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Organometallic Thermochemistry Database

J. A. Martinho Simões

Introduction

Standard enthalpies of formation are the building blocks of any thermochemical database. In the case of organic compounds, and, in particular, those which contain only carbon, hydrogen, oxygen, and nitrogen, the enthalpies of formation are most commonly derived from combustion calorimetry experiments. For many organic compounds this technique affords very accurate data, not only because their reaction with oxygen is well characterized and occurs with nearly 100% yield, but also because the standard enthalpies of formation of the products (e.g. carbon dioxide and water) are well known. In other words, for a large variety of organic compounds, combustion calorimetry could well be called an "absolute" method, since the enthalpy of formation of the "fuel" is the only unknown in the combustion enthalpy balance.

Unfortunately, combustion calorimetry is not suited to probe the thermochemistry of many other important classes of substances. This is due to several reasons, but perhaps the most common is that the combustion reaction is not always well characterized. For example, the combustion of an organometallic compound of formula CxHyMz may lead to non-stoichiometric metal oxides, which requires a detailed analysis of these reaction products if the enthalpy of formation of CxHyMz is to be derived. While some difficulties related to ill-defined combustion products can be overcome by using rotating-bomb combustion calorimeters, most of the available thermochemical data for organometallic substances, unlike the case of organic compounds, have not been determined from their reaction with oxygen but from their reaction with some other reactant. The drawback in using alternative experimental methods, such as reaction-solution calorimetry, is that these reactions often involve two or more species whose enthalpies of formation are not available. It will be noted that this situation is frequently met in the present database.

The Organometallic Thermochemistry Database includes thermochemical properties (enthalpies and entropies of reaction) of neutral organometallic compounds (see below), as reported in the literature. Whenever available, or appropriate, the following information is provided in each record: (1) The molecular formula of the compound (or, in the case of a reaction, the molecular formula of one of the compounds); (2) the structural formula of the compound; (3) the physical state of the compound to which the data refer; (4) the solvent, if any; (5) the reaction to which the data refer, or, in the case of a combustion reaction, the product metal oxide and other products that need to be specified; (6) the main experimental method which afforded the thermochemical data; (7) the enthalpy of the reaction, followed by the literature reference(s) where the value was reported; (8) the entropy of the reaction, followed by the literature reference(s) where the value was reported; (9) the standard enthalpy of formation in the condensed state; (10) the standard enthalpy of sublimation or vaporization of the compound, followed by the literature reference(s) where the value was reported; (11) the method used to determine the standard enthalpy of sublimation or vaporization; (12) the standard enthalpy of formation in the gas phase; (13) comments and additional information.

A systematic inventory of enthalpies of reaction is obviously more difficult to organize than a compilation based on enthalpies of formation. The approach followed in the Organometallic Thermochemistry Database, was that, whenever possible, the enthalpy of each reaction would be listed in two different records: one involving all reactants and products in their standard states and the other involving all reactants and products in solution. The former record will enable a future calculation of an enthalpy of formation value if the required auxiliary data become available. The latter is deemed to be useful to derive metal-ligand bond dissociation enthalpies in solution. Both will be helpful in estimating new values or assessing the existing data [1, 2, 3, 4, 5]. Additional enthalpies of solution may be available in the literature (including the reference(s) indicated in a given record), enabling the calculation of reaction enthalpies with the species in different physical states. At any rate, reliably estimating the contribution of enthalpies of solution to a given reaction enthalpy may not be a difficult exercise, particularly in non-polar solvents.

The physical state of many species involved in reactions in solution is not rigorously defined in the Database, reflecting the same problem in the original literature, where accurate information about the concentration of reactants and products is frequently absent. Also, there are abundant examples where substances like hydrogen, methane, etc., are described as being exclusively in solution or in the gas phase and no correction for gas/liquid partition was made. Although this lack of information is obviously unfortunate, it is believed that the uncertainties due to the ill-defined compositions are usually smaller than the uncertainties assigned to the experimental enthalpies of reaction.

