## Abstract

Inelastic neutron scattering (INS) studies, electronic structure calculations, and molecular mechanics have been carried out on a series of molecular hydrogen complexes, M(CO)_{3}(η^{2}-H_{2})(PR_{3})_{2} (M = Mo, W, R = c-C_{6}D_{11}; M = W, R = i-C_{3}D_{7}), in order to determine relative electronic versus steric (ligand bulk) effects on the barrier to rotation of the H2 ligand. Low-lying vibrational excitations were identified with INS, and high-resolution spectrometers were used to measure the rotational tunneling splitting of the librational ground state on the solid complexes at 4 K. Replacement of the W by Mo changed the latter splitting by about a factor of 3, from 0.89 to 2.82 cm^{-1}. Variation of the phosphine on the other hand changed the frequency by less than 20%. The torsional transitions observed in the range 300-400 cm_{-1} are consistent with the tunneling transitions for a simple double-minimum potential with one angular degree of freedom for the rotation. The barrier heights hindering the H_{2} rotation were determined from these measurements to be 2.4 kcal/mol (M = W, R = i-Pr), 2.2 kcal/mol (M = W, R = i-Pr), and 1.5-1.7 kcal/mol (M =Mo, R = Cy). Ab initio electronic structure calculations showed that the electronic component yields barriers of 1.4-1.8 kcal/mol for M = W and R = H, of 0.8 kcal/mol for M = W and R = Me, and of 0.6 kcal/mol for M = Mo and R = H. The present calculations show the simple double-minimum potential with the minima parallel to the P-M-P axis, which is indeed observed to be the equilibrium position for the H_{2} in the crystallographic studies. Molecular mechanics (MM2) calculations showed no direct steric effects arising from the bulky phosphine ligands on the H_{2} rotational barrier but did show an additional orientational preference (0.6-1.4 kcal/mol) for the H_{2} along the P-M-P axis. The sum of the calculated ab initio and MM2 barriers agreed remarkably well with the observed INS values.

Original language | English |
---|---|

Pages (from-to) | 2324-2332 |

Number of pages | 9 |

Journal | Journal of the American Chemical Society |

Volume | 112 |

Issue number | 6 |

DOIs | |

State | Published - Jan 1990 |

## Fingerprint

Dive into the research topics of 'Molecular Hydrogen Complexes. 6. The Barrier to Rotation of η^{2}-H

_{2}in M(CO)

_{3}(PR

_{3})

_{2}(η2-H

_{2}) (M = W, Mo; R = Cy, i-Pr): Inelastic Neutron Scattering, Theoretical, and Molecular Mechanics Studies'. Together they form a unique fingerprint.