The thermochemistry of organometallic compounds

M-85 1 J. Chem. Thermodyytramics 1978, 10, 309-320

The thermochemistry compounds HENRY A. SKINNER University of Manchester,

of organometallic

Manchester

Ml3

9PL, U.K.

Being the second ROSSINI

LECTURE

delivered at the FIFTH INTERNATIONAL CONFERENCE ON CHEMICAL THERMODYNAMICS Under the Commission on Thermochemistry and Thermodynamics International Union of Pure and Applied Chemistry Ronneby, Sweden 25 August 1977

of the

1. hltroductio!l

Organometallic chemistry is one of the growth areas of chemical investigation at the present time, and has been so for the past two decades. Due mainly to the intensive efforts of inorganic preparative chemists, numerous new compounds, in particular of the transition metals, have been isolated and investigated in respect of their molecular structures and characteristic spectra. The thermochemist is now presented with an area for investigation almost unexplored, challenging his technical skills, and beginning to be met. Readers of the Bulletin of Chemical Thermodynamics will certainly have noticed the increasing size of the Index in recent years, no small part of which has been due to the number of entries classified as “organometallic compounds”. It is pertinent to recall that the very thorough compilation of thermochemical data, published by Bichowsky and Rossini’r) in 1936, made reference to five compounds only which might be classified as “organometallic” in character.

2. Types of organometallic compounds It is convenient to begin by listing the types of organometallic compounds for which thermochemical data have been determined, or are now becoming available. (1) METAL

ALKYLS

Berthelot”) reported measurements of the energy of combustion of dimethyl- and diethylmercury in 1899, but no data of real value appeared until after the end of World War II. By the mid-1960’s, enthalpies of formation had been determined for several OO21-9614/78/0401-0309 19

$02+OlO.

8 1978 Academic Press Inc. (London)

Ltd.

310

H. A.

SKINNER

alkyl and aryl compounds of metals from Groups IIB (Zn, Cd, Hg), IIIB (Ga, In), IVB (Ge, Sn, Pb), and VB (As, Sb, Bi) of the Periodic Table. At this time, the synthesis of alkyl derivatives of the transition metals had not been achieved, and indeed was predicted to be most improbable. (3) This no doubt induced greater effort, and alkyl derivatives of several transition metals (e.g. Ti, Zr, Hf, Ta, W, Re) have been obtained,(4,‘) reviving interest in this particular area of thermochemistry concerned with the strength of metal-carbon o-bonds. (2) METAL

CARBENES

The carbenes of certain transition metals are important bring about the catalytic metathesis of olefins, e.g.

because of their ability to

b + f\&-*/d g’ No thermochemical (3) METAL

OLEFIN,

M-C ab = I I fg C-C de

‘4 =Y

+

II

A f g

’ e

A d

e

(1)

data on these compounds have yet been reported. DIENE

AND

Thermal studies have been made on Ni, Ir, and on di-ene complexes (** ‘) Pd, Fe, MO, and Cr. Enthalpies of heptatriene tricarbonyls of Cr, MO, carbonyl.(gy lo) (4) METAL

b

a

M=C
POLYENE

COMPLEXES

various n-olefin complexes@’ ‘) of Fe, Pt, Rh, (butadiene, cyclohexadiene, norbornadiene) of formation have been measured for the cycleand W, and for (q&o-octatetraene) iron tri-

ARENES

The metallic arenes-exemplified by “sandwich” molecules such as (bis-benzene) chromium-and derivatives of the type (arene)M(CO),, have been investigated thermochemically by several different groups. Of special interest here (as with olefin complexes) is the effect of substituents in the benzene ring on the stability and reactivity of arene-metal complexes.(g* ’ r) (5) METALLOCENES;

METAL

CYCLO-PENTADIENYL

COMPLEXES

The thermochemical group of Rabinovich and Tel’noi at Gorki have paid special attention to the metallocenes of the first transition series,“” and energies of combustion are reported for the bis-cyclopentadienyls of SC, Ti, V, Cr, Mn, Fe, Co, and Ni. These measurements, using the “static-bomb” calorimeter, demanded thorough analysis of the solid combustion residues, which usually contained the metal oxides mixed with unburnt metal, in variable proportions.