The thermochemical convention on uncertainties [6] is unfortunately ignored in many current publications. Moreover, information on how the uncertainties were calculated is often omitted, which hinders the evaluation of "correct" error bars for each experimental quantity. The option taken in the Organometallic Thermochemistry Database was simply to quote the uncertainties given in the original publications.

Another important option that had to be considered concerns the use of ancillary data to derive enthalpies of formation and, in a few cases, also enthalpies of reaction. It must be stressed that parameters like enthalpies of reaction or combustion are the true experimental quantities, whereas enthalpies of formation are computed from those parameters and rely on enthalpies of formation of other compounds. Each author has preferences regarding these auxiliary values, so it may happen that two literature values for the enthalpy of formation of a given compound differ markedly only because they were derived with different ancillary data. Unless stated otherwise, all the enthalpies of formation in the Organometallic Thermochemistry Database have been recalculated on the basis of a single set of ancillary data, in order to insure that the value calculated for a given enthalpy of reaction, by using the enthalpies of formation listed in the Database, will not be affected by inconsistencies in those data. Therefore, in summary, the Organometallic Thermochemistry Database includes the experimental enthalpies of reaction, as reported in the literature, but presents recalculated values for the enthalpies of formation. The thermochemical "consistency" between the Database and the auxiliary values is better than ±1 kJ/mol. The auxiliary data records are provided, together with the literature references from which they were quoted. Estimated values are enclosed in parentheses.

A note regarding data reliability seems appropriate. It is believed, although with some optimism, that most of the enthalpy values presented are accurate within the attached error bars. Unfortunately, there are too few cases where the thermochemistry of the same organometallic compound or of the same reaction have been probed by at least two groups or by at least two different experimental techniques. Whenever comparisons are possible and the data differ noticeably, a recommendation was normally provided, although it must be stressed that "selected data" does not necessarily mean "high quality" data. In some other cases, no reason was found to decide for a given value and a simple or weighted average was recommended. Thermochemical data evaluation for organometallic compounds is still incipient [1, 2, 3, 4, 5] and it may well be possible that different selections will be made in future updates of the Database. It should, finally, be pointed out that authors sometimes make corrections of their own reported data in a later publication. Whenever this is the case, only the corrected values are given in the record, but all the literature references are indicated.

Some values included in the Database have only historical interest. For example, it has been shown that static-bomb combustion calorimetry is unsuitable to probe the thermochemistry of a variety of transition and non-transition element compounds [7]. For example, as remarked by G. Pilcher, static-bomb results for compounds of Si, Al, and Pb "should be rejected and it does not seem at all worthwhile for such measurements on compounds of these elements to be attempted in the future". This and other conclusions in the same paper should be kept in mind when using the Database, particularly when a single record, relying on static-bomb combustion calorimetry, is available for a given compound.

An additional comment regarding data from static-bomb combustion calorimetry must be made: in the case of Ge compounds, although the rotating-bomb method is clearly preferred, static-bomb experiments may afford reliable data. This depends, however, on a correct assignment of the state of the product oxide, GeO2 (amorphous, hexagonal or tetragonal) [7]. In the present Database, all the enthalpies of formation of Ge compounds derived from static-bomb experiments were recalculated by assuming that amorphous GeO2 is formed [7].

In general, the estimated enthalpies of sublimation listed in the Organometallic Thermochemistry Database are probably the most unreliable information provided. To our knowledge, there is no general method for estimating those quantities for most classes of compounds, in contrast to the variety of available procedures for predicting enthalpies of vaporization [6, 7, 8]. Usually, estimated sublimation enthalpy values are, at best, educated guesses, often based on an experimental (or an estimated!) value for a similar compound. The approach followed in the Database was merely to quote the values that have been published, unless an experimental value has been reported. A word of caution on the reliability of many gas-phase enthalpies of formation is thus in order.

