Linkage Isomerism - Advances in Chemistry (ACS Publications)

20 Linkage Isomerism R O B I N T. M . F R A S E R

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch020

The University of Kansas, Lawrence, Kan.

A ligand which contains more than one coordinating group can bond to metal ions in more than one way; however, if only one of the coordinating groups can combine with a particular metal at one time, the ligand is called "am bidentate" and may form linkage isomers, differing only in the point of ligand attachment. The first mononuclear pair (the xantho and isoxantho ions [Co(NH ) NO +2]) was prepared in 1857. The second was characterized in 1962. Since then, 10 or so pairs of complexes exhibiting the phenomenon called by Werner "salt isomerism" have been found by reinvestigating older literature preparations, by applying synthesis concepts based on mechanism studies, or by examining series of complexes containing re lated ligands differing in their class a or class b affinities. 3

5,

2

ligand which contains two different donor groups, only one of which can coordinate to a particular metal ion at one time, is termed " a m bidentate." Such ligands give rise to complexes exhibiting linkage isomer ism. The complexes have the same geometric configuration, and their only difference lies i n the point of attachment of the ligand; hence, the isomerism is quite distinct from flexidentate chelation (11), which involves multidentate ligands using varying numbers of coordinating groups. Linkage isomerism involving chelating ligands is extremely uncommon but not unknown (15). Historical Although the xantho and isoxantho complexes of cobalt (III) having the composition ( N H ) 5 C o N 0 . X (where X = CI or 3^S0 ) had been known for some time (17, 21, 22), Werner was the first to recognize that the pair constituted one example of a more general phenomenon and, 3

2

2

4

295 In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

296

WERNER CENTENNIAL


accordingly, included salt isomerism (43) in his seven-fold complex classi fication. Early assignments of structure were based on color and relative stabilities, and more recent infrared studies have confirmed (26, 30) that the xantho complex contains the nitrite group bonded through the nitro gen, while the isoxantho complex contains the nitrite bonded through one oxygen. A n investigation (28) of the mechanism of formation of the nitrito (isoxantho) complex and its subsequent isomerization to the more stable nitro isomer led to the conclusion that the reaction involves attack by dinitrogen trioxide (the anhydride of nitrous acid) on hydroxypentaamminecobalt(III). (NH ) CoH 0 3

5

2

+3

+ H 0 — (NH ) CoOH+' 2

3

+ H 0

5

3

+

2HN0 ^ N 0 + H 0 2

2

3

2

(NH ) CoOH * + N 0 -> (NH ) CoONO + H N 0 3

5

+

2

3

3

5

+2

2

Oxygen tracer experiments (25) confirmed that the cobalt-oxygen bond remained intact throughout the reaction. The nitrito complexes of pentaamminerhodium(III), pentaammineiridium(III), and pentaammineplatinum(IV) have been obtained by the same method (5). In a similar way, adding sulfur dioxide or sodium sulfite to solutions of aquopentaamminecobalt (III) ion at low temperatures and low p H yields the unstable pink oxygen-bonded sulfito complex rather than the stable yellow-brown sulfur-bonded isomer. A second approach has been developed for preparing linkage isomers— that of deliberately modifying the bonding characteristics of the central metal ion through coordination with ligands such as triphenylarsine; in this way the palladium (II) complexes of nitrogen- and sulfur-bonded thiocyanate have been obtained (8). In some cases the synthesis has yielded a mixture of the two linkage isomers; this is certainly the case with thiosulfatopentaamminecobalt (III) chloride (82) and possibly with hydrogen thiosulfatopentacyanocobaltate (III) also (84). It has also been found that an equilibrium between the two isomers is sometimes established in the solid (6) or in solution (19, 40, 44); in the latter the position of equilibrium depends greatly on the solvent. Constituent Parts of Linkage Isomers M e t a l Ions. Metal ions may be divided into two groups: (a) those that form their most stable complexes with ligands bonding through the first atom of any group in the periodic table and (b) those which form their most stable complexes with ligands bonding through subsequent atoms of any periodic group (3). The transition between a and b type behavior is more or less gradual, and b metals are found clustered in the later stages of each transition series, occupying a triangular area (the

In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

20.

