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Dynamic Behavior in Solution of the Trans ‐Hydridodihydrogen Complex [OsHCl( n 2 ‐H 2 )(CO)(P i Pr 3 ) 2 ]: Ab Initio and NMR Studies
Author(s) -
Bakhmutov Vladimir I.,
Bertrán Juan,
Esteruelas Miguel A.,
Lledós Agustí,
Maseras Feliu,
Modrego Javier,
Oro Luis A.,
Sola Eduardo
Publication year - 1996
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19960020714
Subject(s) - chemistry , ligand (biochemistry) , kinetic isotope effect , reaction rate constant , equilibrium constant , deuterium , crystallography , dihydrogen complex , hydrogen , analytical chemistry (journal) , stereochemistry , hydride , kinetics , physics , atomic physics , biochemistry , quantum mechanics , receptor , organic chemistry , chromatography
Reaction of complex [OsHCl‐(CO)(P i Pr 3 ) 2 ] ( 1 ) with hydrogen gives the trans ‐hydridodihydrogen complex [OsH‐Cl( n 2 ‐H 2 )(CO)(P i Pr 3 ) 2 ] ( 2 ). The H‐H distance in the dihydrogen ligand, determined by variable‐temperature 1 H T 1 measurements, is 0.8 Å. The fast‐spinning nature of the dihydrogen ligand of 2 has been deduced by evaluating the deuterium quadrupole coupling constant for the n 2 ‐D 2 ligand of [OsDCl( n 2 ‐D 2 )(CO)(P i Pr 3 ) 2 ] ([D 3 ] 2 ). Measurements of the equilibrium constants for formation of 2 ( K ) give Δ H 0 = – 14.1 ± 0.5 kcal mol −1 and Δ S 0 = – 30 ±1 e.u. An equilibrium isotope effect K d / K of 2.8 is found for this reaction. The activation parameters for the H 2 loss from 2 are Δ H ‡ = 14.6 ± 0.2 kcal mol −1 and Δ S ‡ = 9.9 ± 0.5 e.u. Hydrogen exchange between the hydrido and n 2 ‐H 2 ligands of 2 takes place at a slow rate ( k obs 2 ) at high temperatures. Activation parameters Δ H ‡ = 17.4 ± 0.5 kcal mol −1 and Δ S ‡ = 1.3 ± 1 e.u., and a kinetic isotope effect ( k obs 2 / k obs 2d ) of 4.6 at 333 K have been determined for this exchange. Ab initio calculations on the model system [OsH‐Cl(H 2 )(CO)(PH 3 ) 2 ] confirm that the trans ‐hydridodihydrogen complex [OsH‐Cl( n 2 ‐H 2 )(CO)(PH 3 ) 2 ] ( 4 ) is the most stable species ( r HH = 0.84 Å). In addition, a cis ‐hydridodihydrogen complex 5 , with a relative energy of 13.8 kcal mol −1 , occupies a local minimum in the potential hypersurface. The energy of other possible trihydrido isomers have also been evaluated. On the basis of the thermodynamic and kinetic data, and the results of the ab initio calculations, the possible mechanism for the H/ n 2 ‐H 2 exchange is discussed.