A Spectroscopic Study of Charge Distribution in Transition Metal Bridging Hydride Complexes

Author

Shixiong Chen, Eastern Illinois University

Degree Name

Master of Science (MS)

Semester of Degree Completion

1989

Thesis Director

Ellen A. Keiter

Abstract

A series of transition metal bridging hydrides of general formula C[μ-HM₂(CO)₁₀] (C = tetraethylammonium, Et₄N⁺; tetraphenylphosphonium, Ph₄P⁺; of bis(triphenylphosphine)iminium, PPN⁺; M = Cr or W) were synthesized from M(CO)₆ and NaBH₄ bt known methods. As a means of obtaining information about the charge distribution in these complexes, they were allowed to interact in solution and the solid state with triphenylphosphine oxide (TPPO) on the premise that this electron donor could identify sites of relative electron deficiency. Evidence for interaction was sought via solution and solid state infrared (IR) spectroscopy and solution ¹H nuclear magnetic resonance (NMR) spectroscopy.

Solution IR spectra for mixtures of TPPO and complexes in a number of ratios were examined for changes in the bands associated with CO stretching, PO stretching, and MCO bending motions. For comparison, spectra of mixtures of TPPO with Ph₄PCl and Et₄NBr were also collected. Spectral alterations suggest that TPPO associates with both the cation and anion in [Ph₄P][μ-HM₂(CO)₁₀] (M = Cr, W). Evidence is presented that more than one site on the anion in these complexes is involved and that one of these is the carbon atom of an equatorial CO ligand. This result supports proposed mechanisms for reactions of transition metal carbonylates that are catalyzed by TPPO. In contrast, spectra for mixtures with [Et₄N]-[μ-HW₂(CO)₁₀] suggest that there is little or no association with TPPO in these samples. This is interpreted as arising because interactions with TPPO are inhibited by substantial ion pairing for this cation-anion combination. All of the spectra show conclusively that TPPO does not associate with a metal atom in any of the mixtures studied. The solution IR spectra provided no direct information regarding interaction of TPPO with the bridging hydrogen atom. The solid state IR spectra of mixtures of the complexes and TPPO inseveral ratios show no evidence for adduct formation.

In ¹H NMR spectra of the mixtures, the chemical shifts of the bridging hydrogen atoms are altered only slightly from their values in the pure complexes. In view of the presumed low sensitivity of this parameter to weak interactions involving a bridging hydrogen, this result is regarded as neither strongly supporting nor ruling out an association of TPPO with the bridging atom.

Recommended Citation

Chen, Shixiong, "A Spectroscopic Study of Charge Distribution in Transition Metal Bridging Hydride Complexes" (1989). Masters Theses. 2344.
https://thekeep.eiu.edu/theses/2344