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297

Linkage Isomerism

Chatt-Leden triangle (23)) in the periodic table, with the most pronounced class b behavior shown by elements in its center: Cu Rh

Pd

Ag

Tr

Pt

Au

Hg

Some elements (for example, M n , Fe, Co, N i ; M o , Tc, R u , C d ; W , Re, Os, T l , Pb, B i , Po) show varying degrees of a and b character and fall in a borderline region of rather diffuse boundary. Factors influencing metal behavior in this region seem to be oxidation number (20), the resulting elec tronic configuration (copper(I) shows class b tendencies while copper(II) is borderline), or the other ligands associated with the metal in its complexes. The most numerous examples of linkage isomerism are expected among complexes of metals of this borderline group or of metals showing weak class a or class b behavior, with the further requirement that at least for pure preparations of the individual isomers, the complexes must be inert (dz, low-spin d , d systems). It is not surprising then that the isomerism has been observed for complexes of cobalt(III), rhodium(III), iridium(III), platinum(IV), and manganese (I) (all d ) and of palladium(II) and platinum(II) (d ) in the solid state, but not for complexes of chromium(III) or iron(III), and for cadmium(II) only in solution.


K

6

s

6

s

The Ligands. In general, the coordinating centers of an ambidentate ligand will not possess the same affinity for a particular metal ion; the more polarizable and larger the coordinating atom or group and the more empty orbitals it possesses, the greater its affinity for class b metals, while the smaller the atom or the more t electrons available for donation to the metal, the greater its affinity for class a metals. Class a affinity increases from left to right i n the periodic table and from bottom to top, so that thiocyanate bonds to class a metal ions through the nitrogen atom but to class b metal ions through the sulfur; the point of attachment of this ligand has, in fact, been used to establish the class behavior of a number of metal ions. Table I lists the ligands that have been found in linkage isomers, and Table II lists some other common ambidentate ligands and representative complexes. It may be seen that (with the exception of the selenocyanate ion) all the anions use one of the adjacent atom pairs in bonding. C

«-*

Si

P

Ge

As

O

^>

S X

Se


298

WERNER CENTENNIAL

Table I.

Ligands Known to Form Linkage Isomers

Ligand

Complex

Nitrite Thiocyanate


Sulfite Thiosulfate Cyanide

M — N , M -- 0

(NH ) MX+

M-N, M-N, M—N, M-N, M-N, M-S M-0 M-S, M-C M-C,

en MX'X+ (bipy)MX (As(C H ) ) MX MX ~ (C0) MX (NH ) MX+ (NH ) MX+ (NH ) MX+ MX ~ MXs'X-3

M -- 0 M -- S M -- s M -- s M -- s

M -- 0

3

5

2

2

6

5

3

2

2

4

5

3

5

3

5

3

5

3

6

M -- N

2

2

Metal

Reference

Co Rh, Ir, Pt Co Pd Pd Cd Mn Co Co Co Co, Cr, Fe, M n Co

21, 22, 80, 48 5 4 8 8 19, 40 44 27 89 32, 83 7, 13, 18

Clearly, the smaller the differences in affinities of the coordinating groups, the more probable the formation of linkage isomers. Examining the two tables also suggests that anions containing nitrogen and phosphorus or sulfur and selenium atoms might well participate in linkage isomerism. Table II.

Other Ligands Exhibiting Ambidentate Behavior

Ligand

Complex

Urea

Thiourea

•Selenocyanate

Dimethyl sulfoxide

M-0 M-N M-0 M-N M-0 M-N M-S M-S M-S M-N M-Se M—N M-Se M-N M-Se M-0 M—0 M-S M-0

MC1 X MC1 X 2

2

2

2

MCl4X

MB X MX +3 r 4

Metal

2

2

6

MCl4X

2

MClzX, MCl4X

2

MX (NH ) MX+ (bipy)MX MXr MXr MX -» MX -» 4

+ 2

3

5

2

2

2

6

6

MC1 X MC1 X MC1 X MC1 X 2

3

2

2

2

2

2

2

Reference

Cu, Zn Pd, Pt Sn, Ti Sn Cr, Fe Ti Pt, Zn Sn Pd, Pt Co Pd, Pt Co, Fe, Zn Hg, Pd, Pt Cr(III), Y(III), Ni(II) Rh(III), Pt(IV)

31 81 12 12 81 85 45 12 45 9 9

Co, N i Cd, Hg Pd, Pt Cu

16, 16, 16, 16,

10,42 10,42

10

10,42 37 37 37 37

The other ligands present in the complex can affect the isomerism in two ways: they can modify the class behavior of the metal, and they can impose steric restrictions on the coordination site of the ambidentate ligand.


