Importance of MetalâLigand Bond Energies in Organometallic

Chapter 1

Importance of Metal-Ligand Bond Energies in Organometallic Chemistry: An Overview Tobin J. Marks

Downloaded by 206.214.8.105 on November 27, 2015 | http://pubs.acs.org Publication Date: June 25, 1990 | doi: 10.1021/bk-1990-0428.ch001

Department of Chemistry, Northwestern University, Evanston, IL 60208

The acquisition and analysis of metal-ligand bond energy information in organometallic molecules represents an active and important research area in modern chemistry. This overview begins with a brief historical introduction to the subject, followed by a discussion of basic principles, experimental methodology, and issues, and concludes with an overview of the Symposium Series volume organization and contents. Finally, a bibliography of thermodynamic data compilations and other source materials is provided. One need only browse through any general chemistry or introductory organic chemistry text to appreciate j u s t how fundamental the notions of bonding energetics are to modern chemistry. I t i s these compilations of bond energy data f o r simple organic and inorganic molecules that a f f o r d students t h e i r f i r s t quantitative ideas about the strengths of chemical bonds as w e l l as the p o s s i b i l i t y of understanding the course of chemical transformations i n terms of the strengths of bonds being made and broken. Likewise, the genesis of valence ideas as basic as the e l e c t r o n e g a t i v i t i e s of atoms can be traced back to perceived i r r e g u l a r i t i e s i n bond energy trends (1). Over the past several decades, major advances have occurred i n the accurate measurement and systematization of thermochemical data f o r organic and r e l a t i v e l y simple (binary and ternary) inorganic molecules. The former represent a cornerstone of modern p h y s i c a l organic chemistry while the l a t t e r provide a useful t o o l f o r understanding large segments of main group, t r a n s i t i o n element, and felement reaction chemistry. A l l such information i s of obvious technological importance f o r process design and p r e d i c t i n g product characteristics. The past several decades have also witnessed the phenomenal development of contemporary organometallic chemistry. This f i e l d has had a major impact on our understanding of structure/bonding/rea c t i v i t y r e l a t i o n s h i p s i n metal-centered molecules, i n p r a c t i c i n g and/or modelling homogeneous and heterogeneous c a t a l y s i s , i n s t o i c h i 0097-6156/90AM2S-0001$06.00A) © 1990 American Chemical Society

In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


2

BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS

ometric synthetic organic chemistry, i n metal ion biochemistry, and i n the synthesis of important e l e c t r o n i c and ceramic materials. While one can only be dazzled by the plethora of unprecedented reactions and equally b e a u t i f u l molecular structures, and while our understanding of bonding and r e a c t i o n mechanisms has advanced considerably, i t i s f a i r to concede that a p a r a l l e l understanding of bonding energetics and the thermodynamics of reactions i n v o l v i n g most organometallic compounds does not yet e x i s t . Indeed, i n most cases, we do not know the strengths of the bonds holding these f a s c i n a t i n g molecules together nor do we even know whether these molecules are k i n e t i c or thermodynamic products of the reactions that produce them. Although the thermodynamics of organometallic substances i s j u s t i f i a b l y a topic of considerable current i n t e r e s t , a c t i v i t y i n t h i s area i s by no means new. Thus, Guntz reported the heat of formation of dimethyl zinc (determined by combustion c a l o r i m e t r i c methods) i n 1887 (2) and Berthelot the heats of formation of several mercury a l k y l s (by s i m i l a r techniques) i n 1899 (3). In 1928, Mittasch reported the heat of formation of i r o n pentacarbonyl, again determined by combustion calorimetry (4). The 1930's and 1940's saw important developments i n instrumentation which allowed f a r more accurate calorimetry (5) and a r a p i d l y expanding data base of information on organic (6) and simple inorganic (1) compounds. Nevertheless, only a handful of organometallic compounds had been studied p r i o r to 1940, and the 1939 "The Nature of the Chemical Bond" contains no bond energy information on organometallic compounds except f o r organosilanes (1). The period a f t e r the Second World War saw g r e a t l y accelerated a c t i v i t y i n the study of organometallic compounds by calorimetry and by a growing number of gas phase techniques. By 1964, Skinner was able to p u b l i s h the f i r s t s u b s t a n t i a l review a r t i c l e on the strengths of metal-to-carbon bonds (7). Further advances were evident i n the 1970 "Thermochemistry of Organic and Organometallic Compounds," by Cox and P i l c h e r (8) , as w e l l as i n the key 1977 review a r t i c l e by Connor on the thermochemistry of t r a n s i t i o n metal carbonyls and r e l a t e d compounds (9). The post-1980 period has been one of height ened i n t e r e s t i n bond energy information f o r a v a r i e t y of important reasons. The developing s o p h i s t i c a t i o n of s t r u c t u r a l and mechanistic organometallic chemistry has r a i s e d increasing numbers of thermo dynamic questions, sometimes as fundamental as why a p a r t i c u l a r r e a c t i o n does or does not occur (10-12). The power of contemporary quantum chemistry to map out the energies and s p a t i a l c h a r a c t e r i s t i c s of molecular o r b i t a l s i n complex organometallic systems i n turn r a i s e s q u a n t i t a t i v e questions about the strengths of the bonds being portrayed. F i n a l l y , the impressive experimental advances i n gas phase, s o l u t i o n phase, and surface chemical physics have allowed studies of metal-ligand i n t e r a c t i o n s i n heretofore i n a c c e s s i b l e environments and on heretofore inaccessible timescales. Conceptual bridges to more t r a d i t i o n a l organometallic chemistry are only j u s t emerging and should have a major impact. Basic Concepts. Measurement Approaches, and Issues. In a s t r i c t spectroscopic sense, the bond d i s s o c i a t i o n energy, D , f o r a diatomic molecule AB can be defined as the change i n i n t e r n a l energy accompanying homolytic bond d i s s o c i a t i o n (Equation 1) at Τ - 0 Q


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Metal-Ligand Bond Energies in Organometallic Chemistry

3

Κ i n the gas phase (13). The equilibrium bond d i s s o c i a t i o n energy, D , measures the depth of the Morse p o t e n t i a l w e l l d e s c r i b i n g AB, and e

AB(g,0 K)

> A(g,0 K) + B(g,0 Κ)

(1)

d i f f e r s from D by the zero-point energy (Equation 2). Here x i s the anharmonicity constant and ω i s the harmonic s t r e t c h i n g frequenQ

e

σ

x

1


D

D

- o

e

+ 2

e

"[1 - —)*»o

(2)

cy. Such parameters are commonly derived from a Birg-Sponer a n a l y s i s of spectroscopic data (13). For thermochemical purposes, the enthalpy required to h o m o l y t i c a l l y d i s s o c i a t e AB at 298 Κ i n the gas phase (Equation 3) i s commonly r e f e r r e d to as the "bond d i s s o c i a t i o n energy," the "bond d i s r u p t i o n enthalpy," the "bond energy," the "bond AB(g,298 K)

