A study of labile ligands in transition metal complexes

• Main Author: Aris, Keith Roy
• Published: University of Warwick 1972
• Subjects: 540; QD Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.594970

The original aim of this research was to prepare a series of metal - olefin complexes, and investigate the effect of the metal on the olefin. In particular, whether sufficient electron density could be withdrawn from the olefin, to make it susceptible to attack by carbon nucleophiles. Initial results with the n-propene cation, [n-C5H5Fe(CO)2C3H6]+ indicated that attack took place on the metal, in preference to the ligand. Attempts, in order to change the steric conditions around the metal, to prepare the analogous phosphine substituted complexes, for example, [n-C5H5Fe(CO)P¢'3C3H6]+ resulted in unexpected results. As a result the chemistry of the phosphine substituted system was studied in greater detail, particularly the properties of the unknown δ -propyl and δ-allyl derivatives, n-C5H5Fe(CO)r¢3R. These studies showed that hydride abstraction of the δ -propyl derivative, .n~C5rr5Fe(CO)r¢3CH2CH2CH3, did not give the n-propene cation,[n-c5H5Fe(CO)P¢3C3H6]+. This cation was, however, prepared on protonation of the d-allyl derivative, n-C5H5Fe(CO)r¢3CH2crr==CH2' but shown to be very unstable, readily losing propene. Similar n-propene cations have been shown to be sensitive to excess acid. A general mechanism is proposed, in which the n-propene cation loses propene, to give the unsaturated cation, which can then either react further with the nucleophiles, for example the acid anions, to yield covalent products, or, in the case of the tetrafluoroborate or hexafluorophosphate salts, can be isolated. The reaction of bromocarbonyltriphenylphosphine-n-cyclo- pentadienyliron, n-C H 5 Fc( CO )P¢3Br , with Lewis acids, was invesstigated as a route to the unsaturated cation, [n--C5R5Fe(CO)P¢3]. The reaction, however, cave a mixture of cationic species. The three that have been isolated so far are, [n-C5H5Fe(Co)P¢3J+, [n-G 5 II 5 Fe(CO)2P¢3]+' 'and [TC-C 5 H S Fe(CO )2-Br-( CO )2Fe(n-C 5 H 5 )] +. The phosphine lability observed in these reactions was typical of all derivatives in this system, for example, δ -allylcarbonyl- triphenylphosphine-n-cyclopentadienyliron, n-C5H5Fe(CO)P¢3CH2CHi=CH2 was thermally unstable, yielding triphenylphosphine and n-allyl- carbonyl-n-cyclopentadienyliron, 1t-C 5 H 5 Fe(CO)-TC-C 3 H 5. The kinetics of this reaction were studied, and used to suggest a mechanism for the reaction.