## Dissertations from ProQuest

#### Title

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

1990

Article

#### Degree Name

Doctor of Philosophy (Ph.D.)

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

COinS