Bonding Energetics in Organometallic Compounds

311 Pages·1990·5.238 MB·English

by  Tobin J. Marks (Eds.)

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428
A CS SYMPOSIUM S E R I ES
Bonding Energetics
in Organometallic Compounds
Tobin J. Marks,
EDITOR
Northwestern University
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9 Developed from a symposium sponsored
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bk- by the Division of Inorganic Chemistry
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02 at the 198th National Meeting
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10. of the American Chemical Society,
doi: Miami Beach, Florida
90 | September 10-15, 1989
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American Chemical Society, Washington, DC 1990
Library of Congress Cataloging-in-Publication Data
Bonding energetics in organometallic compounds
Tobin J. Marks, editor
p. cm.—(ACS Symposium Series, ISSN 0065-6156; 428).
"Developed from a symposium sponsored by the Division of
Inorganic Chemistry at the 198th National Meeting of the
American Chemical Society, Miami Beach, Florida, September
10-15, 1989."
Includes bibliographical references and indexes
1 ISBN 0-8412-1791-2
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0
w
8.f bon1d. sO—rgCaonnogmreestsaelsl.i c chemistry—Congresses. 2. Chemical
2
4
0
0- I. Marks, Tobin J., 1944- . II. American Chemical Society.
99 Division of Inorganic Chemistry. III. American Chemical Society.
k-1 Meeting (198th: 1989: Miami Beach, Fla.). V. Series.
b
1/ QD411.B65 1990
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0 547'.05—dc20 90-36268
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0. CIP
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d The paper used in this publication meets the minimum requirements of American
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Foreword
1 The ACS SYMPOSIUM SERIES was founded in 1974 to provide a
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w0 medium for publishing symposia quickly in book form. The
8.f format of the Series parallels that of the continuing ADVANCES
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1/ the supervision of the Editors with the assistance of the Series
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Preface
As CONTEMPORARY ORGANOMETALLIC CHEMISTRY HAS GROWN in
sophistication and as we have learned more about reactivity, reaction
mechanisms, molecular architecture, and electronic structure, it is only
natural that we should inquire more deeply about the strengths of the
01 bonds holding organometallic molecules together and how these are
0
pr altered in various chemical transformations. At present, the acquisition
8.
42 and understanding of metal-ligand bond energy information for
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0- organometallic molecules is an active and important area of chemical
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1 research. It impacts directly upon central issues in contemporary
k-
b organometallic, inorganic, organic, physical, enzymatic, and catalytic
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02 chemistry.
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0. This volume and the symposium that served as its basis represent an
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oi: effort to bring together leading experimental and theoretical researchers
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0 | concerned with organometallic bonding energetics in the gas phase, in
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9 solution, and on well-defined surfaces. There has traditionally been
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5, minimal interaction between these diverse communities of activity, and
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ne the present volume attempts to convey the essence of the oral presenta
u
e: J tions and active dialogue that took place at the symposium. In addition,
Dat an overview chapter has been added to introduce basic concepts and
on experimental methodologies, as well as to provide a bibliography of
cati relevant review articles, textbooks, compilations of thermodynamic data,
ubli and other source materials. This book is intended for all chemical
P
researchers who are interested in a broad, in-depth survey of bonding
energetics in organometallic molecules.
I am grateful to the Division of Inorganic Chemistry for generous
financial support of this symposium and to the participants for their
enthusiastic dedication. I also thank the ACS Books Department staff
for their diligence and excellent advice.
TOBIN J. MARKS
Department of Chemistry
Northwestern University
Evanston, IL 60208
March 14, 1990
xi
Chapter 1
Importance of Metal-Ligand Bond Energies
in Organometallic Chemistry: An Overview
Tobin J. Marks
Department of Chemistry, Northwestern University, Evanston, IL 60208
01 The acquisition and analysis of metal-ligand bond
0
h energy information in organometallic molecules
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8. represents an active and important research area
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4 in modern chemistry. This overview begins with a
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0- brief historical introduction to the subject,
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19 followed by a discussion of basic principles,
k- experimental methodology, and issues, and
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1/ concludes with an overview of the Symposium
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0 Series volume organization and contents.
1
0. Finally, a bibliography of thermodynamic data
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oi: compilations and other source materials is
d provided.
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9
1 One need only browse through any general chemistry or introductory
5, organic chemistry text to appreciate just how fundamental the
2
e notions of bonding energetics are to modern chemistry. It is these
n
Ju compilations of bond energy data for simple organic and inorganic
e: molecules that afford students their first quantitative ideas about
at
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on understanding the course of chemical transformations in terms of the
ati strengths of bonds being made and broken. Likewise, the genesis of
c
bli valence ideas as basic as the electronegativities of atoms can be
Pu traced back to perceived irregularities in bond energy trends (1).