> A(g,298 K) + B(g,298 K)

(3)

enthalpy," and the "bond strength (7, 14)." As given i n Equation 4, i t r e f e r s to fragments which are i n relaxed (equilibrium) states, and D

AB

- AHf(A,g,298 K) + AHf(B,g,298 K) - AHf(AB,g,298 K)

(4)

i s r e l a t e d to D v i a Equation 5. Here N i s Avagadro's number. Some authors use abbreviations of the form ΔΗ(ΑΒ) rather than D B (13). Q

A

A

D - N D + RT A

(5)

Q

The d e s c r i p t i o n of bonding energetics f o r polyatomic molecules i s more complex. For homoleptic systems such as MX (X - an atom), the enthalpy of atomization can be defined as i n Equation 6, where a temperature of 298 Κ i s normally assumed. From t h i s quantity, i t i s n

AHatom " AHf(M,g) + nAHf(X,g) - AHf(MX ,g) n

(6)

also p o s s i b l e to define a mean bond d i s s o c i a t i o n energy (or enthal py) , D; which describes the average M-X bond enthalpy (Equation 7), as w e l l as f i r s t , second, e t c . stepwise bond d i s s o c i a t i o n energies ÔMX - A H

atom

/n

(7)

(or enthalpies) (Equations 8, 9, e t c ) . I t i s not i n general correct D! - A H f ( M X . ) + AHf(X,g) - AHf(MX ,g) n

D

2

llg

- AHf(MX _ ,g) + AHf(X,g) - Δ Η ( Μ Χ . n

2

(8)

n

£

η

1>β

)

(9)

to assume that D - D]_, nor that e i t h e r D or w i l l be the same i n a l l MX and M Y ^ n ^ molecules. The l a t t e r issue of whether Djix values are transferable among d i f f e r e n t environments i s of great i n t e r e s t not only f o r developing bond energy parameters of broad a p p l i c a b i l i t y but also f o r understanding a n c i l l a r y l i g a n d s t e r i c and m


4


e l e c t r o n i c e f f e c t s . The adherence of r e d i s t r i b u t i o n processes (e.g., Equation 10) to a purely s t a t i s t i c a l model i s one straightforward MY

n

+ MX

n

^

MY . X n

x

x

+ etc.

(10)

t e s t of t r a n s f e r a b i l i t y . For elaborate polyatomic organometallic molecules, the atomizat i o n enthalpy of taking the entire molecule to gaseous atoms i s not a p a r t i c u l a r l y informative quantity, and i t i s more meaningful to use the terminology of d i s r u p t i o n enthalpies which describe d i s s o c i a t i o n into i d e n t i f i a b l e fragments. For a homoleptic compound, MR , this term i s defined as i n Equation 11 f o r the process depicted i n Equation 12 (7, 14). There i s now a considerable body of n

^ d i s r u p t » AHf(M.g) + nAHf(R*,g) - AHf(MR ,g) Downloaded by 206.214.8.105 on November 27, 2015 | http://pubs.acs.org Publication Date: June 25, 1990 | doi: 10.1021/bk-1990-0428.ch001

n

MRnCg)

M

> ( g ) + nR-(g)

(11) (12)

AHf(M,g) and AHf(R-,g) data available (15-17). Because of uncertaint i e s i n t r a n s f e r a b i l i t y surrounding AH^isrupt values and because chemical s i t u a t i o n s are more commonly encountered i n which the enthalpies of single d i s s o c i a t i o n / d i s r u p t i o n processes as i n Equation 13 are more u s e f u l , the author suggests defining the M-R bond disrupI^MR

> L^M + R-

(13)

t i o n enthalpy (or d i s s o c i a t i o n energy) as i n Equation 14 (12). A l though problems of t r a n s f e r a b i l i t y are s t i l l important, i t should be D(L M-R) - AHf(LnM) + AHf(R») - AHf (I^MR) n

(14)

recognized that many processes of greatest chemical i n t e r e s t w i l l take place w i t h i n the same coordination sphere (e.g., Equation 15) and that D(L M-R) i m p l i c i t l y contains the information needed to understand/predict such processes. Ways to test parameter transfern

I^MR

> I^MR'

(15)

a b i l i t y i n such systems and to account for the lack of useful v o l a t i l i t y i n many of the molecules of i n t e r e s t w i l l be discussed s h o r t l y (vide i n f r a ) . Returning to Equations 11 and 12 and s i m i l a r s i t u a t i o n s , problems a r i s e when AHf values of disruption/atomization fragments are not a v a i l a b l e . Various schemes e x i s t to estimate such quantities by apportioning the energetics among the constituent bonds (7., 14). The r e s u l t i n g , estimated M-R energies are generally referred to as bond enthalpy contributions and denoted f o r an MR molecule by Ë(MR) . Importantly, such values do not incorporate the enthalpies of reorganization involved i n r e l a x a t i o n of the newly formed fragments to t h e i r equilibrium configurations. As such, Ê values are less r e a d i l y applied to transformations of the type depicted i n Equation 15. The c l a s s i c a l approach to deducing the thermodynamic properties of a substance has t r a d i t i o n a l l y been combustion calorimetry (18) . Here the substance of i n t e r e s t i s completely burned, and the heat of n


1.

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5


combustion apportioned among the known/assumed and unknown c o n s t i t uent bond energies. While such approaches are w e l l - s u i t e d for simple organic molecules, the apportioning problem becomes exceeding l y d i f f i c u l t f o r complicated organometallic molecules. In addition, organometallic molecules are notoriously d i f f i c u l t to burn cleanly, so that major errors can be introduced i n data a n a l y s i s (despite many advances i n calorimeter design and combustion procedures). A u s e f u l a l t e r n a t i v e approach i s sometimes high temperature calorimetry with other oxidants (e.g., halogens) (19). Various types of s o l u t i o n r e a c t i o n calorimetry have proven e s p e c i a l l y u s e f u l i n thermochemical studies of organometallic molecules. Provided the reactions employed are clean, rapid, and q u a n t i t a t i v e , i t i s possible to employ processes as i n Equation 16 to express r e l a t i v e M-R bond d i s r u p t i o n enthalpies i n terms of the measI^MR

+ X

> I^MX + RX

2

(16)

ured heat of r e a c t i o n and generally a v a i l a b l e or r e a d i l y estimated d i s s o c i a t i o n energy parameters (Equation 17). Particularly useful reactions f o r t h i s purpose are halogenolysis and protonolysis (12, D(L M-R) n

soln

» ΔΗ

Γ χ η

+ 0(Ι^Μ-Χ)

+ D(RX)

5θ1η

s o l n

- D(X ) 2

s o l n

(17)

