Experimental and Theoretical Studies of Nonclassical d0 Cyclopentadienyl Polyhydride Complexes of Molybdenum and Tungsten
Year: 1998
Type: article
Abstract: Low-temperature protonation of compounds Cp*MH 5 (PMe 3 ) (M = Mo, 1; W, 2 ) by HBF 4 ·Et 2 O in CD 2 Cl 2 or CDFCl 2 affords the thermally unstable “hexahydride” derivatives [Cp*MH 6 (PMe 3 )] + (M = Mo, 3; W, 4 ). The corresponding protonation of 1 - and 2 -d 5 affords 3 - and 4 -d 5, respectively. The Δδ on going from H 6 to HD 5 is small for both compounds, but positive for 3 and negative for 4, and no isotopic perturbation of resonance (IPR) is observed. The T 1min at 400 MHz for [Cp*MH 6 (PMe 3 )] + apparently doubles on going from Mo to W (52 ms for 3 and approximately 100 ms for 4 ). Optimized geometries at the restricted Hartree−Fock (RHF) and second-order Møller−Plesset (MP2) levels and energy calculations at higher levels of theory show that these complexes are dihydrogen complexes [Cp*M(H 2 )(H) 4 (PR 3 )] + . The theoretical determination of a dihydrogen complex is consistent with the fact that the experimental T 1min values lie within the expected range for dihydrogen complexes. Examination of the potential energy surface at the MP2 level gives two mechanisms for hydride motion: exchange through a “trihydrogen anion” transition state and rotation of the dihydrogen ligand about its axis. The barrier for the hydride exchange (∼4 kcal/mol) is consistent with the inability to decoalesce the proton NMR signal.
Source: Organometallics
DOI: 10.1021/om9803523
Language: English
Subfield: Organic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2040382000