Marino Basato

Reaction of di-µ-phenylthio-bis(tricarbonyliron)(Fe–Fe) with triphenyl-phosphine: a detailed kinetic and mechanistic study

Di-µ-phenylthio-bis(tricarbonyliron)(Fe–Fe) undergoes a two-step CO substitution reaction with triphenyl-phosphine in decalin. The substitution does not go to completion in the presence of carbon monoxide and the kinetics of the forward and reverse reaction for each step have been studied. The unsubstituted complex undergoes direct attack by PPh3 either on the predominant anti-form or on the very reactive syn-form which is produced in the rate-determining anti–syn-isomerisation. The monosubstituted complex, which is also present mainly in the anti-form in equilibrium with a reactive syn-form, reacts with carbon monoxide through an SN2 mechanism, but a CO-dissociative mechanism is involved in its reaction with the bulkier PPh3. The bis(phosphine) complex so obtained is unable to undergo SN2 displacement even by carbon monoxide and must previously lose one molecule of phosphine. Relative rate constants for bimolecular attack on the co-ordinatively unsaturated intermediate by carbon monoxide and triphenylphosphine have been obtained. Equilibrium and activation parameters for these reactions are reported.

Reaction mechanisms of metal–metal bonded carbonyls. Part VI. Reactions of µ-carbonyl-µ-diphenylgermanediyl-bis(tricarbonylcobalt) with carbon monoxide, triphenylphosphine, and tri-n-butylphosphine

The reversible ‘ring-opening’ reaction of the complex [(OC)3Co(µ-GePh2)(µ-CO)Co(CO)3], (I), with carbon monoxide in decalin to form (µ-GePh2){Co(CO)4}2, (II), proceeds by a path first order in [Complex] and [CO], and the reverse reaction is first order only in [Complex]. Activation and equilibrium parameters have been obtained. Reaction with triphenylphosphine forms the complex (µ-GePh2){Co(CO)3L}2, (III; L = PPh3), probably via[(OC)3Co(µ-GePh2)(µ-CO)Co(CO)2PPh3], produced in a rate-determining CO-dissociative process and subsequently attacked by a second phosphine molecule in a rapid ring-opening reaction. Bimolecular attack by triphenylphosphine also occurs and leads directly to the complex (µ-GePh2){Co(CO)3L}{Co(CO)4}, (IV; L = PPh3). Reaction of the latter with triphenylphosphine produces complex (III; L = PPh3) by a process first order only in [Complex]. Reaction of complex (II) with triphenylphosphine proceeds via rate-determining formation of (I) which then reacts rapidly with the phosphine as described above. Tri-n-butylphosphine can attack the complexes (II) and (IV; L = PBu3) by bimolecular processes. The mechanisms of these reactions are discussed in terms, especially, of relative rate constants for bimolecular attack by carbon monoxide and triphenylphosphine on the complexes or reactive intermediates involved.

Reaction mechanisms of metal–metal bonded carbonyls. Part IV. The substitution reaction of µ-diphenylacetylene-bis(tetracarbonylcobalt) with tri-n-butylphosphine

µ-Diphenylacetylene-bis(tetracarbonylcobalt) undergoes two successive substitution reactions with tri-n-butylphosphine in decalin and the kinetics of these reactions have been studied. Each step shows pseudo-first-order behaviour with kobs=k1+k2[PBu3] under an atmosphere of argon. Activation parameters have been obtained for each path in each step and the effect of carbon monoxide on the paths governed by k1 is consistent with a CO-dissociative mechanism. Relative rate constants for bimolecular attack on the co-ordinatively unsaturated intermediates by carbon monoxide and tributylphosphine have been obtained. The rate parameters for the two dissociative paths are quite similar but the second bimolecular path is governed by a much higher value of ΔH‡, and a much less negative value of ΔS‡, than the first.