20). When t h i s procedure i s c a r r i e d out i n a vacuum-tight i s o p e r i b o l (quasi-adiabatic) batch t i t r a t i o n calorimeter (2J.) , there i s the added advantage of anaerobic security f o r h i g h l y s e n s i t i v e samples, a r e a d i l y incorporated protocol for measuring heats of s o l u t i o n , and a means ( v i a t i t r a t i o n ) of s e q u e n t i a l l y i n v e s t i g a t i n g a series of bonds i n a molecule as w e l l as providing a b u i l t - i n check on r e a c t i o n stoichiometry and calorimeter system i n t e g r i t y . As should be evident i n Equation 16, derived r e l a t i v e D(L M-R) parameters i n such a determination are "anchored" to a p a r t i c u l a r D(L M-X) value. Conversion from a series of r e l a t i v e bond enthalpies to a scale approximating absolute can be achieved i f D(L M-X) can be estimated. Use of the corresponding D]^(MX ) value appears to be r e a l i s t i c f o r M - an early t r a n s i t i o n metal, lanthanide, or a c t i n i d e i n the same formal oxidation state, and X = halogen (22). Absolute D(M-R) values can be obtained v i a one- and two-electron redox sequences as shown i n Equations 18-23 and 24-28 (23-22). The a v a i l a b i l i t y of lower-valent, less c o o r d i n a t i v e l y saturated l^M species i s of course e s s e n t i a l for n

n

n

n

LnMR + X I^M-X

> I^MX + RX

2

> LnM + 1/2 X X·

> 1/2 X

R-X LnMR D(LnM-R) - Δ Η t h i s approach.

Γ χ ( 1 6 )

+ AH

(18) (19)

2

(20)

2

> R- + X·

(21)

> LnM + R*

(22)

r x ( 1 9 )

- 1/2 D(X ) + D(R-X) 2

(23)

In addition, note that Equations 24-29 as w r i t t e n


6


L MR2 + 2 X n

n

2

2 Χ·

LnMR

2

D(L M-R ) - A H n

2

r x ( 2 4 )

2

-> I^M + X

Ι^ΜΧ

2R-X

(24)

-> L MX + 2 RX

2

-> X

(25)

2

(26)

2

•> 2 R* + X·

(27)

-> L^M + 2 R*

(28)

4- Δ Η

Γ Χ ( 2 5 )

- D(X ) + 2

D(RX)

2

a f f o r d a D(M-R) value, while stepwise cleavage of M-R groups w i l l y i e l d D(L M(R)-R) and D(LnM(X)-R) parameters referenced to a D(M-X). Bond enthalpy values derived from the aforementioned c a l o r i metric approaches and a l l other D(M-R) q u a n t i t i e s determined i n s o l u t i o n (vide i n f r a ) are not s t r i c t l y gas phase q u a n t i t i e s as s p e c i f i e d i n the formal d e f i n i t i o n s (Equations 1,3,4). However, since most chemistry of i n t e r e s t to organometallic chemists occurs i n the s o l u t i o n phase, i t can be argued that t h i s i s not an undesirable state of a f f a i r s . There i s also good evidence that D(R-H) values f o r simple organic molecules are the same i n nonpolar solvents as i n the gas phase (27), and that the heats of s o l u t i o n ( A H i ) of uncharged organometallic molecules are generally rather small i n nonpolar solvents (-2-4 kcal/mol) (12, 21, 24). Conversion of s o l u t i o n phase D(M-R) data to the gas phase requires Δ Η χ and heat of sublimation ( A H ) data f o r the species involved (e.g., on both sides of Equation 18). Such parameters are r e a d i l y a v a i l a b l e f o r small inorganic and organic molecules, while A H i values can be e a s i l y measured f o r the organometallic molecules of i n t e r e s t . In contrast, AH data are not e a s i l y measured f o r organometallic compounds and may be inaccessible f o r compounds of low v o l a t i l i t y and/or thermal stability. Hence, i t i s u s u a l l y assumed that AH b i s the same f o r a l l organometallic molecules involved. Where t h i s assumption has been checked by experiment (19, 28), i t has been found to be reason able, but not extremely accurate. Other s o l u t i o n c a l o r i m e t r i c techniques which have been applied to the study of organometallic molecules have included batch, flow, heat f l u x (Calvet), and pulsed, time-resolved photoacoustic c a l o r i metry. The former method i s a simpler i s o p e r i b o l v a r i a n t of the aforementioned batch t i t r a t i o n technique and involves breaking ampoules of the compound of i n t e r e s t into a r e a c t i o n dewar containing an excess of reagent. I t i s not chemically as s e l e c t i v e as a method i n which the reagent i s added i n a t i t r a t i o n mode, and i n some systems the excess reagent i s l i k e l y to cause side reactions. The heat f l u x or Calvet technique i s an isothermal approach, and has the advantage that reactions can be studied at elevated temperatures and pressures (19., 29, 30). I t has also been employed i n a solventless mode using halogen reagents as a substitute f o r combustion c a l o r i metry i n studying metal carbonyls (19, 30). As i n the batch tech nique, the reagent cannot be added incrementally. I t i s , i n prin c i p l e , adaptable to the study of extremely a i r - s e n s i t i v e compounds. Pulsed, time-resolved photoacoustic calorimetry employs a pulsed uv l a s e r source to induce a photochemical r e a c t i o n i n the molecule of n


(29)

s o

5 0

n

η

SUD

s o

n

S U D

su


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i n t e r e s t (e.g., Equation 30). The absorbed photochemical energy which i s subsequently released as thermal energy i s measured by hi/


MLn

> MLn.x + L

(30)

acoustic techniques (from the r e s u l t i n g shock waves), and combined with quantum y i e l d information on nonradiative processes, y i e l d s the M-L bond enthalpy as w e l l as k i n e t i c information on recombination processes (31-33.) . Comparison of photoacoustic D(M-L) data with gas phase data f o r the same molecules obtained by other techniques (vide i n f r a ) has been p a r t i c u l a r l y informative i n q u a n t i f y i n g the ener getics of weak M-solvent interactions (32). Disadvantages of organometallic systems f o r photoacoustic calorimetry may include the lack of clean, well-defined photochemistry, low nonradiative quantum y i e l d s , and the p i t f a l l s of multiphoton processes at high l i g h t fluences (32). Equilibrium and k i n e t i c techniques have also been successfully applied to studying metal-ligand bonding energetics i n s o l u t i o n . In the former approach, r e l a t i v e metal-ligand bond enthalpies are obtained by studying (usually v i a NMR, IR, or o p t i c a l spectroscopy) the positions of various e q u i l i b r i a (e.g., Equation 31). Study of the equilibrium constant as a function of temperature combined with LnM-R + R'H

LnM-R' + RH

(31)

