Isolation of an Extremely Labile Dihydrogen Complex, Cr(CO)3(PPri3)2(H2), Containing the Shortest Ligated H-H Bond

Gregory J. Kubas, Janet E. Nelson, Jeffrey C. Bryan, Juergen Eckert, Linda Wisniewski, Kurt Zilm

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First-row members of the group 6 series of transition metal dihydrogen complexes Cr(CO)3(PR3)22-H2) (R = Cy, Pri) have been synthesized and examined by X-ray crystallography and NMR, infrared, and inelastic neutron scattering spectroscopies. The complexes completely dissociate H2 in solution to give Cr(CO)3(PR3)2, which contains an agostic C-H interaction. An X-ray structure of Cr(CO)3(PPri3)2(H2) showed a side-bonded dihydrogen ligand with H-H = 0.67(5) Å, and proton NMR of the HD isotopomer showed a J(HD) value of 35 Hz. Solid-state 1H NMR spectroscopy of the PCy3 complex indicated that the H-H separation is 0.85(1) Å, which is believed to be a more precise and accurate measure of this key parameter. The barrier to rotation of the η2-H2 has been recalculated to be 1.17(10) kcal/mol, as compared with 1.32(10) and 1.9(1) kcal/mol for the Mo and W analogues, based on H-H distances from solid-state NMR. Correlations of properties of the group 6 species indicate that the H2 → M σ-bonding strength appears to increase as Mo < Cr ~ W but that dπ → H2 σ* back-bonding increases down the group. It might be generally concluded that back-bonding is more of a factor than σ-bonding in influencing stability, H-H distance, and possibly overall bond strengths. Crystallographic data: space group P21/c, a = 13.457-(3) Å, b = 11.493(3) Å, c = 16.639(4) Å, β = 102.25(2)°, Z = 4, R = 3.32%.

Original languageEnglish
Pages (from-to)2954-2960
Number of pages7
JournalInorganic Chemistry
Issue number13
StatePublished - Jun 1 1994


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