Over the past several decades, major advances have occurred in the
accurate measurement and systematization of thermochemical data for
organic and relatively simple (binary and ternary) inorganic mole
cules. The former represent a cornerstone of modern physical
organic chemistry while the latter provide a useful tool for under
standing large segments of main group, transition element, and f-
element reaction chemistry. All such information is of obvious
technological importance for process design and predicting product
characteristics.
The past several decades have also witnessed the phenomenal
development of contemporary organometallic chemistry. This field
has had a major impact on our understanding of structure/bonding/re
activity relationships in metal-centered molecules, in practicing
and/or modelling homogeneous and heterogeneous catalysis, in stoichi-
0097-6156/90AM2S-0001$06.00A)
© 1990 American Chemical Society
2 BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS
ometric synthetic organic chemistry, in metal ion biochemistry, and
in the synthesis of important electronic and ceramic materials.
While one can only be dazzled by the plethora of unprecedented
reactions and equally beautiful molecular structures, and while our
understanding of bonding and reaction mechanisms has advanced
considerably, it is fair to concede that a parallel understanding of
bonding energetics and the thermodynamics of reactions involving most
organometallic compounds does not yet exist. Indeed, in most cases,
we do not know the strengths of the bonds holding these fascinating
molecules together nor do we even know whether these molecules are
kinetic or thermodynamic products of the reactions that produce them.
Although the thermodynamics of organometallic substances is
justifiably a topic of considerable current interest, activity in
this area is by no means new. Thus, Guntz reported the heat of
formation of dimethyl zinc (determined by combustion calorimetric
methods) in 1887 (2) and Berthelot the heats of formation of several
01 mercury alkyls (by similar techniques) in 1899 (3). In 1928,
h0 Mittasch reported the heat of formation of iron pentacarbonyl, again
c
8. determined by combustion calorimetry (4). The 1930's and 1940's saw
2
4 important developments in instrumentation which allowed far more
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0- accurate calorimetry (5) and a rapidly expanding data base of
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9 information on organic (6) and simple inorganic (1) compounds.
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k- Nevertheless, only a handful of organometallic compounds had been
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1/ studied prior to 1940, and the 1939 "The Nature of the Chemical Bond"
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0 contains no bond energy information on organometallic compounds
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0. except for organosilanes (1).
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oi: The period after the Second World War saw greatly accelerated
90 | d bayc tiav igtry owiinn gt hen umsbtuedry ooff goarsg apnhoamseet altleicch ncioqumepso.u ndsB yb y1 9c6a4,l orSikmientnreyr anwda s
19 able to publish the first substantial review article on the strengths
5, of metal-to-carbon bonds (7). Further advances were evident in the
2
e 1970 "Thermochemistry of Organic and Organometallic Compounds," by
n
Ju Cox and Pilcher (8) , as well as in the key 1977 review article by
e: Connor on the thermochemistry of transition metal carbonyls and
at
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on ened interest in bond energy information for a variety of important
ati reasons. The developing sophistication of structural and mechanistic
c
bli organometallic chemistry has raised increasing numbers of thermo­
Pu dynamic questions, sometimes as fundamental as why a particular

reaction does or does not occur (10-12). The power of contemporary
quantum chemistry to map out the energies and spatial characteristics
of molecular orbitals in complex organometallic systems in turn
raises quantitative questions about the strengths of the bonds being
portrayed. Finally, the impressive experimental advances in gas
phase, solution phase, and surface chemical physics have allowed
studies of metal-ligand interactions in heretofore inaccessible
environments and on heretofore inaccessible timescales. Conceptual
bridges to more traditional organometallic chemistry are only just
emerging and should have a major impact.
Basic Concepts. Measurement Approaches, and Issues.
In a strict spectroscopic sense, the bond dissociation energy, D,
Q
for a diatomic molecule AB can be defined as the change in internal
energy accompanying homolytic bond dissociation (Equation 1) at Τ - 0
1. MARKS Metal-Ligand Bond Energies in Organometallic Chemistry 3
Κ in the gas phase (13). The equilibrium bond dissociation energy,
D, measures the depth of the Morse potential well describing AB, and
e
AB(g,0 K) > A(g,0 K) + B(g,0 Κ) (1)
differs from D by the zero-point energy (Equation 2). Here x is
Q e
the anharmonicity constant and ω is the harmonic stretching frequen-
σ
1 xe
D - Do +"[1 - —)*»o (2)
e 2
cy. Such parameters are commonly derived from a Birg-Sponer analysis
of spectroscopic data (13). For thermochemical purposes, the
enthalpy required to homolytically dissociate AB at 298 Κ in the gas
phase (Equation 3) is commonly referred to as the "bond dissociation
01 energy," the "bond disruption enthalpy," the "bond energy," the "bond
0
h
c
8. AB(g,298 K) > A(g,298 K) + B(g,298 K) (3)
2
4
0
90- enthalpy," and the "bond strength (7, 14)." As given in Equation 4,
19 it refers to fragments which are in relaxed (equilibrium) states, and
k-
b
1/ D - AHf(A,g,298 K) + AHf(B,g,298 K) - AHf(AB,g,298 K) (4)
02 AB
1
10. is related to D via Equation 5. Here N is Avagadro's number. Some
doi: authors use abbQreviations of the form ΔΗA(ΑΒ) rather than DAB (13).
90 | D - ND + RT (5)
9 A Q
1
25, The description of bonding energetics for polyatomic molecules
June itsh e moerneth aclopmyp loefx .a toFmoirz ahtoimoonl ecpatnic b es ydsetfeimnse d suacsh ians E qMuXan ti(oXn -6 ,a nw haetroem )a,
ate: temperature of 298 Κ is normally assumed. From this quantity, it is
D
n
o AHatom " AHf(M,g) + nAHf(X,g) - AHf(MX,g) (6)
ati n
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bli also possible to define a mean bond dissociation energy (or enthal­
u
P py) , D; which describes the average M-X bond enthalpy (Equation 7),

as well as first, second, etc. stepwise bond dissociation energies
ÔMX - AH /n (7)
atom
(or enthalpies) (Equations 8, 9, etc). It is not in general correct
D! - AHf(MX. ) + AHf(X,g) - AHf(MX,g) (8)
n llg n
D - AHf(MX_,g) + AHf(X,g) - ΔΗ(ΜΧ. ) (9)
2 n 2 £ η 1>β
to assume that D - D]_, nor that either D or will be the same in
all MX and MY^n^ molecules. The latter issue of whether Djix
m
values are transferable among different environments is of great
interest not only for developing bond energy parameters of broad
applicability but also for understanding ancillary ligand steric and