0

a van't Hoff analysis, y i e l d s ΔΗ° and AS values f o r the process (34). Of course, data must be acquired over a s u f f i c i e n t l y wide temperature range, which may be prohibited by thermal i n s t a b i l i t y . A l t e r n a t i v e l y , i t can be assumed that AS ~ 0 f o r the e q u i l i b r a t i o n process, so that ΔΗ « AG (34-36) . Major disadvantages of the e q u i l i b r a t i o n technique f o r organometallic systems include the lack of s u i t a b l y clean or rapid e q u i l i b r i a i n many systems, the inherent inaccuracy i n measuring large equilibrium constants (only small steps i n ΔΗ values can be used i n constructing scales of r e l a t i v e bond enthalpies), and the i n a b i l i t y to measure absolute bond enthalpies. K i n e t i c techniques f o r measuring metal-ligand bond enthalpies focus on measuring the a c t i v a t i o n enthalpies f o r homolytic processes as i n Equation 32. The r e s u l t i n g organic r a d i c a l i s u s u a l l y trapped. I^MR

> LnM + R-

(32)

I f i t i s reasonably assumed that ΔΗ + f o r the reverse, recombination r e a c t i o n i s small (ca. 2-3 kcal/mol), then the absolute L^M-R bond enthalpy can be d i r e c t l y calculated from ΔΗΦ f o r the forward reaction (37-42). Radical cage d i f f u s i o n e f f e c t s cannot be ignored i n such k i n e t i c analyses, however they appear only to be important i n more viscous media. Good agreement has recently been noted between k i n e t i c a l l y - d e r i v e d Co-R bond enthalpies and those obtained using one-electron oxidative ( c f . , Equations 18-23) c a l o r i m e t r i c techniques (26). These k i n e t i c techniques are of course only applicable to systems with r e l a t i v e low M-R bond enthalpies and clean, homolytic d i s s o c i a t i o n chemistry. D i f f e r e n t i a l scanning calorimetry has been applied to thermo dynamic studies of organometallic compounds i n the s o l i d state (43).


7

8


Thus, the enthalpies of various decomposition reactions (e.g., gaseous o l e f i n loss) can be r e a d i l y measured f o r rather small samples (a few mg.). Drawbacks of t h i s technique include the necessity of c o r r e c t i n g data to 298 K, the uncertain influence that s o l i d state e f f e c t s may have on measured ΔΗ values, and the p o s s i b i l i t y of side reactions at elevated temperatures. The l a t t e r e f f e c t s can be assayed by thermogravimetric and v o l a t i l e product a n a l y s i s . Electrochemical techniques have also been applied to determining metal-metal bond energies i n dinuclear organometallic molecules (44). Using a combination of redox e q u i l i b r a t i o n experiments with reagents of known p o t e n t i a l s and f a s t scan c y c l i c voltammetry, i t i s possible to obtain E° data f o r the kinds of reactions shown i n Equations 33 and 34, respectively. The r e s u l t i s the free energy (but not the Mn (CO)

> 2 Mn(CO) -

(33)

2Mn(CO) -

> 2 Mn(CO)

(34)

Mn (CO)

> 2 Mn(CO)


2

10

5

5

2

10

5

5

(35)

enthalpy) of Equation 35. In Equations 36-39, an approach to measuring metal-hydrogen bond enthalpies i n a c e t o n i t r i l e s o l u t i o n i s shown which uses pKa, electrochemical, and tabulated gas phase e l e c t r o n a f f i n i t y data (45). When approximate corrections f o r s o l v a t i o n and entropy of s o l v a t i o n e f f e c t s are made, good agreement I^MH

> LnM" + H

LnM" H I^MH

+

+

(36)

> I^M

(37)

> Η·

(38)

> I^M + Η·

(39)

i s obtained with ϋ(Ι^Μ-Η) values measured by other techniques. C r u c i a l to t h i s method i s the amenability of the subject compounds to both the pK and electrochemical (Equation 37) measurements. Solvation e f f e c t s are expected to be important i n coordinatively unsaturated systems. Gas phase approaches to the study of organometallic metal-ligand bonding energetics can be roughly divided into those employing ionmolecule reactions and those which employ neutral molecular precur sors. In the most common embodiment of the former approach, bare gaseous metal ions are created by evaporation/ionization or pulsed l a s e r desorption/ionization of bulk metal sources or by d i s s o c i a t i o n / i o n i z a t i o n of v o l a t i l e molecular precursors (e.g., by electron impact). These ions are then brought to well-defined k i n e t i c energies e i t h e r by mass f i l t e r i n g i n a guided ion beam (tandem mass spectrometer) apparatus (46, 42) or by trapping i n c i r c u l a r o r b i t s i n an ion cyclotron resonance (ICR) spectrometer (48, 49) . The l a t t e r device can be r e a d i l y coupled to a highly s e n s i t i v e Fourier transform mass spectrometry detection system. A wide v a r i e t y of informative a


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experiments can then be performed i n these two experimental configurations . In a common type of ion-molecule experiment (46), a guided beam of monoenergetic metal ions i s impinged upon gaseous neutral molecules under single c o l l i s i o n conditions, and the reaction products analyzed by mass spectrometry as the k i n e t i c energy of the metal ions i s varied. For the r e l a t i v e l y straightforward case of endothermic reactions (e.g., Equation 40), the MH y i e l d i s analyzed as a function of M k i n e t i c energy to obtain the threshold E-p for the +

+


M

+

+ RH

> MH+ + R*

(40)

reaction. Making the reasonable assumption (which has been v e r i f i e d i n several simple cases) that the endothermic r e a c t i o n has a n e g l i g i b l e k i n e t i c a c t i v a t i o n b a r r i e r , then Equation 41 applies. For processes as i n Equation 42, the r e l a t i o n s h i p of Equation 43 holds, where IP i s the i o n i z a t i o n p o t e n t i a l . In addition to the assumption +

D(M-H ) - D(R-H) - E M+ + RH

(41)

T

> MH + R+

D(M-H) - D(R-H) + IP(R) - IP(M) - E

(42) (43)

T

of n e g l i g i b l e a c t i v a t i o n b a r r i e r s , other uncertainties i n the above analyses include the cross-section model used to c a l c u l a t e E-p and i n t e r n a l energy uncertainties i n the reactants. The l a t t e r issue includes the p o s s i b i l i t y that M i s formed and used i n an electroni c a l l y excited state. In addition, large molecular ions w i l l have many degrees of freedom so that cross-section versus k i n e t i c energy p l o t s w i l l be rather f l a t and E-p values d i f f i c u l t to accurately determine i n such cases. Although the nature of the cross-section model and the data analysis are quite d i f f e r e n t , bond enthalpy information can also be derived from exothermic ion-molecule reactions by studying product k i n e t i c energy release d i s t r i b u t i o n s (.50) . ICR-FTMS experiments have been employed to derive thermodynamic information using several approaches (48). These include studies of exothermic and endothermic ion-molecule reactions, e q u i l i b r a t i o n studies, competitive c o l l i s i o n - i n d u c e d d i s s o c i a t i o n reactions, and photodissociation studies. Exothermic reactions i n v o l v i n g thermalyzed (cooled by an i n e r t buffer gas) ions provide brackets on metal-ligand bond enthalpies, as i l l u s t r a t e d by Equation 44 which implies that D(Fe-C£H4 ) > 66 kcal/mol. Endothermic reactions can also be studied i n the manner described above for ion beam experi+