THE THERMOCHEMISTRY (6) METAL

ALLYL

OF ORGANOMETALLIC

COMPOUNDS

311

COMPLEXES

Few compounds of this type have been studied thermochemically. Examples include the dimer, {(allyl)PtCl),, for which d.s.c. methods were used to study the thermal decomposition,(13’ and the complex (allyl)Fe(CO),I, for which microcalorimetric measurements have been made on thermal decomposition, and on iodination.(“’ (7) METAL

CARBONYLS;

POLYNUCLEAR

METALLIC

CARBONYLS

Enthalpies of formation are now determined for most of the transition metal carbonyls, and for several polynuclear metal carbonyls. (r4) Measurements of energies of combustion by static-bomb combustion calorimetry, and in one noteworthy case, by rotating-bomb calorimetry, (15) have been augmented by thermal Mn2(Wlo9 decomposition studies using “hot-zone” calorimetry,” 6, and by high-temperature microcalorimetric techniques. (8) PYRIDINE-METAL

.COMPLEXES

The enthalpies of formation of several crystalline complexes of the type ML,X, (where M is a transition metal, L is pyridine or a substituted pyridine; IZ = 2 or 4; and X = Cl, Br, or I) have been measured, mainly by use of d.s.c. techniques.(“) More recent studies have centred on pyridine metal carbonyl complexes, including the tris-pyridine tri-carbonyls of MO and W, and the similar acetonitrile complexes. (9) METAL-DIALKYLAMIDES

Measurements of enthalpies of alcoholysis(“) and of hydrolysis(‘g’ have provided values for the enthalpies of formation of dialkylamides of Ti, Zr, and Hf (e.g. Ti(NMe,),) and of Ta(NMeJ, and W(NMe&. (10) METAL-ACETYLACETONES

Irving and co-workers (‘O) have applied solution-calorimetric methods in a series of careful studies from which the enthalpies of formation of the tris-(acetylacetonato) complexes of Al, Ga, Cr, Mn, and Fe were determined. A satisfactory bombcombustion technique has now been described for compounds of this type.(2’) (11) METAL

ACETATES

There is currently a lively interest in the structures of certain transition metal acetates, which are dimeric and contain metal-metal bonds. (22) The structural interest has induced thermochemical studies on a series of these compounds, e.g. tetraacetato dichromium, with the objective of evaluating the strength of metal-metal bonding.

3. Typical examples: techniques and special problems (I) ACETYLACETONATE

COMPLEXES

OF METALS

The combustion in oxygen of compounds such as tris-(acetylacetonato) AI(II1) ought not to present a difficult problem, in that the metal atom is already fully oxidized prior to combustion. Nevertheless, disparate combustion results are to be found in the literature. Cave11 and Pilcher t21) have found that satisfactory combustion can be achieved with a conventional bomb calorimeter by moderating the combustion

H. A. SKINNER

312

process: benzoic acid is suitable for this purpose. Combustion without a moderator tends to be explosive, and often results in “holing” of the platinum crucible. Some typical comparisons are shown below: AHF1k.l mol-1

Al(acac)o Cr(acac), Ga(aca&

solution(20)

combustion

combustion/ moderatedtzl)

-(1789 + 6) -(1533 k 5) -(1483 + 6)

(1749 s 10) -(1480 & 14) -

-(1793 f 2) -(1476 & 5)

(2) TETRA-p-ACETATO

DIMOLYBDENUM (II) Solution calorimetry (oxidative hydrolysis by a strongly acidic solution of Fe(II1) chloride) was applied (23) to measure the standard enthalpies of formation of crystalline Mo,(acetate)4 and of crystalline Mo(acac),, leading to the values: c> AH;{Mo~(O&CH~)~, c> = -(1976.5 + 8.5) kJ mol-‘, and AH,“{Mo(CSH,02),, = -(1324.8 f 3.9) kJ mol-I. Attempts to measure the energy of combustion of MO, (acetate), often gave low values although analysis of the combustion products indicated virtually complete combustion of the metal (MO -+ MOO,) and of the acetate (C -+ COz). Because of the extreme sensitivity of Mo,(acetate), to air and moisture, the thin Melinex bag containers may not have offered sufficient protection to prevent partial oxidation of the samples during the equilibration period within the bomb prior to firing.

