Allyl and carbonyl complexes of molybdenum and tungsten.

  • Kathleen E. Hill

Student thesis: Doctoral ThesisPhD


This thesis reports an investigation of the solution properties of the compounds [(n3-C3H4R)M(CO)2(MeCN)2X] (R = H, M = Mo or W, X = C1, Br or I; R = Me, M = Mo, X = C1) and their reactions with various phosphorus and arsenic nucleophiles. The ionisation of [(n3-C3H5)W(CO)2C1] to [(n3-C3H5)W(CO)2(MeCN)3]+ [(n3-C3H5)2W2(CO)4C13]- and free nitrile is strongly solvent dependent and slight compared with the molybdenum analogue. No autoionisation process has been detected for the bromo- or iodo-derivatives, nor for the 2-methylallyl molybdenum derivative [(n3-2-MeC3H4)Mo(CO)2(MeCN)2C1] In some donor solvents solvolysis occurs resulting in the partial formation of [(n3-C3H4R)M(CO)2(MeCN)2(solvent)] +X- and/or exchange of coordinated MeCN with solvent in the neutral species. The importance of these processes in the reactions of these compounds is discussed. The reaction of PMePh2 with [(n3-C3H5)M(CO)2(MeCN)2Cl] (M = Mo or W) is solvent dependent. In methanol the adducts [(n3-C3H5)M(CO)2(PMePh2)2Cl] are obtained whilst in acetonitrile reductive elimination of the allyl chloride occurs and the metal (O) compounds [M(CO)2 (MeCN) (PMePh2)3] are formed. The new anions [(n3-C3H5)M(CO)2(PMePh2)Cl2]- were detected in the course of this latter reaction and have been isolated as their o-allylphosphonium salts. Reaction of each of the compounds [(n3-C3H5)Mo(CO)2 ( PMePh2)2Cl], [n3-C3H5)Mo(CO)2 (MeCN)3]BF4, Ph4As [n3-C3H5)2Mo2(CO)4C13] and [PMePh2(o-C3H5)] [(n3-C3H5)Mo(CO)2(PMePh2)Cl2] with excess PMePh2 in acetonitrile results in the final formation of the same metal (O) product [Mo(CO)2(MeCN)(PMePh2)3]. The significance of these observations on the mechanism of the reduction of [n3-C3H5)Mo(CO)2(MeCN)2Cl] is discussed. The compounds [M(CO)2(MeCN)(PMePh2)3] (M = Mo or W) slowly decompose in solution to form mer-[M(CO)3(PMePh2)3] as isolable products. Under suitable conditions the fac-isomers can also be obtained. Both Raman and infrared spectral data have been recorded for these compounds. Triphenylphosphine causes reduction of [(n 3-2-MeC3H4)Mo(CO)2(MeCN)2Cl] and the n3-C3H5 analogue, in both methanol and acetonitrile yielding [Mo(CO)2(MeCN)2(PPh3)2]. The only intermediate isolable from these reactions was [(n3-2-MeC3H4)Mo(CO)2(MeCN)(PPh3)Cl].MeCN. Differences in the pattern of reactivity of PPh3 compared with PMePh2 are discussed in terms of steric and electronic effects. Reactions of [(n3-C3H5)W(CO)2(MeCN)2X] with the ditertiary arsines Ph2AsCH2CH2AsPh2 (dae) Ph2AsCH2AsPh2 (dam) and with the secondary phosphine PHPh2, yield complexes of stoichiometry [(n3-C3H5)W(CO)2(L2)X] (L2 = 2PHPh2, X = Cl; L2 = dae, X = Cl. Br or I) and [(n3-C3H5)2W2(CO)4X2 (dam)] (X = Cl or Br). The latter compounds contain dam bridges which are readily cleaved by pyridine (py), Ph2PCH2CH2PPh2 (dpe), Ph2PCH2PPh2 (dpm) or chloride ions to give [(n3-C3H5)W(CO)2(L2)X] (L2 = dpm, dpe or 2py) and [(n3-C3H5)2W2(CO)4Cl3]- respectively. Under forcing conditions the nitrile complexes react with dpm, dpe and PHPh2 to give low yields of the tungsten (O) derivatives cis-[W(CO)2(L2)2] (L2 = dpm, dpe or 2PHPh2). The molybdenum compounds [(n3-C3H5)Mo(CO)2(PHPh2)2Cl] and cis-[Mo(CO)2(PHPh2)4] have also been prepared for comparative purposes. These compounds are stereochemically non-rigid in solution at ambient temperatures. An X-ray single crystal structure determination of [(n3-C3H5)Mo(CO)2{lcub}P(OMe)3{rcub}2Cl] showed it to be of a structural type not previously known for this class of d4 molybdenum (II) compounds. Its geometry is best described as a pentagonal bipyramid in which the chlorine atom and a carbonyl group are in axial positions and the allyl ligand occupies two adjacent sites in the equatorial girdle. The two dynamic cationic complexes [(n3-C3H4R)Mo(CO)2{lcub}P(OMe)3{rcub}3] BF4 (R = H or Me) have also been characterised and shown to possess a similar pentagonal bipyramidal structure in which the chloride ion is replaced by a P(OMe)3 ligand.
Date of Award1982
Original languageEnglish
Awarding Institution
  • University of Bath

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