+

Fe+ + C H C1 6

5

> FeC H + + HC1 6

(44)

4

+

ments (vide supra). However, the greater uncertainty i n M energies i n ICR-FTMS experiments renders t h i s approach somewhat less accurate than that described above. ICR-FTMS experiments can y i e l d r e l a t i v e metal-ligand bond enthalpy information by measuring equilibrium constants f o r ligand e q u i l i b r a t i o n processes (e.g., Equation 45) and examining other reactions where a n c i l l a r y thermodynamic information e x i s t s (e.g., Equation 46). These approaches assume that AS « 0 and


9

10


ML + L

5=* ML

+ L

(45)

y i e l d r e l a t i v e metal-ligand bond enthalpies unless an absolute anchor + \ > y Fe

FeNH + 2

+

^3

46

( )


point has been established. The gaseous metal-ligand ion reactants for such studies are normally prepared by ion-molecule reactions or by the decomposition of neutral organometallic precursors. Compet i t i v e c o l l i s i o n - i n d u c e d d i s s o c i a t i o n studies (e.g., Equation 47) can also be employed to estimate metal-ligand bond enthalpies by observ ing the d i s t r i b u t i o n of fragments f o l l o w i n g c o l l i s i o n s with neutral molecules. F i n a l l y , photo-dissociation techniques can be used to obtain metal-ligand bond enthalpies by determining the photon AM+B

> AM+ + Β or

(47) > MB+ + A

energetic threshold f o r d i s s o c i a t i o n (Equation 48). Such an approach i s only workable i f the ion absorbs the photon, i f the metal-ligand hi/

ML+

> M+ + L

(48)

ion undergoes clean photodissociation ( i t i s i n some cases possible to disentangle side r e a c t i o n s ) , and the metal-ligand ion has a high density of low-lying excited states so that d i s s o c i a t i o n i s thermodynamically- rather than spectroscopically-determined. In a l l of the above ion-molecule experiments, i t should be evident that s t r u c t u r a l ambiguities can a r i s e i n products which are assigned purely on the basis of an M/e r a t i o . In some cases, a d d i t i o n a l product s t r u c t u r a l information can be obtained by i s o t o p i c l a b e l l i n g , c o l l i s i o n - i n d u c e d d i s s o c i a t i o n , and photodissociation experiments. The l a t t e r techniques r e l y upon understanding the nature of the fragments produced when the metal-ligand ion i s dissociated. I t should also be noted that ICR-FTMS experiments are by no means l i m i t e d to small metal-ligand ions, and that bracketing studies leading to r e l a t i v e metal-ligand bond enthalpies ( c f . , Equations 45, 46) can be s u c c e s s f u l l y performed on ions as elaborate as (C H ) ZrR+ (50). Flowing afterglow techniques are yet another approach to studying ion-molecule reactions of organometallic species (51, 52). Here ion-molecule reactions are conducted i n a flow of i n e r t buffer gas (usually He) , so that a l l reagents and products are completely thermalyzed. Representative experiments have employed bracket ing/competition experiments ( c f . , Equations 45-47) to obtain informa t i o n such as hydride a f f i n i t i e s of metal carbonyls and t r a n s i t i o n metal formyl complex s t a b i l i t i e s (51, 52.) . Another class of gas phase experiments begins with the molecular organometallic compounds of i n t e r e s t and measures the added energy required f o r ligand d i s s o c i a t i o n . The mass spectrometric appearance p o t e n t i a l method r e l a t e s the metal-ligand bond energy to the mass 5

5

2


1.

MARKS


spectrometer e l e c t r o n a c c e l e r a t i n g p o t e n t i a l needed to observe the metal-ligand fragment of i n t e r e s t and the fragment i o n i z a t i o n p o t e n t i a l (e.g., Equation 49). Major u n c e r t a i n t i e s i n t h i s approach +


DiOlLn) - ΑΡ(ΜΙ^.! ) - ΙΡίΜΙ^.χ)

(49)

concern the p r e c i s i o n with which the threshold can be measured and the possible formation of thermally "hot" products. Photodissocia t i o n approaches were discussed i n the previous section. Very low pressure p y r o l y s i s k i n e t i c techniques measure ( v i a mass spectrometry) the a c t i v a t i o n energy f o r l i g a n d d i s s o c i a t i o n under c o l l i s i o n l e s s conditions (53, 54). As i n the s o l u t i o n k i n e t i c approach (vide supra), i t i s assumed that the k i n e t i c b a r r i e r f o r recombination i s small. This approach suffers from problems assoc i a t e d with c a t a l y t i c sample decomposition on the hot walls of the apparatus and r a d i c a l chain reactions. A recent improvement i n t h i s technique (£3, 54) , the LPHP (laser powered homogeneous p y r o l y s i s ) method, employs a pulsed i n f r a r e d (CO2) l a s e r to heat an SFg "sensi t i z e r " i n the r e a c t i o n mixture containing a low pressure of the compound of i n t e r e s t . An N thermalizing bath and an i n t e r n a l M(CO) "thermometer" of known thermolytic c h a r a c t e r i s t i c s are also present so that sample p y r o l y s i s i s r a p i d and homogeneous, reactor w a l l s remain c o o l , and the i n t e r n a l temperature i s known accurately. Sample decomposition i s allowed to proceed f o r 10-15 before rapid expansion cooling and mass spectrometric a n a l y s i s . This technique has been s u c c e s s f u l l y applied to a wide v a r i e t y of organotransition metal compounds. Measurements of metal-ligand bonding energetics on clean metal surfaces are generally c a r r i e d out by e q u i l i b r i u m or k i n e t i c methods (.55). In the former approach, adsorbate coverage, Θ, i s measured as a function of e q u i l i b r i u m pressure at varying temperatures. The enthalpy of adsorption i s r e l a t e d to these isotherm data v i a Equation 50. In k i n e t i c methods, desorption rates are measured as a function 2

n

(

aIn ρ ~) ^-constant d T

A H

ads — RT

(50)

2

of temperature and A H j c a l c u l a t e d assuming a n e g l i g i b l e a c t i v a t i o n energy f o r the (reverse) adsorption process. These approaches are complicated by the r e a l i s t i c p o s s i b i l i t i e s that A H j i s dependent upon Θ, that the adsorbate bonding mode may be temperature-dependent, and that the surface i t s e l f may be heterogeneous. Surface thermochemical measurements are c l e a r l y most informative when analyzed i n conjunction with d e t a i l e d adsorbate surface s t r u c t u r a l information (56, 57). ac

s

a(

s

Symposium Volume Motivation and Organization. The present Symposium Series volume and the September 1989 symposium from which i t derives are part of an e f f o r t to b r i n g together leading investigators a c t i v e i n the diverse s u b d i s c i p l i n e s concerned with bond energies i n organometallic compounds. There has t r a d i t i o n a l l y been l i t t l e i n t e r a c t i o n between such researchers, and the present work attempts to convey the essence of the o r a l presentations and