Reaction mechanisms of metal–metal bonded carbonyls. Part VII. Reaction of alkynes with µ-carbonyl-µ-diphenylgermanio-bis(tricarbonylcobalt)(Co–Co)

Alkynes [C2Ph2, MeC2Ph, PhC2H, and C2(CO2Et)2] displace the bridging GePh2 group from [(OC)3[graphic omitted]o(CO)3], (I), to form the well known complexes [(OC)3[graphic omitted]o(CO)3] together with (GePh2)n(n= 4–7). The kinetics of reaction of complex (I) with diphenylacetylene in decalin have been studied over a range of temperature. The rate of reaction is first order in [Complex] and [C2Ph2] and the reaction is greatly retarded by carbon monoxide. The results are consistent with a reaction mechanism that involves initial reversible ring opening to form [(OC)3Co(µ-GePh2)Co(CO)4],† a process that is also involved in a reactions of complex (I) with carbon monoxide and triphenyl- and tributyl-phosphine. A mechanism involving intermediates with terminally co-ordinated GePh2 groups cannot be conclusively ruled out but is considered to be less probable.

Reaction of di-µ-diethylphosphido-bis(tetracarbonylmolybdenum)(Mo–Mo) with tri-n-butylphosphine: kinetics and mechanism of a reaction involving seven-co-ordinate complexes

In the absence of light, di-µ-diethylphosphido-bis(tetracarbonylmolybdenum)(Mo–Mo), (I), undergoes a two-step carbonyl substitution reaction with tri-n-butylphosphine in decalin, giving [(Bu3P)(OC)3[graphic omitted]o(CO)4], (II), and [(Bu3P)(OC)3[graphic omitted]o(CO)3(PBu3)], (III). The substitution reaction does not go to completion in the presence of carbon monoxide and the kinetics of the forward and reverse reaction for each step have been studied. All the substitutions occur by a dissociative mechanism involving the reactive intermediate [L(OC)3[graphic omitted]o(CO)3](L = CO or PBu3) which has a co-ordinatively unsaturated six-co-ordinate molybdenum atom. Values of the competition ratio kco/kPBus for bimolecular attack on this intermediate, at 80 °C, range from 67.0 for L = CO to 2.62 × 104 for L = PBu3. Thus the co-ordinatively unsaturated metal centre shows an unexpected high sensitivity to the nature of the incoming ligand and to steric and/or electronic variations on the adjacent metal atom. The substitution of one CO group by PBu3 in (I) does not affect the rate of dissociation of CO, whereas (II) has a different rate of dissociation of PBu3 compared to (III). Activation parameters for the rate constants and competition ratios, together with equilibrium data, for these reactions are reported.

Reaction mechanisms of metal–metal bonded carbonyls. Part V. A kinetic study of the reaction of diphenylacetylene with hexacarbonylbis(tri-n-butylphosphine)dicobalt

The kinetics of reaction of diphenylacetylene with hexacarbonylbis(tri-n-butylphosphine)dicobalt in decalin at 100 °C have been studied. The reaction proceeds in two stages, the initial product being the acetylene-bridged complex [(OC)3Co(µ-C2Ph2)Co(CO)2PBu3]. This then undergoes a substitution reaction with tributylphosphine, released in the first stage, to form (µ-C2Ph2){Co(CO)3PBu3}2. The first stage proceeds by two main paths, one of which rises to a limiting rate with increasing [C2Ph2], the other being first order in [C2Ph2]. Both paths are retarded by carbon monoxide and by tributylphosphine but quantitative study of the latter effect is made difficult by the occurrence of direct attack by phosphine on the complex, apparently to form a more highly substituted cobalt carbonyl. Nevertheless, it can be concluded that the two paths are probably distinguished by whether the acetylene attacks a reactive intermediate before or after reversible dissociation of both a phosphine and a carbon monoxide ligand. When dissociation occurs before attack by acetylene it appears that carbon-monoxide dissociation followed by phosphine dissociation occurs at least ten times more frequently than the reverse order. The reactive intermediate involved when attack by the acetylene occurs before dissociation can be formulated as a carbonyl-bridged complex, formed by metal migration, and the sequence of dissociation is again predominantly CO followed by phosphine.