Thesis (Ph.D., Chemistry) -- University of Idaho, December 2014 | Ligand exchange reactions between various metal carbonyl derivatives and incoming ligands such as phosphine and phosphites have been extensively studied. It is known that prototypical organometallic compounds such as (?5 -C5H5)Mn(CO)3 are virtually unreactive to thermal exchange of carbonyl ligands while its indenyl derivative (?5 -C9H7)Mn(CO)3 undergoes carbonyl exchange at moderate temperatures. Several examples of rate acceleration by indenyl ligands relative to cyclopentadienyl ligands have now been reported and the observed “indenyl effect” attributed to the ability of the indenyl ligand being able to
shift its binding from ?5 to ?3 during associative reactions. This ?5 to ?3 haptotropic rearrangement lowers the energy of activation. A similar effect has been observed for metal nitrosyl compounds. In these cases, the nitrosyl ligand is normally observed to be bound linearly to the metal and formally treated as a three-electron donor. Associative reactions of metal carbonyl, nitrosyl compounds with ligands have been shown to be accelerated relative to all carbonyl compounds. This acceleration is attributed to a “nitrosyl effect” in which the nitrosyl ligand can go from a linear, three-electron donor to a bent, one-electron donor in the associative reaction intermediate. The family of Group VI compounds ? 5 - C5H5)M(CO)2(NO) and their indenyl derivatives offer an interesting platform to examine both indenyl and nitrosyl effects in the same molecules The (?5 -C5H5)M(CO)2(NO) derivatives for Cr, Mo and W are known, but only (?5
-C9H7)Cr(CO)2(NO) has been reported for the indenyl derivatives. The chromium compound was prepared using CrCl(CO)2(NO)Py2 as a precursor. This research set out to prepare the series of Group VI compounds, (?5 -C9H7)M(CO)2(NO), by conventional routes using the reaction of (?5 - C9H7)M(CO)3 anions with known NO donors such as Diazald. Subsequent carbonyl exchange kinetics studies were planned with the goal of investigating the indenyl and nitrosyl effects down the Group VI family of compounds. (?5 -C9H7)Cr(CO)2(NO) was successfully prepared by the proposed route, albeit in low yield, but neither the Mo or W compounds could be prepared. A simple alkylation reaction to prepare (?5 -C9H7)M(CO)3(CH2C6H5) was carried out after many failures to ensure ourselves that the intermediate (?5 -C9H7)M(CO)3 anions were being successfully prepared. Our kinetics studies were therefore limited to (?5 -C9H7)Cr(CO)2(NO). As noted above, the corresponding (?5 -C5H5)Cr(CO)2(NO) derivative has been reported to be thermally unreactive with PPh3 although derivatives of the form (?5
-C5H5)Cr(CO)(NO)L, where L = phosphine or phosphite, can be prepared photochemically. We have carried out a series of pseudo-first order kinetics studies of (?5 -C9H7)Cr(CO)2(NO) with PMe3, PBu3, PMe2Ph and P(OMe)3. This series represents a range of ligand nucleophilicity and cone angle (steric bulk). Energies of activation were determined by measurements of the rates of reaction across a range of temperatures, while the entropy and enthalpies of activation were extracted from Eyring plots. The strongly negative entropies of activation are consistent with an associative reaction. The data do not allow us to parse out the relative contributions of indenyl and nitrosyl effects, but the observation that ?S? values are on the order of -49 eu, while those of (?5 -C9H7)Mn(CO)3 are around -26 eu argues for an associative mechanism.