11


12


active dialogue which occurred at the symposium. A l l contributions have been c a r e f u l l y reviewed by experts i n the respective f i e l d s , and a l l e f f o r t s have been made to expedite the e d i t o r i a l process so that coverage i s as up-to-date as possible. This volume begins with a discussion of r e l a t i v e l y simple systems i n the gas phase. Thus, Armentrout, Beauchamp, Bowers, and F r e i s e r focus upon the course and thermodynamics of both endothermic and exothermic gas phase reactions between metal ions and simple organic or inorganic molecules. Metal-ligand bond enthalpy data are reported f o r a wide range of ions, n e u t r a l s , and metal-metal bonded species. Important trends i n bonding energetics are revealed and convincingly explained. Richardson next describes gas pjiase organozirconium chemistry of the molecular ion (C5H5)2ZrCH3 and draws u s e f u l connections to known s o l u t i o n phase chemistry. He also compares the gas and s o l u t i o n phase redox properties of metallocenes and metal acetylacetonates. Lichtenberger reports a new way to estimate metal-ligand bond energies based upon i o n i z a t i o n p o t e n t i a l s and simple molecular o r b i t a l concepts. The approach i s s u c c e s s f u l l y applied to several classes of metal carbonyl complexes. The focus then s h i f t s to s o l u t i o n phase studies, and Halpern presents a d e t a i l e d study of metal-alkyl bond energies derived from k i n e t i c measurements. Close a t t e n t i o n i s given to supporting l i g a n d e f f e c t s on D(M-alkyl) as w e l l as to the possible errors incurred i n such analyses. A complementary c o n t r i b u t i o n by Koenig further discusses the determination of bond energies by k i n e t i c methods and the importance of r a d i c a l p a i r cage e f f e c t s i n such determinations. Hoff and Kubas report c a l o r i m e t r i c and k i n e t i c studies of metal-ligand binding energetics and k i n e t i c s i n a f a s c i n a t i n g s e r i e s of molecular N2 and H2 complexes. Wayland next describes the CO and H2 chemistry of several series of rhodium porphyrin complexes, e l u c i d a t i n g the thermodynamics and k i n e t i c s of several unprecedented migratory CO i n s e r t i o n and reduction processes. Metal-ligand bond energies, a n c i l l a r y l i g a n d e f f e c t s , and t h e i r consequences f o r unusual r e a c t i v i t y are then surveyed by Marks for a broad s e r i e s of organolanthanides. Drago next discusses the a p p l i c a t i o n of Ε and C parameters to accurately p r e d i c t i n g l i g a t i o n thermodynamics i n several classes of organometallic/coordination compounds. Photoacoustic calorimetry i s an a t t r a c t i v e and r e l a t i v e l y new technique f o r studying metall i g a n d bonding energetics i n s o l u t i o n , and Yang applies t h i s method to Mn-L and C r - o l e f i n bond energies i n a s e r i e s of (C5H5)Mn(CO)2L and Cr(C0)5(olefin) complexes. F i n a l l y , Simoes presents a " c l a s s i c a l " ( c a l o r i m e t r i c ) study of metal-ligand bonding energetics i n a series of (C5H5)2MR2 and (C5H5)2ML complexes, as w e l l as a " n o n - c l a s s i c a l " (photoacoustic c a l o r i m e t r i c ) study of Si-Η bond energies i n a series of s i l a n e s . In the area of surface phenomena, Somorjai discusses the r e s t r u c t u r i n g processes which occur when clean metal surfaces undergo chemisorption, and the major consequences that such s t r u c t u r a l reorganizations have for the measured thermodynamics of chemisorp t i o n . The binding of small molecules to chemically modified surfaces i s next surveyed by S t a i r . Dramatic e f f e c t s on the chemisorption thermodynamics of Lewis bases and on heterogeneous c a t a l y t i c a c t i v i t y are observed when clean molybdenum surfaces are modified with carbon, oxygen, or s u l f u r . The t h e o r e t i c a l s e c t i o n begins with a presenta t i o n by Pearson on the use of absolute e l e c t r o n e g a t i v i t y and hard/-


1. MARKS


s o f t concepts f o r understanding bond energies and chemical r e a c t i v i t y i n organometallic systems. These rather simple concepts are shown to have considerable p r e d i c t i v e power. Harrison and A l l i s o n next focus on t r a n s i t i o n metal-N bond energy systematics i n a series of gas phase ions. Correlations are drawn between experimental data and the r e s u l t s of h i g h - l e v e l e l e c t r o n i c structure c a l c u l a t i o n s . Finally, Z i e g l e r discusses the use of l o c a l density f u n c t i o n a l c a l c u l a t i o n a l methods to probe M-CH3, M-H, and M-CO bonding trends as a function of the p o s i t i o n of M i n the Periodic Table. Important and convincing explanations of trends i n s o l u t i o n experimental data can be made.


Acknowledgments The author thanks a l l of the "Bond Energies and the Thermodynamics of Organometallic Reactions" symposium p a r t i c i p a n t s f o r t h e i r enthusiasm i n t h i s enterprise and f o r the prompt preparation of outstanding Symposium Series chapters. He likewise thanks the manuscript reviewers and the ACS Books s t a f f f o r t h e i r conscientious e f f o r t s i n ensuring a h i g h - q u a l i t y symposium volume, and Ms. Rachel H a r r i s f o r expert s e c r e t a r i a l assistance. The support of NSF under grant CHE8800813 i s also g r a t e f u l l y acknowledged.

Literature Cited 1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14. 15.

