex forma tion. As expected on the basis of the bonding model discussed, the osmium ά and H σ* orbitals gain electron population, while the osmium d and H σ orbitals lose electrons. Given the inherent limitations of the Mulliken analysis (16, 48), we suggest that the actual amounts of charge transferred should be viewed only as a qualitative to semiquantitative guide to the importance of the charge-transfer mechanism. A n alternative approach is the direct study of the energetic effects of charge transfer using the Complete Spatial Orbital Varia tion (CSOV) method (49), as carried out by Craw et al. (16), which suggested that charge transfer is indeed the dominant contribution to the O s - H interac tion energy. The binding energies of H in the complexes, Δ Ε , that is, the energy of the reaction


3

2

2

G

2

n

2

σ

n

2

2

2

2

Β

[Os(NH3) L ]^ ) + H - > [ O s ( N H ) L ^ - H ) ] ^ ) 4

z

+2

+

2

3

2

4

+ 2

2

+

σ*

/

+0.57

+0.67 V

-0.64 -0.40 Os

H

2

Figure 4. Partitioning of the difference in gross Mulliken populations between [Os(NHj L (r?-H^f y complexes and the [OsfiH^L ]^ )* and H into σ and π contributions (averaged over the series of comphxes studied). 4

z

z+

+

2

2


2

30


provides a quantitative measure of the strength of the O s - H bond. The M P 2 and D F T estimates are given in Table H I and show quite clearly that the bond between Os and H is quite strong. The trends in binding energy with U, as calculated by M P 2 and D F T methods, are quite similar, although the D F T predictions are significantly lower, approximately 70% of the M P 2 values. The binding energies are expected to correlate with the H - H distance ( r ) . The plot of the Δ Ε values against the H - H distance, shown in Figure 5, suggests a reasonable degree of correlation, but with considerable scatter at the lowenergy side. The binding energies i n Table III are generally much larger than those obtained for other dihydrogen complexes such as [W(CO) (r| -H )] (36), i n which the binding energy of H (before zero-point energy corrections) is only -19.8 kcal mol" . The reason for this difference is most likely the high positive charge of Os (formal charge of 2e), in comparison with W, which appears with zero formal charge i n the complex just cited. Indeed, according to our recent work (50) on the [\ν(0Ο) (ΡΗ ) (η -Η )] complex, the binding energy of H is -15.6 kcal mol" , when calculated at the D F T (BLYP) level, whereas the corre sponding M P 2 value is -26.7 kcal mol" . Commensurate with the smaller bind ing energy in this tungsten complex, the H - H distance is 0.85 Â at the D F T level, indicating a stretch of only 0.09 Â, in stark contrast with those computed for the Os(II) complexes. 2

2

HH


Β

5

2

2

2

1

3

3

2

2

2

2

1

1

H-D

Spirir-Spin Coupling Constants

L i and Taube (7) have remarked on the sensitivity of the H - D spin-spin cou pling constant, /

, to the trans ligand in a variety of Os(II) dihydrogen corn-

H D

Table III. The Calculated Binding Energies ofH [Os(NH ) (L^)^ -H )]( ) 2

3

2

4

2

z+2

+

- Δ Ε (kcal/mol) β

L

z

(CH^CO H 0 CH COO" ciHC H N CH CN CNNH OH NH 2

3

5

5

3

2

3

MP2

DFT

64.0 57.7 59.0 60.8 40.2 46.7 46.6 40.5 48.2 49.1

45.9 49.7 44.5 45.0 22.9 32.6 33.0 23.7 36.5 37.5

N O T E : -ΔΕ is calculated binding energy of H [Os(NH ) L fa -H )]fc > for each ligand, V. Β

3

4

z

2

2

+2

2

in

+


BYTHEWAY ET AL. Molecular Hydrogen Complexes of Os(II)

2.

31

-37.5 CN"#

·Η~


-42.5 h

X

NH OH \ •

-47.5 h m US

Structure and Bonding in Molecular Hydrogen Complexes of Osmium

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