The structure of tetra-aceto dimolybdenum (and of other related Mo(I1) dimeric complexes) is characterized by the short MO-MO separation (x0.21 nm), thought to indicate a quadruple bond between the metal atoms.(24’ Attempts to evaluate the contribution of the metal-metal bonding to the total chemical binding energy in the

THE THERMOCHEMISTRY

OF ORGANOMETALLIC

COMPOUNDS

313

molecule, met with initial difficulties in that reliable experimental data are not available for the enthalpies of formation of the (acetate) and (acac) radicals. On the basis of “reasonable” estimates for these, the MO zz MO bond-enthalpy contribution has been assessed in the region of 500 kJ mol-’ : this provisional value, although thought to be an upper limit, is indicative of multiple metal-metal bonding in Mo,(acetate),. (DIMETHYLAMINO)TUNGSTEN AND HEXA (DIMETHYLAMINO) DITUNGSTEN The di-tungsten compound, W,(NMe,),, is representative of several containing a metal-metal triple bond. It is both air and moisture sensitive, but sublimes in U~CUOat elevated temperatures without decomposition. Measurements of the energy of combustion have been made,(25’ the samples being sealed in Melinex bags, and covered with hydrocarbon oil in the platinum crucible. The recovery of CO, from the bomb gases indicated complete combustion of the organic content, but not all the tungsten was oxidized to WO,, some black residue (insoluble in alkali) remaining in the crucible. On correction for this, values for -AU,O/kJ mole1 ranged from 11528 to 11455, the mean value, - (11502 + 55), corresponding to AH;(c) = - (31 + 55) kJ mol- ‘. Later measurements using “double-bagging” of the sample, have given higher values for -AU:; preliminary solution-calorimetric measurements of the enthalpy of oxidative hydrolysis (reaction of W,(NMe,), with K2Crz0, in acid solution) are in fair agreement with the enthalpy of formation derived from later combustion measurements, which place AH,“(c) for W,(NMe,& at about + 50 kJ mol- ‘. The enthalpy of formation of W(NMe& was determined from solution-calorimetric measurements (hydrolysis in acid solution), and the vacuum-sublimation microcalorimetric method was applied to measure AH,,,. These studies(’ 9, gave AH,“(g) = (270 + 12) kJ mol-‘, leading to (D)(W-NMe,) = (220 + 5) kJ mol-’ for the mean bond-dissociation energy in W(NMe,),. Transference of this value, unchanged, to the d&tungsten analogue, leads to the surprisingly high value, (D)(W=W) x 940 kJ mol-‘. It is, however, doubtful that the direct transfer of bond-energy term values in this case is a valid procedure.

(3) HEXA

(4) HEXAMETHYL TUNGSTEN The permethyl metal compounds of Ta, W, and Re are unstable in the solid state,ct6) but can be handled safely in isopentane solution in the cold. Measurements of the enthalpy of hydrolysis (5) of W(CH,), were therefore made at 273 K, using aqueous ammonia as reagent and iso-octane as the diluent solvent, the overall process being: WMe,(isopentane)

+2H,O(l)

+20H-(aq)

= WO:-(aq) + 6CH,(g/soln,

isooctane).