Pauling, L. "The Nature of the Chemical Bond," Cornell University Press: Ithaca, NY, 1939. Guntz, A. Compt. Rend., 1887, 105, 673. Berthelot, M. P. E. Compt. Rend., 1899, 129, 918. Mittasch, A. Z. Angew. Chem., 1928, 41, 827-833. See, for example: Rossini, F. D., ed., "Experimental Thermochemistry," Wiley: New York, Vol. 1, 1956, and references therein. See, for example: Parks, G. S.; Huffman, H. M. "The Free Energies of Some Organic Compounds," American Chemical Society Monograph Series, Chemical Catalog Co.: New York, 1932. Skinner, H. A. Advan. Organometal. Chem., 1964, 2, 49-114. Cox, J. D.; Pilcher, G. "Thermochemistry of Organic and Organometallic Compounds," Academic Press: London, 1970. Connor, J. A. Topics Curr. Chem., 1977, 71, 72-109. Halpern, J. Acc. Chem. Res. 1982, 15, 238-244. Ibers, J. Α.; DiCosimo, R.; Whitesides, G. M. Organometallics, 1982, 1, 13-20. Bruno, J. W.; Marks, T. J . ; Morss, L. R. J. Am. Chem. Soc. 1983, 105, 6824-6832. See, for example: Atkins, P. W. "Physical Chemistry," 3rd ed., Freeman: New York, 1986, Chapts. 4, 17. Pilcher, G.; Skinner, H. A. in "The Chemistry of the Metal -Carbon Bond"; Hartley, F. R.; Patai, S., Eds.; Wiley: New York, 1982; pp. 43-90. Wagman, D. D., et al, "The NBS Tables of Chemical Thermo dynamic Properties. Selected Values for Inorganic and C and C Organic Substances in SI Units," J. Phys. Chem. Ref. Data, 1982, 11, Supplement No. 2. 1

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21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

Pedley, J. B.; Naylor, R. D.; Kirby, S. P. "Thermochemical Data of Organic Compounds," Second Ed., Chapman and Hall: London, 1986. McMillan, D. F.; Golden, D. M. Ann. Rev. Phys. Chem. 1982, 33, 493-532. Sunner, S.; Mansson, Μ., Eds. "Combustion Calorimetry"; Pergamon Press: Oxford, 1979. Connor, J. Α.; Zafarani-Moattar, M. T.; Bickerton, J . ; Saied, Ν. I.; Suradi, S.; Carson, R.; Takhin, G. Α.; Skinner, H. A. Organometallics 1982, 1, 1166-1174, and references therein. Marks, T. J . : Gagné, M. R.; Nolan, S. P.; Schock, L. E.; Seyam, A. M.; Stern, D. Pure Appl. Chem., 1989, 16, 16651672. Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110. 7701-7715. Nolan, S. P.; Stern, D.; Hedden, D.; Marks, T. J., this volume. Schock, L. E.; Seyam, A. M.; Marks, T. J . , Polyhedron, 1988, 7, 1517-1530. Nolan, S. P.; Stern, D.; Marks, T. J. J. Am. Chem. Soc., 1989, 111, 7844-7853. Dias, A. R.; Galena, M. S.; Simoes, J. A. M.; Pattiasina, J. W.; Teuben, J. J. Organometal. Chem., 1988, 346, C4-C6. Toscano, P. J . ; Seligson, A. L.; Curran, M. T.; Skrobutt, A. T.; Sonnenberger, D. C. Inorg. Chem., 1989, 28, 166-168. Castelhano, A. L.; Griller, D. J. Am. Chem. Soc., 1982, 104, 3655-3659. Yoneda, G.; Lin, S.-M.; Wang, L.-P.; Blake, D. M. J. Am. Chem. Soc., 1981, 103, 5768-5771. Nolan, S. P.; Lopez de la Vega, R.; Hoff, C. D. J. Am. Chem. Soc., 1986, 108, 7852-7853, and references therein. Connor, J. Α.; Skinner, Η. Α.; Virmani, Y. J. Chem. Soc., Faraday Trans I, 1972, 1754-1763. Rudzki, J. E.; Goodman, J. L.; Peters, K. S. J. Am. Chem. Soc., 1985, 107, 7849-7854. Yang, G. K.; Peters, K. S.; Vaida, V. Chem. Phys. Lett., 1986, 125, 566-568. Yang, G. K.; Vaida, V.; Peters, K. S. Polyhedron, 1988, 7, 1619-1622. Bulls, A. R.; Bercaw, J. E.; Manriquez, J. M.; Thompson, M. E. Polyhedron, 1989, 7, 1409-1428. Bryndza, H. E.; Fong, L. K.; Paciello, R. Α.; Tam, W.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 1444-1456. Stoutland, P. O.; Bergman, R. G.; Nolan, S. P.; Hoff, C. D. Polyhedron, 1988, 7, 1429-1440. Halpern, J. Polyhedron, 1988, 7, 1483-1490. Hay, B. P.; Finke, R. G. Polyhedron, 1988, 7, 1469-1481. Koenig, T. W.; Hay, B. P.; Finke, R. G. Polyhedron, 1988, 7, 1499-1516. Wayland, Β. B. Polyhedron, 1988, 7, 1545-1555. Collman, J. P.; McElwee-White, L.; Brothers, P. J . ; Rose, E. J. Am. Chem. Soc., 1986, 108, 1332-1333. Bakac, Α.; Espenson, J. H. J. Am. Chem. Soc., 1984, 106. 5197-5202.



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15

43. Puddephatt, R. J. Coord. Chem. Rev., 1980, 33, 149-194, and references therein. 44. Pugh, J. R.; Meyer, T. J. J. Am. Chem. Soc., 1988, 110, 8245-8246. 45. Tilset, M.; Parker, V. D. J. Am. Chem. Soc., 1989, 111, 6711-6717. 46. Armentrout, P. Β., this volume. 47. Ervin, Κ. M.; Armentrout, P. B. J. Chem. Phys., 1985, 83, 166-189. 48. Freiser, B. S., this volume. 49. Buckner, S. W.; Freiser, B. S. Polyhedron, 1988, 7, 15831603. 50. van Koppen, P. A. M.; Bowers, M. T.; Beauchamp, J. L.; Dearden, D. V., this volume. 51. Lane, K. R.; Squires, R. R. Polyhedron, 1988, 7, 1609-1618. 52. Lane, K. R.; Lee, R. E.; Sallans, L.; Squires, R. R. J. Am. Chem. Soc., 1984, 106, 5767-5772. 53. Lewis, Κ. E.; Golden, D. M.; Smith, G. P. J. Am. Chem. Soc., 1984, 106, 3905-3912, and references therein. 54. Smith, G. P. Polyhedron, 1988, 7, 1605-1608. 55. Somorjai, G. A. "Chemistry in Two Dimensions: Surfaces," Cornell University Press: Ithaca, 1981, Chapts. 2, 6. 56. van Hove, Μ. Α.; Wang, S.-W.; Ogletree, D. F.; Somarjai, G. A. Advan. Quantum Chem., 1989, 20, 1-184. 57. Somorjai, G. Α., this volume. Bibliography The following is a listing by subject area of recent data compilations, review articles, and monographs dealing with thermo dynamic properties, bond energies, and associated measurement techniques. Data Compilations. Organic Compounds 1. Pihlaja, K. in "Molecular Structure and Energetics," Liebman, J. F.; Greenberg, Α., Eds., VCH Publishers: New York, 1987, Vol. 2, Chapt. 5. (Thermodynamic properties; bond energies). 2. Chickos, J. S. in "Molecular Structure and Energetics," Liebman, J. F.; Greenberg, Α., Eds., VCH Publishers: New York, 1987, Vol. 2, Chapt. 3. (Heats of sublimation). 3. Pedley, J. B.; Naylor, R. D.; Kirby, S. P. "Thermochemical Data of Organic Compounds," Second Ed., Chapman and Hall: London, 1986. (Thermodynamic properties; bond energies). 4. Smith, B. D.; Srivastava, R. "Thermodynamic Data for Pure Compounds, Part A. Hydrocarbons and Ketones," Elsevier: Amsterdam, 1986. (Thermodynamic properties). 5. Smith, B. D.; Srivastava, R. "Thermodynamic Data for Pure Compounds, Part B. Halogenated Hydrocarbons and Alcohols," Elsevier: Amsterdam, 1986. (Thermodynamic properties). 6. McMillan, D. F.; Golden, D. M. Ann. Rev. Phys. Chem. 1982, 33, 493-532. (Bond energies). 7. Benson, S. W. "Thermochemical Kinetics," 2nd ed.; John Wiley and Sons: New York, 1976. (Thermodynamic properties; bond energies).


