Title

Kinetic and thermodynamic studies of chromium, molybdenum and tungsten organometallic complexes

Date of Award

1990

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Committee Member

Carl D. Hoff, Committee Chair

Abstract

The rates of reaction of (PC$\sb6$H$\sb{11}$)$\sb3$W(CO)$\sb3$(L) (L = H$\sb2$, D$\sb2$ and N$\sb2$) with pyridine have been studied using stopped flow kinetics. The molecular nitrogen system shows simple first order loss of N$\sb2$ with a rate constant of 75 s$\sp{-1}$ at 25$\sp\circ$C. The reactions of the H$\sb2$ and D$\sb2$ complexes are complicated due to the presence of both molecular hydrogen (deuterium) and dihydride (dideuteride) complexes. These data are resolved and interpreted in terms of a rapid loss of molecular hydrogen (k = 469 s$\sp{-1}$ for H$\sb2$ and 267 s$\sp{-1}$ for D$\sb2$ at 25$\sp\circ$C). The hydride complex reacts an order of magnitude slower than the molecular hydrogen complex (k = 37 s$\sp{-1}$ for H$\sb2$ and 33 s$\sp{-1}$ D$\sb2$ at 25$\sp\circ$C).The substitution of py by P(OMe)$\sb3$ in the complexes (P(C$\sb6$H$\sb $)$\sb3$)$\sb2$M(CO)$\sb3$(py), M = Cr, Mo, has been studied. These results are combined with the calorimetric measurements, and with the data on the W complex, to give a complete picture for ligand substitution in these sterically crowded complexes. The relative rates of dissociation of the M-py bond are in the order Cr $>$ Mo $>$ W in the approximate ratio 4,800/300/1. The relative rates of reacton of the formally coordinatively unsaturated complexes (P(C$\sb6$H$\sb $)$\sb3$)$\sb2$M(CO)$\sb3$ with trimethyl phosphite is Mo $>$ W $>$ Cr in the approximate ratio 80/36/1. The activation energies are about 5 kcal/mol higher than the ground state energies for all three metals, indicating similar rearrangements in the transition state. The rate of reaction of (C$\sb5$Me$\sb5$) (CO)$\sb3$Cr$\sp{\cdot}$ with (C$\sb5$H$\sb5$) (CO)$\sb3$MoH and (C$\sb5$H$\sb5$) (CO)$\sb2$(PPh$\sb3$)CrH have been studied by stopped flow kinetics. Two different hydrogen atom transfer pathways have been observed.The solution hydrogen-bonded complexes, (CO)$\sb5$MoN$\sb2$H$\sb4$ and (CO)$\sb5$MoNH$\sb3$ have been studied by calorimetry and FT-IR spectroscopy. The energy of hydrogen bonding is $-$5.49 $\pm$ 0.1 kcal/mol for (CO)$\sb5$MoN$\sb2$H$\sb4$ and $-$3.52 $\pm$ 0.1 kcal/mol for (CO)$\sb5$MoNH$\sb3$ in THF.

Keywords

Chemistry, Physical

Link to Full Text

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