(2)

The mean bond-dissociation value, (D)(W-CH,) = (159 If: 7) kJ mol- ‘, derived from the measured enthalpy of formation, is consistent with the instability of the permethyl compound, in which there appears to be significant steric strain from interference between methyl groups.(27) Enthalpies of formation are now available for each of the compounds Tax, and WX6, (where X = F, OCH,, Cl, N(CHJ)2, and CH,), enabling calculation of the

314

H. A. SKINNER

I

I

M-F FIGURE

I

M-OCH3

I

I

M-Cl

M-NMe,

1. Mean bond-dissociation

I

I

M-CH3 energies.

mean-bond dissociation energies, (D)(Ta-X) and (@(W-X) in these compounds. The values are plotted in figure 1; the pattern shown, (n)(M-F) > (D)(M-OCH,) % (D)(M-CI) > (D)(M-N(CH,),) > (D)(M-CH,), is almost the same for M = TaandW. A plot similar to figure 1 is given by the (D)(M-X) values in MX4 (M = Ti, Zr, Hf). The implication is that the pattern followed by (D)(Ta-X) in Tax, can serve as a model for similar metals (M = V, Nb), and likewise that followed by (D)(W-X) in WX6, can serve for the metals M = Cr, MO, and U. On the basis, (D)(M-CH,) values are predictable from known (D)(M-F) values; the metal-methyl bonds in Mo(CH& should be less stable, and those in U(CH,), more stable than in W(CH,),; the bonds in Nb(CH,), are expected to be weaker than in Ta(CH,),; V(CH,), is predicted to be too unstable to isolate. (5) HEXAMETHYLBENZENE MOLYBDENUM TRICARBONYL The enthalpy of formation of crystalline (C,Me6)Mo(C0)3 has been determined”’ from calorimetric measurements of (i), combustion in oxygen, with benzoic acid as combustion moderator; (ii), thermal decomposition in argon, using the drop-microcalorimetric technique at elevated temperatures; and (iii), thermal decomposition in iodine vapour, also by use of the drop-microcalorimetric technique. Attempts to measure the enthalpy of sublimation by the vacuum-sublimation method(“) were not successful, as evacuation of the sample induced thermal decomposition at temperatures as low as 400 K. This in marked contrast to the analogous chromium complex, which can be heated to temperatures >500 K without decomposition.

THE THERMOCHEMISTRY (6) TRIS-PYRIDINE

OF ORGANOMETALLIC

MOLYBDENUM

COMPOUNDS

315

TRICARBONYL

Microcalorimetric measurements (28) have been made on the thermal decomposition of the complex, and on its synthesis from reaction of Mo(CO), with excess pyridine in the vapour phase at elevated temperatures (417 to 422 K). Thermal decomposition is markedly accelerated at temperatures in the range 420 to 450 K, by evacuation of the reaction vessel; figure 2 is a typical thermogram showing this effect. The decomposition at temperatures ~430 K is only partial however, some Mo(CO), vapour escaping the hot-zone, i.e. PY,WW,(~)

= W(g) I

I

+tWCWd

+-No(c).

evacuate cell 1

4

I 20

I 30

--I

(3)

dropsample 4

10

f/min FIGURE 2. Heat flow (recorderresponse) 0 plotted againsttime t for the sublimationand thermaldecomposition of MoP~,(CO)~at 455K. (7) TRIS-ACETONITRILE

TUNGSTEN TRICARBONYL

Several different approaches have also been used in order to determine A~;{(CH,CN),W(W,, c}. Vrieze and co-workers(2g) used d.s.c. techniques to measure the enthalpy of the ligand displacement reaction : ((CWWJWO),, c} +3C’W = 3CWWg) +WW,(g), (4) and microcalorimetric methods were applied (“) to measure the enthalpies of thermal decomposition in argon gas, and in iodine vapour. (8) (ALLYL)-IRON

TRICARBONYL

IODIDE

Microcalorimetric measurements (lo) of the enthalpy of thermal decomposition in argon gas, and of the enthalpy of reaction with iodine vapour, gave values for A.H,“((C,H,)Fe(CO),I, c> in good agreement with one another. The thermal decomposition in inert gas leads to dissociation of all ligands, but not of iodine, from the metal: (C3H,)Fe(C0)31 = 3C0 +*&Hi0 +FeI, (5)

H. A. SKINNER

316

and in presence of excess iodine, the products include FeI,, and there is iodination (at least in part) of the ally1 radical, (CsH, + C,H,I).