16


8.


9.

Cox, J. D.; Pilcher, G. "Thermochemistry of Organic and Organometallic Compounds," Academic Press: London, 1970. (Thermodynamic properties). Stull, D. R.; Westrum, E. F.; Sinke, G. C. "The Chemical Thermodynamics of Organic Compounds"; Wiley: New York, 1969. (Thermodynamic properties; bond energies).

Data Compilations. Inorganic Compounds. 1. Morss, L. R. in "The Chemistry of the Actinide Elements," 2nd ed.; Katz, J. J . ; Seaborg, G. T.; Morss, L. R., Eds., Chapman and Hall: London, 1986, Chapt. 17. (Thermodynamic properties of actinide compounds). 2. Chase, M. W., Jr., et al, "JANAF Thermochemical Tables," 3rd ed., Part I, Al-Co J. Phys. Chem. Ref. Data, 1985, 14, Supplement No. 1. (Thermodynamic properties). 3. Chase, M. W., Jr., et al, "JANAF Thermochemical Tables," 3rd ed., Part II, Cr-Zr, J. Phys. Chem. Ref. Data, 1985, 14, Supplement No. 1. (Thermodynamic properties). 4. Christensen, J. J . ; Izatt, R. M. "Handbook of Metal Ligand Heats," 3rd ed., Marcel Dekker: New York, 1983. (Thermo dynamic properties of coordination compounds). 5. Wagman, D. D., et al, "The NBS Tables of Chemical Thermo dynamic Properties. Selected Values for Inorganic and C and C Organic Substances in SI Units," J. Phys. Chem. Ref. Data, 1982, 11, Supplement No. 2. (Thermodynamic proper ties). 6. Drobot, D. V.; Pisarev, E. A. Russ. J. Inorg. Chem., 1981, 26, 3-16. (Metal-halogen, metal-oxygen, and metal-metal bond energies). 7. Glidewell, C. Inorg. Chim. Acta, 1977, 24, 149-157. (Bond energies in oxides and oxo-anions). 8. Huheey, J. E. "Inorganic Chemistry," 2nd ed.; Harper and Row: New York, 1978; pp. 824-850. (Bond energies). 1

2

Data Compilations and Reviews. Organometallic Compounds. 1. Simoes, J. A. M.; Beauchamp, J. L. Chem. Rev., in press. (Large compilation of bond energies). 2. Marks, T. J . , Ed. "Metal-Ligand Bonding Energetics in Organotransition Metal Compounds," Polyhedron Symposium-in -Print, 1988, 7. (Short review articles containing much data). 3. Skinner, Η. Α.; Connor, J. A. in "Molecular Structure and Energetics," Liebman, J. F.; Greenberg, Α., Eds., VCH Publishers: New York, 1987, Vol. 2, Chapt. 6. (Compila tions of bond energies). 4. Pilcher, G.; Skinner, H. A. in "The Chemistry of the Metal -Carbon Bond"; Hartley, F. R.; Patai, S., Eds.; Wiley: New York, 1982, pp. 43-90. (Extensive compilation of bond energies). 5. Connor, J. A. Top. Curr. Chem. 1977, 71, 71-110. (Exten sive compilation of bond energies). 6. Halpern, J. Acc. Chem. Res. 1982, 15, 238-244. (Review article). 7. Mondal, J. U.; Blake, D. M. Coord. Chem. Rev. 1983, 47, 204-238. (Extensive compilation of bond energies).


1. MARKS


8.

Skinner, H. Α.; Connor, J. A. Pure Appl. Chem. 1985, 57, 79-88. (Compilation of bond energies). 9. Pearson, R. G. Chem. Rev. 1985, 85, 41-59. (Review article). 10. See also Cox and Pilcher above.


Data Compilations. Gas Phase Basicities and Proton Affinities. 1. Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Suppl. 1. 2. Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data, 1984, 13, 695-808. Gas Phase Studies. 1. Russell, D. Η., Ed. "Gas Phase Inorganic Chemistry," Plenum Press: New York, 1989. (Much data on organometallic systems). 2. Allison, J. Prog. Inorg. Chem., 1986, 34, 627-676. (Extensive compilation of data on organometallic systems). 3. Bowers, M. T., Ed. "Gas Phase Ion Chemistry," Academic Press: New York, 1979, Vols. 1, 2. (Much information on gas phase techniques and results). 4. Lehman, T. Α.; Bursey, M. M. "Ion Cyclotron Resonance Spectrometry," Wiley: New York, 1976. (Introductory text). Calorimetry. 1. Wiberg, Κ. B. in "Molecular Structure and Energetics," Liebman, J. F.; Greenberg, Α., Eds., VCH Publishers: New York, 1987, Vol. 2, Chapt. 4. (Review of experimental methods). 2. Grime, J. Κ., Ed. "Analytical Solution Calorimetry," Wiley: New York, 1985. 3. Hemminger, W.; Höhne, G. "Calorimetry," Verlag-Chemie: Weinheim, 1984. 4. Ribeiro da Silva, Μ. Α. V. "Thermochemistry and Its Applications to Chemical and Biochemical Systems," Reidel: Dordrecht, 1982. (NATO Advanced Study Institute mono graph). 5. Sunner, S.; Mansson, Μ., Eds. "Combustion Calorimetry"; Pergamon Press: Oxford, 1979. 6. Barthell, J. "Thermometric Titrations," Wiley: New York, 1975. 7. Vaughn, G. A. "Thermometric and Enthalpimetric Titrimetry," Van Nostrand-Reinhold: New York, 1973. 8. Tyrrell, H. J. V.; Beezer, A. E. "Thermometric Titrimetry," Chapman and Hall: London, 1968. Surface Studies 1. Van Hove, Μ. Α.; Wang, S.-W.; Ogletree, D. F.; Somarjai, G. A. Advan. Quantum Chem. 1989, 20, 1-184. 2. Somorjai, G. A. "Chemistry in Two Dimensions: Surfaces," Cornell University Press: Ithaca, 1981, Chapts. 2-6. RECEIVED January 22, 1990


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