J. Paul Fawcett

Reaction mechanisms of metal–metal-bonded carbonyls. Part 23. Thermal homolytic fission of decacarbonyldirhenium (Re–Re)

The kinetics of thermal decomposition of [Re2(CO)10] in decalin under oxygen have been examined. Taken together with data for the substitution reaction with triphenylphosphine the results are qualitatively and quantitatively consistent with initial reversible homolytic fission followed by further reaction of the [Re(CO)5] radicals. Activation enthalpies obtained from the data show that [Re(CO)5] is considerably more stable than [Mn(CO)5]. The substitution reaction of [Re2(CO)10] with PPh3 must also occur via initial homolytic fission and evidence is presented to suggest that substitution of PPh3 into [Re(CO)5] can be much more rapid than reaction with O2 and proceeds via an associative pathway.

Reaction mechanisms of metal–metal bonded carbonyls. Part 12. Reactions of decacarbonylmanganeserhenium, decacarbonylditechnetium, and their bis(triphenylphosphine) derivatives

The kinetics of the following reactions in decalinhave been studied : the complexes [MnRe(CO)10] and [Tc2(CO)10] with triphenylphosphine, [MnRe(CO)8(PPh3)2] and [Tc2(CO)8(PPh3)2] with carbon monoxide, and decomposition of [Tc2(CO)10] under oxygen. The results are consistent with initial reversible homolytic fission of the metal–metal bonds so that the activation enthalpies for the rate-limiting steps can be taken as a kinetic measure of the strengths of the metal–metal bonds. Attention is drawn to correlations between the activation enthalpies for the decacarbonyls and spectroscopic parameters related to the bond strengths.

Reaction mechanisms of metal–metal bonded carbonyls. Part XI. Reactions of nonacarbonyl(triphenylphosphine)dirhenium and octacarbonylbis(triphenylphosphine)dirhenium

The Kinetics of the reversible reaction [Re2(CO)9(PPh3)]+ PPh3⇌[Re2(CO)8(PPh3)2]+ CO in decalin have been studied in each direction and activation parameters obtained. The Kinetic behaviour is quite simple and is consistent with a ligand-dissociative mechanism. However, it is also consistent with metal migration and homolytic-fission mechanisms. The reaction of [Re2(CO)9(PPh3)] with carbon monoxide has quite different activation parameters from the reaction with triphenylphosphine and these reactions cannot, therefore, both go via simple rate-determining homolytic fission. Reaction of [Re2(CO)8(PPh3)2] with PPh3 leads to mononuclear carbonylphosphine products and is half order in [Re2(CO)8(PPh3)2]. This reaction most probably does go via reversible homolytic fission. Reactions with oxygen, nitrogen monoxide, and iodine are also described.

Kinetic and spectroscopic parameters for some metal–metal bonded carbonyls

Activation enthalpies for reactions of several metal-metal bonded carbonyls are shown to correlate well with force constants for the metal-metal vibration, and with energies of a u.v. absorption band tht can be assigned to a σ→σ* transition between orbitals of the metal-metal bond.

Reaction mechanisms of metal–metal bonded carbonyls; substitution via homolytic fission

Evidence is presented that the substitution by triphenyl phosphite, in decalin or cyclohexane of one triphenylphosphine in the complex [Mn(CO)4PPh3]2 occurs via initial reversible homolytic fission of the Mn–Mn bond followed by an associative interchange reaction of triphenyl phosphite with the Mn(CO)4PPh3 radical.

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 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 mechanisms of metal–metal-bonded carbonyls. Part 19. Homolytic fission of bis[tetracarbonyl(triphenylphosphine)manganese]-(Mn–Mn) as a path for thermal substitution

The complex [{Mn(CO)4(PPh3)}2] reacts with P(OPh)3 in cyclohexane at 40–50 °C to form [Mn2(CO)8(PPh3)-{P(OPh)3}]. A detailed study of the dependence of the rate on the concentrations of complex, P(OPh)3, and PPh3 shows that the kinetics are fully consistent with a mechanism involving initial, reversible, homolytic fission. This is followed by reversible bimolecular substitution of PPh3 in [Mn(CO)4(PPh3)] by P(OPh)3 before final formation of the product by combination of two unlike radicals. This is the first clearly demonstrated example of a mechanism of this type operating in a simple thermal-substitution reaction.

Reaction mechanisms of metal–metal-bonded carbonyls. Part 15. Reactions of nonacarbonyl(triphenylphosphine)dimanganese and octacarbonylbis( triphenyiphosphine)dimanganese

Kinetics of reactions of [{Mn(CO)4(PPh3)}2] with carbon monoxide, oxygen, and triphenylphosphine are reported and the major reaction path in each case is concluded to involve initial reversible homolytic fission. Infrared evidence is adduced for the formation of substantial amounts of [Mn(CO)3(PPh3)2] in cyclohexane after reaction with PPh3. Reaction with oxygen proceeds by an additional path, not yet fully characterised but of almost equal importance. Data for reaction of [Mn2(CO)9(PPh3)] with CO, PPh3, and oxygen are also reported. Kinetic, and some thermodynamic, parameters for reactions of the decacarbonyls [M2(CO)10](M2= Mn2, Tc2, Re2, or MnRe) and their triphenylphosphine derivatives are compared, and the importance of steric effects in determining the kinetic strengths of the metal–metal bonds is pointed out.