3. The strengths of metal-carbon and metal-ligand bonds Some general trends The enthalpies of formation of organometallic compounds serve as starting-point (in favorable cases) from which to evaluate the “strengths” of metal-carbon and metal-ligand bonds. Pilcher (3o) has examined the trends shown by metal-alkyl bonds in terms of their mean “bond dissociation enthalpies”t for the B-subgroup metals of Groups II, III, IV, and V; in all cases, the values (D)(M-CH,) full in a given Group as the atomic number of the metal increases. For the A-subgroup metals of Group IV, this trend is reversed (figure 3). However, on replotting the same (D)(M-CH,) values against AH,“(M, g) (figure 4), both A- and B-subgroup metals show the same general trend, i.e. (D)(M-CH,) increases with increasing AH,“(M, g). The trend in figure 4 is meaningful in that AHi(M, g) values regect the strengths of M-M bonds themselves.

z FIGURE

3. Plot of bond-dissociation

energy against atomic number Z.

The bond-enthalpy contributions of arene-metal and cycloheptatriene-metal bonds in compounds LM(C0)3 (where L = mesitylene, hexamethylbenzene, cycloheptatriene; M = Cr, MO, W) increase in magnitude as M changes from Cr-+Mo+W, and are plotted in figure 5 against AH&M, g). The trend noted in figure 4 is again apparent; additionally, it is noteworthy that the values (D)(L-M) for arene ligands (L = C,H,, C,H,Me,, C,Me,) attached to a given metal are increased by methyl substitution into the benzene ring.” ll g, tThe mean bond dissociation enthalpy, (M-R) in MR, (g) is defined as AH/n where AH refers to the disruption process: MR,(g) = M(g) + nR(g).

THE THERMOCHEMISTRY

FIGURE

COMPOUNDS

317

----~l------T---Ti-~ '

r---1-

I

OF ORGANOMETALLIC

{ Pb 200

,

I L-l-.--600 400 L\HfO(M,g)/kJ ml--

4. Plot of bond-dissociation

-J

energy against enthalpy of formation.

Although the (D)(L-M) values for arene complexes of MO and W tricarbonyls indicate stronger bonds than in the corresponding chromium complexes, it is the latter which are thermally the more stable. The arene tricarbonyl complexes of MO and W decompose on heating in the condensed state (possibly even before melting), and the products of decomposition include Mo(CO), and W(CO),. The thermal decomposition in the condensed state may take place initiaLly by a co-operative mechanism involving the formation of M=M and M-CO-M bridge bonds, i.e.

-

80 60

400

500

600

700

AIf;CM.gb'kJ

FIGURE

5. Plot of bond-dissociation

800

900

Mel-'

energy against enthalpy of formation.

318

H. A. SKINNER

and the release of CO then replacing ligand L in LM(CO), to form M(CO),. Calculated values for the enthalpies of partial decomposition, (at 298 K) LM(CO),(c) = L(c or 1)++M(CO),(c) +$M(c), (7) for M = Cr, MO, W are listed below; Compound

AH/kJ mol-1 $19.7 -25.0 +17.6 -25.0 -66.5 -35.1 -52.7 -99.6

(Me3H&$W333 W&WePWX% (Me3H3C3W(C0)3 (cycle-C,H,)Cr(CO), (cycle-C,Ha)Mo(CO), (cycbC,Hs)W(CO)3

Of the compounds listed, only two are stable enough at elevated temperatures (>425 K) to sublime without decomposition. For each of these, AH (listed above) is positive. One objective of general interest is to determine the order of “bonding power” of different ligands when attached to a given metal. An approach to this identifies “bonding power” with the quantity (D)(L-M)/n, where n denotes the number of electrons donated by the ligand L to the metal in the complex. Available (D)(L--M) data are too few to provide more than a provisional order of bonding power for some of the more common ligands, presented in tabular form below :