Up to a certain extent, the previous remarks also apply to some enthalpies of liquid-vapor phase transitions, which have been reported at boiling temperatures instead of 298.15 K. Although the differences will usually be small, ca. less than 4 kJ/mol, a reassessment of all the available enthalpies of vaporization is clearly desirable. This task is now being undertaken and the results will be included in a future release of the Database.

Whenever possible, the values listed in the Organometallic Thermochemistry Database refer to 298.15 K. However, the thermochemistry of some reactions has been studied at a different temperature, e.g. 293 or 303 K. No mention of this is made in the record, since the corrections to 298.15 K are negligible. However, in those cases where the enthalpy was determined at temperatures further from 298.15 K and no correction was possible, the experimental temperature is indicated. The temperature ranges of equilibrium experiments (leading to van't Hoff plots) are given, unless no information was found in the literature. It will be noticed that this "Second Law method" of obtaining thermochemical quantities has been used for many organometallic reactions. Unfortunately, the data have sometimes poor quality, often because the plot relies on a small number of points. Reaction entropies are particularly sensitive to the errors and may indicate that a result is, at least, questionable. Very simple methods to assess the accuracy of organometallic reaction entropies have been suggested [9].

Although the present Database attempts to be comprehensive for the elements and ligands indicated below, and it is unlikely that important data have been inadvertently missed, gaps will be corrected in future editions. Future extensions of the Database to other elements and families of molecules (e.g. coordination compounds) are envisaged. In any case, an effort to list all the available information on the energetics of neutral organometallic compounds, no matter the source or the experimental technique, has been made. Even kinetics-based data were included, whenever there was evidence that an activation enthalpy would be a good approximation (a lower or upper limit) of a reaction enthalpy. Thermochemical data derived from gas-phase ion techniques will be the subject of a different database. Only a very limited amount of values from those experiments has been included in the Organometallic Thermochemistry Database.

Elements covered in the Database

Groups 1 (except Hydrogen), 2, 3 (including lanthanides and actinides), 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 (except Boron), 14 (except Carbon and Silicon), and 15 (Antimony and Bismuth only).

Types of compounds included in the Database

All the compounds which have at least one metal-carbon bond have been surveyed, with the exception of metal carbides. Several other families of molecules of special importance to organometallic chemistry, such as metal alkoxides, have also been included.

Future additions to the Database

The Database will be updated at least yearly. Future additions will include organo- phosphorus, arsenium and silicon compounds.

References

  1. Martinho Simões, J.A.; Minas da Piedade, M. E. In Energetics of Organic Free Radicals, Martinho Simões, J.A.; Liebman, J.F.; Greenberg, A., Editors, Blackie: London, 1996, chapter 6.
  2. Martinho Simões, J.A. In Energetics of Organometallic Species, Martinho Simões, J.A., Ed.; NATO ASI Series C Vol. 367, Kluwer: Dordrecht, 1992, and references cited therein.
  3. Benson, S. W. Acc. Chem. Res. 1992, 25, 375.
  4. Drago, R. S.; Wong, N. M. Inorg. Chem. 1995, 34, 4004.
  5. Angelici, R. J. Acc. Chem. Res. 1995, 28, 51.
  6. Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic Press: London, 1970.
  7. Pilcher, G. In Energetics of Organometallic Species Martinho Simões, J. A., Editor; NATO ASI Series C Vol. 367; Kluwer: Dordrecht, 1992.
  8. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed. McGraw-Hill: New York, 1988.
  9. Minas da Piedade, M. E.; Martinho Simões, J. A. J. Organometal. Chem. 1996, 518, 167.

Reference 1 is a survey of the methods that have been used to estimate and assess thermochemical data of organometallic compounds. References 2-5 describe additional methods and/or examples. Reference 7 is an authoritative critical review of combustion calorimetry applied to organometallic compounds. Methods of estimating enthalpies of vaporization are described in references 6 and 8. A very simple procedure for assessing or estimating reaction entropies is proposed in reference 9.

List of abbreviations

Group or molecule

Method

Method (Vaporization)

State