Metal carbonyls, bond enthalpies

Metal-Hydrogen Bond Enthalpy Contributions. Metal Carbonyl... 

Table 12a. Standard enthalpy of formation, A/ff(g), enthalpy of disruption, AHjy, and metal-halogen bond enthalpy contribution, (M-X), in metal carbonyl halides (kJ mol-1)...

Table VIII shows how metal-metal and metal-CO bond enthalpy contributions for Os3(CO), 2 compare with those for adjacent metal carbonyls and with Fe and Ru analogs. These values indicate that CO dissociation and M—M bond cleavage for Os3(CO)12 are more endothermic than for Ru3(CO)12 (288). Substitution reactions of Ru3(CO)12 have associative as well as dissociative rate terms (289), whereas Os3(CO)12 has substitution that is independent of the entering ligand. The first-order rate coefficient for reaction in decane at 92°C with PPh3 is 1.12 + 0.14 second-1 A// is 161 + 2 kJ mol-1 and AS is 101 + 6 J K-1 mol-1, and a simple dissociative mechanism seems likely (290).

Because the enthalpies of formation of such molecular metal carbonyl carbide clusters have yet to be measured or calculated accurately, it has not been possible to calculate their metal-carbon bond enthalpies. However, enough thermochemical information is available on some bulk metal carbides to allow the strengths of both their metal-metal and metal-carbon bonds to be assessed, as indicated in the next section. 

Table 2. Bond length, d, and bond enthalpy, AW, data for metals and metal carbonyls, including calculated metal-ligand bond enthalpies.

Reference to Table 2 indicates the scale of increase expected for metal-ligand bond enthalpy terms as the ratio of ligand to metal decreases. Taking "(Os-CO) for Os(CO)4 units as 201 kJmol", as found for Os3(CO)i2, we estimate that a realistic value of (Os-CO) for Os(CO)3 units will be some 4% higher, i.e. 209 kJmor. Using these values, one can estimate values of the enthalpy of disruption of all the known osmium carbonyls Os.v(CO)j. into gaseous metal atoms and carbon monoxide molecules ... 

Table 1. Standard enthalpy of formation of metal carbonyls [Mm(CO) ] in the gas phase. Bond description and bond enthalpy contributions (T, M and B) to the enthalpy of disruption, AHq...

The benzoyl derivative, PhCOMn(CO)5 and related acyl derivatives have intrinsic interest because they can be prepared by alkyl group migration to coordinated CO, a formal internal nucleophilic attack. This reaction, which is often referred to as carbonyl insertion86), is reversible in certain instances to give the metal alkyl. The enthalpy of disruption for PhCOMn(CO)s (Table 13) can be divided up to give the b.e.c of the Mn-COPh bond if the values of T (Mn-CO) shown in Table 13 are used, then the Mn-COPh enthalpy lies in the range 105 10 kJ mol-1, the lower value being preferred as outlined earlier. 

Table 24. Metal-metal bond enthalpies (kJ mol 1) in binuclear metal carbonyl complexes determined by electron impact and reaction kinetic methods...

The principal assumption of transferability (Section 1.3.) seems to be acceptable on the basis of the data accumulated so far, but clearly requires further testing where organic ligands are concerned. No alternative assumption suggests itself in relation to M-CO bond enthalpies, but the resulting values of T and M could be improved when measurements are made on the more exotic polynuclear metal carbonyls that have been prepared in the last few years. 

Table 4.5. Average values (kJ/mol) of the metal—metal bond enthalpies estimated from metal carbonyls (Mco) and from bulk metals (Mmet) (adapted from Mingos and Wales (1990)).

At this point it is fair to ask. what is the driving force for carbonyl insertion These reactions involve breaking a metal-carbon bond and formation of a carbon-carbon bond. In addition, a bond is formed between the metal and the incoming I. ewis base (CO in the foregoing examples, but frequently a phosphine or an aminel. The enthalpy change, AH. for the reaction... 

Carbon monoxide is a colorless, odorless, flammable, almost insoluble, very toxic gas that condenses to a colorless liquid at — 90°C. It is not very reactive, largely because its bond enthalpy (1074 kj-mol ) is the highest for any molecule. However, it is a Lewis base, and the lone pair on the carbon atom forms covalent bonds with d-block atoms and ions. It is also a Lewis acid, because its empty antibonding tr-orbitals can accept electron density from a metal (Fig. 14.39). This dual character makes carbon monoxide very useful for forming complexes, and numerous metal carbonyls are known. An example of this behavior is its reaction with nickel to give nickel carbonyl, a toxic, volatile liquid ... 

Thermochemical Studies of Organo-Transition Meta) Carbonyls and Related Compounds Table 10. Bond enthalpy contributions, ZJ(M-ArH) kj mol-1, in bis(arene) metal compounds... 

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