Ligand

n

Fe

Cc& co

5 2 2 3 2 2 6 6 6 2 4 6

59.4 59

PF3 C3Hs

F’yridine CH,CN C&lea GMd& Cd-h Cd-L Butadiene cycle-C?Hs

5; 1 48 46 -


kJ mol-’ MO

W

(71) (311 76 73 67 48.7 46.6 -

(79) 01) 89.5 86.5 84

(47) 44

z 52

5T7 -

THE THERMOCHEMISTRY

OF ORGANOMETALLIC

COMPOUNDS

319

CO, PFs, c@.+C5H5, allyl, and pyridine appear to be “stronger” ligands than arenes and olefins, but it is too early to conclude that this order applies to all transition metals.

4. Future developments Many recent thermochemical data on organometallic compounds have been obtained by the application of microcalorimetric techniques, which have the very practical advantage of consuming no more than a few milligrams of sample for each measurement made. There is need for confirmation of these by independent studies, preferably by application of precision combustion calorimetry, but there remains the constraint that many organometallic compounds of particular interest are not generally available, and are both difficult and expensive to prepare. The successful development of miniature bomb-calorimeters (32) offers the means to operate with small samples for combustion measurements, and the novel design of a miniature “rotating-bomb” calorimeter, now announced by Mansson,‘Randzio, and Sunner,(33) should enable satisfactory combustion studies on rare organometallic compounds to be made in the near future. In many cases, enthalpies of sublimation are lacking for solid organometallic compounds for which AH,“(c) values have been determined. Some of these compounds are unstable at elevated temperatures, so that vapour-pressure measurements made at higher temperatures are misleading. More useful results at lower temperatures could probably be obtained by Knudsen-cell techniques, but as yet few such studies have been made. The further development of organometallic thermochemistry almost requires that there be an active co-operation between the calorimetricians concerned and inorganic chemists skilled in the art of synthesis of new and interesting compounds. There is perhaps a parallel to be drawn with the state of biochemical thermochemistry a decade or so ago. The remarkable growth of activity since then had its origins in joint discussions designed to bring biochemists and calorimetricians together at that time;(j4) there followed the inclusion of biothermal contributions at National calorimetry conferences, and, more significantly, at the International Calorimetry Conferences organized by the IUPAC Commission on Thermochemistry and Thermodynamics.

At Manchester University, Dr Pilcher and I have been particularly favoured in recent years by the active collaboration with our colleagues Dr J. A. Connor and Dr C. D. Garner from the Inorganic Department, and through them, have received the support of several organometallic research groups from elsewhere. I would acknowledge especially the assistance from the laboratories of Professors G. Wilkinson, P. A. Pauson, F. A. Cotton, and M. H. Chisholm, Dr P. Timms, Professors J. Lewis, P. Chini, H. Werner, and the late Dr E. A. Koerner von Gustorf.

