Title

Thermochemical studies of (M(eta(5)-cyclopentadienyltricarbonyl)(2) (M = chromium, molybdenum, tungsten) and their derivative complexes

Date of Award

1993

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Carl D. Hoff, Committee Chair

Abstract

Metal-hydrogen, metal-metal and metal-ligand bond strength data are of great importance to understanding catalytic activation of hydrogen. This thesis describes results in all three areas. Bond strengths have been determined for the complexes (M($\eta\sp5$-L)(CO)$\sb2$L) $\sb{\rm n}$ (M = Cr, Mo, W; $\eta\sp5$-L = C$\sb5$H$\sb5$, C$\sb5$Me$\sb5$, C$\sb9$H$\sb7$; L = CO, P(OMe)$\sb3$, PEt$\sb3$, PPh$\sb3$; n = 1 or 2) and their hydrides.The Cr-H bond strengths of the hydrides are little affected by ligand substitution. The relative Cr-H bond strengths span less than 3 kcal/mol. The absolute bond strengths are on the order of 61 kcal/mol. The hydrogen, proton and electron transfer reactions of the studied chromium hydride and anion complexes have been combined to complete a thermochemical cycle. The heats of proton and electron transfer reactions span a much wider range ($\approx$20 kcal/mol) than the heats of hydrogen transfer reactions for the same ligand set. The metal-hydrogen and metal-metal bond strengths increase upon descending the chromium triad for the dimers (M(C$\sb5$H$\sb5$)(CO)$\sb3\rbrack\sb2$ and related hydrides.The highly strained Cr-congeners have a Cr-Cr bond strength of 15 kcal/mol or weaker. As expected, increasing steric demand in the ligand sphere caused further decrease in the metal-metal bond strengths, leading to stable radicals even in the solid state for $\eta\sp5$-L = C$\sb5$H$\sb5$, L = PEt$\sb3$, PPh$\sb3$ and for $\eta\sp5$-L = C$\sb5$Me$\sb5$, L = PMe$\sb3$.Heteronuclear reactions were also studied. The enthalpy of metal-metal metathesis, yielding (C$\sb5$H$\sb5$)(CO)$\sb3$Cr-Mo(C$\sb5$H$\sb5$)(CO)$\sb3$ from the homonuclear dimers, has been determined to be nearly zero, thus the Cr-Mo bond dissociation energy is close to the average of bond enthalpies of the parent homonuclear compounds. These values show little effect of ligand exchange on a M-M$\prime$ bond strength in structurally similar compounds. The M-M and M-H bond strengths for the related dimers and hydrides have been found to behave similarly.The kinetic indenyl effect has been attributed to transition state stabilization. Relative bond enthalpy studies on the complexes HM($\eta\sp5$-L)(CO)$\sb3$ and (M($\eta\sp5$-L)(CO)$\sb3\rbrack\sp-$(M = Cr, Mo, W; $\eta\sp5$-L = cyclopentadienyl, pentamethylcyclopentadienyl, indenyl), reported here, suggest that the indenyl effect is primarily due to a ground state destabilization of the M-($\eta\sp5$-L) bond, when $\eta\sp5$-L is a fused aromatic ring system. (Abstract shortened by UMI.)

Keywords

Chemistry, Inorganic; Chemistry, Physical

Link to Full Text

http://access.library.miami.edu/login?url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:9331490