COMMUNICATIONS TO THE EDITOR
May 20, 1964
between C-5 and C-10 in a (hypothetical) type 8 intermediate. In view of the exclusive conversion 7- 9 under identical irradiation conditions, the choice between the reaction paths 6 2 and Sa -+ 9 is evidently controlled by the size of ring B in the dienones 1 and 7, respectively. Acknowledgment.-We thank Dr. J. Seibl for the measurement of the mass spectra. Financial support of this work from the Schweiz. Nationalfond zur Forderung der wissenschaftlicben Forschung (Project No. 2839) is gratefully acknowledged.
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(13) Unpublished results by R. Weoger.
ORCANISCH-CHEMISCHES LABORATORLUM DER EIDG. TECHNISCHEN HOCHSCHULE ZURICH,SWITZERLAND RECE~VED MARCH12, 1964
G. Bozwio
H. P. THRONDSEN K. SCHAFFNER 0.JEGER
The Importance of Steric Factors in Substitution Reactions of Metal Complexes. A Pseudo-octahedral Complex
Sir: With the rebirth of the crystal field theory' and its application to metal complexes, the importance of the electronic structures in these systems in explaining their properties is well documented. We wish to report here an example where steric factors, not electronic structure, are responsible for the substitution reaction of a metal complex.
Fig. I.-MoIccuk~r
modrl of the pseudo~octahcdral complex
[Pd(Et,dien)CI]+.
The rates of substitution reactions of octahedral metal complexes do not generally depend on the reagent.? For square-planar complexes, the rates of reaction do depend on the reagent.s The question arises as (1) H. Befhe, Ann. Physik. I51 8 , 133 (1828). (2) (a1 F. Banoio and R. G. Pearson, "Meehanirmr of Inorganic Renctions." John Wiley and Sons, New York, N. V., 1858. Chagter 3; (h) M. Eigen, Purr APDI. Chrm., 6. No. 1 . 8 7 (1862). (3) F. Bar010 s n d R. G. Pcarson, "Progress in Inorganic Chemistry," Vol. 4, F. A. Cotton, Ed., latcrseienee Publishers, Inc., New York, N. Y., lQ82.
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COMMUNICATIONS TO THE EDITOR
2076
Acknowledgment.-We wish to thank Professor R. G. Pearson for his interest and helpful advice. U e wish t o thank Mr. G. Haller for the isolation and purification of the compound [Pd(MeEt4dien)C1]PFs. This research is supported in part by the U.S. Atomic Energy Commission under Grant At(ll-l)-lO87 and in part by National Institutes of Health Grant RG7488. DEPARTMENT OF CHEMISTRY WILLIAMH. BADDLEY XORTHWESTERN UNIVERSITY FREDBASOLO EVANSTON, ILLISOIS RECEIVED MARCH12, 1964
Vol. 8(i
2.6
2.5
* mE ff 2.4 E
e -;2.3 .-U
Y
rn
High Frequency Titrimetric Determination of the Electron Deficiency in Lithium Alkyls'
Sir:
22
2.1
Organometallic reagents, including lithium alkyls, are generally referred t o as strong bases because of their "carbanion character" and gross reactions, i.e., proton abstraction or attachment to Lewis acids.2 Various authors have, however, conceived of the structure of certain organometallics in terms of a bonding electron deficiency. This deficiency could result in character-
3.5
30
3 e
2.5
u
4
2.0
uJ
m
.5 1 5
s
s
10
05
I
8 12 E t h e r a d d e d , ml
16
20
Fig. 1.-Curve A shows the change in capacitance of a cell containing 90 ml. of a hexane solution t o which was added increments of ether. Curve C shows same except 0.144 equiv. of butyllithium was present. Curve B is the difference between C and A. Titrations were carried out a t 2.5' (ether = 9.5 A [ ) .
istic acidic properties. Specifically, it recently has been suggested that the basic structural unit of a lithium alkyl is a dimer K2Li2,I4which associates with itself (R2Li2). as a solid and in nonpolar solvents. If this specific suggestion is correct. by application of the 2(n- 1) rule of Longuet-Hig-gins,3ait may be predicted t h a t just two electrons are needed to satisfy the deficiency of each dimer structure. T o investigate this (1) Paper I V in t h e series "Solvent Effects in Organometallic Reactions." Papel- 111' Z K Cheema. G V,' Gihson, and J F E a s t h a m . J A m . C h e m Soc.. 85, 3517 (1963) ( 2 ) Cf R Waack and AT. A I)ot-an, ibid., 8 6 , 4042 (1963), G. W i t t i g , Bui: soc. c h i m Fraitcr. [;I]13.52 (1963). ( 3 ) Cf ( a ) H C I.onguet-Higgins. Quart Art'. (London), 11, 121 (1957); ( b ) G E Coates, "Organw.\letalIic Compounds," 2nd E d . , Afethuen. Lond o n , 1960, pp. 4 4 , .57, 1 3 1 , 1 3 2 ff, I C ) M Weinet- and R . West, .I A m . Chem. Soc , 86, 485 (1963) (4) V . H Ilietrich. Acta C y r i l , 16, 681 (1963)
2
4 6 8 1 Base a d d e d , mmoles.
0
Fig. 2.-Apparent dielectric constants of three hexane solutions, each of 7 ml. volume and containing 11.2 mequiv. of butyllithium, to which Lewis bases were added: D, triethylamine; C, dietliyl ether; E, tetrahydrofuran.
prediction experimentally, we have measured the change in dielectric properties of several lithium alkyls consequent to their titration with Lewis bases. Figure 1 shows the changes in dielectric properties of 90 ml. of hexane solution in a high frequency capacitance cell5 to which increments of diethyl ether (Et20) were added: curve A for hexane originally alone and curve C for hexane containing.0.144 equiv. of RLi, nbutyllithium. Although an RLi solution in hexane is hardly more polar than hexane alone, it is seen t h a t addition of ether causes a relatively steep rise in the dielectric character of the solution (more change in the dielectric than ether alone causes) and t h a t this rise is complete when the ratio of RLi to Et20 is 2 : 1. This latter fact is more apparent from a plot B of the difference between curves A and C. In effect, C is a titration curve for a reaction (eq. 1) in which the lithium alkyl is an "acid." There would seem to be some practical potential in titrations of this type, particularly with a differential curve like B, for nondestructive analysis of alkyllithium solutions.
+ n E t 2 0 -+
(R2Li2)n
n(R2Li2).0Et2 I
(1)
It was conceivable t h a t the Et20 titration curve did not corroborate the prediction of each R2Li2unit accepting just one pair of electrons to satisfy the organometallic's bonding electron deficiency; EtzO does have two pairs of nonbonding electrons which i t could be contributing in structure I. To check this consideration, high frequency titrations of butyllithium in hexane were carried out with triethylamine (Et3N) instead of E t 2 0 as the base. As Fig. 2, curve D, shows, Et3N also causes a rise in dielectric character, and the steep rise is complete with the addition of one pair of nonbonding electrons from the base for every R2Li2unit in solution. Hydrocarbon solutions of several lithium alkyls (ethyl, sec-butyl, t-butyl, n-butyl), each with several different ( 5 ) A j - b f c . signal was applied concentrically t o t h e all-glass cell and measured with t h e circuit of a Sargent Model V oscillometer modified so t h a t capacitance change was linear with dielectric constant change within t h e cell and so t h a t t h e cell could be operated in a contt-olled atmosphere