320

H. A. SKINNER

REFERENCES 1. Bichowsky, F. R.; Rossini, F. D. Thermochemistry of Chemical Substances. Reinhold: New York. 1936. 2. Berthelot, M. P. E. Compt. Rend. 1899, 129,918. 3. JafFe, H. H.; Doak, G. 0. J. Chem. Phys. 1953,21,196. 4. Lappert, M. F.; Patil, D. S.; Pedley, J. B. J. C. S. Chem. Comm. 1975, 830. 5. Adedeji, F. A.; Connor, J. A.; Skinner, H. A.; Galyer, L.; Wilkinson, G. J. C. S. Chem. Comm. 1976, 159. 6. Mortimer, C. T.; McNaughton, J. L.; Puddephatt, R. J. J. C. S. Dalton 1972, 1265. 7. Brown, D. L. S.; Connor, J. A.; Leung, M. L.; Paz-Andrade, M. I.; Skinner, H. A. J. Organometal. Chem. 1976,110,79. 8. Partenheimer, W. Inorg. Chem. 1973, 11,743. 9. Brown, D. L. S. ; Connor, J. A.; Demain, C. P.; Leung, M. L. ; Martinho-Simoes, J. A. ; Skinner, H. A.; Zafarani-Moattar, M. T. J. Orgunometuf. Chem. 1977, 142, 321. 10. Connor, J. A.; Demain, C. P.; Skinner, H. A.; Zafarani-Moattar, M. T. (to be published). 11. Adedeji, F. A.; Brown, D. L. S.; Connor, J. A. ; Leung, M. L.; Paz-Andrade, I. M.; Skinner, H. A. J. Organometal. Chem. 1975, 97, 221. 12. Tel’noi, V. I. ; Kiryanov, K. V. ; Ermolaev, V. I. ; Rabonovich, I. B. Tr. Khim. Khim. Tekhnol. Gorki 1975, 4, 3. 13. Ashcroft, S. J.; Mortimer, C. T. J. Chem. Sot. A 1971,781. 14. Connor, J. A. ; Skinner, H. A. ; Virmani, Y. FaraaIzy Symp. Chem. Sot. 1973,8, 18 15. Good, W. D.; Fairbrother, D. M.; Waddington, G. J. Phys. Chem. 1958, 62, 853. 16. Barnes, D. S.; Pilcher, G.; Pittam, D. A. ; Skinner, H. A. ; Todd, D. ; Virmani, Y. J. Less Common Metals 1974,36, 117; 38,53; and 42,217. 17. Ashcroft, S. J. ; Mortimer, C. T. Thermochemistry of Transition Metal Complexes. Academic Press: London & New York. 1970. 18. Bradley, D. C.; Hillyer, M. J. Trans. Faraday Sot. 1966, 62, 2367 and 2374. 19. Adedeji, F. A.; Chisholm, M. H.; Cotton, F. A.; Connor, J. A.; Skinner, H. A. (to be published). 20. Hill, J. 0.; Irving, R. J. J. Chem. Sot. A 1966,971; 1%7,1413; 1968,1052 and 3116; 1%9,2690. 21. Cavell, K. J.; Pilcher, G. J. C. S. Faraday Zl977, 73, 1590. 22. Cotton, F. A.; Rice, C. E.; Rice, G. W. J. Am. Chem. Sot. 1977, 99, 4704. 23. Garner, C. D.; Pilcher, G.; Cavell, K. J. ; Parkes, S. (to be published) 24. Cotton, F. A. Chem. Sot. Rev. 19754.21. 25. Cavell, S.; Cavell, K. J. (unpublished). 26. Mertis, K.; Galyer, L.; Wilkinson, G. J. Orgunometal. Chem. 1973,97, C65. 27. Galyer, L. ; Wilkinson, G.; Lloyd, D. R. J. C. S. Chem. Comm. 1975,830. 28. Adedeji, F. A.; Connor, J. A. ; Demain, C. P.; Martinho-Simoes, J. A. ; Skinner, H. A. ; ZafaraniMoattar, M. T. J. Organometal. Chem. (in press). 29. Bleijerveld, R. H. T.; Vrieze, K. Znorg. Chim. Acta 1976, 19, 195. 30. Pilcher, G. Inter. Rev. Sci. (Phys. Chem.), Series 2, Vol. 10, (H. A. Skinner, Editor). Butterworths: London & Boston. 1975. 31. Tel’noi, V. I. ; Rabinovich, I. B. ; Kiryanov, K. V. ; Smirnov, A. S. Doklady ANSSSR 1976, 231, 733 (Eng. Trans.). 32. MBnsson, M. J. Chem. Thermodynamics 1973, 5, 721. 33. MBnsson, M.; Randzio, S.; Sunner, S., 5th International Conf. Chem. Thermodynamics, Ronneby, Sweden, 1977, Abstract 54. 34. E.g. the meeting organized by Professor R. Lumry in Prescott, Wisconsin in November 1966.