20.

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Isomerism

299

Of the two, the steric effect is easier to understand—i.e., the closer that bulky ligands can come to the metal the less space left for the ambidentate ion and the more favorable a less branched configuration for the ligand be comes (8, 44)- Sulfur-bonded thiocyanate, because of its angular M — S — C N structure, has larger steric requirements than the nitrogenbonded, with linear M — N — C S . Thus, while platinum(II) (because of its class b tendencies) shows the expected sulfur bonding to thiocyanate in ( N H j 2 P t ( S C N ) , this changes to nitrogen bonding when the ammonia groups are replaced by the larger triphenylphosphines. I n contrast, re placing the latter by triphenylstibines once more permits sulfur bonding because the phenyl groups are further away from the central metal, and steric hindrance is decreased. It is more difficult to account for the electronic effect, partly because it is not easy to investigate without accompanying changes i n steric effects. It has been suggested (20) that the more ligands of high class b affinity that are associated with a particular metal the more b-like the metal becomes and the greater its affinity for still more ligands of this type; for example, the fluoro complex is the most stable of the pentaamminecobalt (III) halides and the iodo the least stable, while the iodo is the most stable of the pentacyanocobalt(III) halides, and the fluoro complex is not known. B y itself this idea of "inorganic symbiosis" is of limited applicability be cause the stable form of the selenocyanatopentacyanocobaltate(III) ion is the nitrogen-bonded form and not the selenium-bonded isomer as would be predicted (9). Another suggestion is that (44) an increase i n negative charge at the metal increases the class a character—an idea that is i n ap parent conflict with the "hard-soft" concept (29), and while i t does ex plain the bonding i n the selenocyanatopentacyanocobaltate(III) ion, it does not explain the bonding i n thiocyanatopentacyanocobaltate (III). The third hypothesis (41) is that 7r-electron acceptor ligands in the com plexes tend to reduce the electron density about the central metal, enhanc ing its class a behavior; support for this is found in the thiocyanato com plexes of platinum(II) and palladium (II). There is no one theory at present which will adequately cover all the experimental observations, but in spite of this a preliminary survey of series of complexes containing ligands varied i n some systematic way appears to be a powerful tool i n preparing linkage isomers. 2

3


Linkage

Structure

Determination

and Rates of Isomerism

Most structures have been assigned on the basis of the infrared absorp tion spectra of the complexes, taking into account both the symmetry of the ligands involved and the spectra of related ions. Table I I I lists the various peaks which have been used in the determinations, together with their (tentative) assignments. Frequencies have been used i n the past


300

WERNER CENTENNIAL

Table III.

Ligand

Vibration and Frequency, cmrExample

Reference

1

Nitrite


Infrared Assignments

v (N0 ) 1375 1310 1420 1410 1330 1065 1048 1060 1065 995 1250 814 794 8

Thiocyanate

RCo-N0 ° RRh-N0 RIr-N0 + RPt-N0 RCo-ONO RCr-ONO+ RRh-ONO+ RIr-ONO RPt-ONO NaN0 *rans-CH -ONO 2

2

26 5 5 5 26 26 5 5 5

+2

+2

2

2

+ 3

2

+2 2

2

+2

+3

2

3

trans-H-ONO

36 36 8 8 8 8 36 36 36

2

4

2

4

2

2

2

2

2

2

2

4

2

4

2

MO-S-O) 970 983 1050-1150 (902, 862)

*(S-0) 5.(08-0) 970 498 947» 960 519 (989) (460)

6

Thiosulfate

CH3-NO2

w(CS) S(N-C-S) 837 482 Zn(NCS) ~ 844 481 Co(NCS) ~ — 844 PdY (NCS) « — 842 Pd(bipy)(NCS) — — PdY (SCN) — 700 Pd(bipy)(SCN) 698 429 Pd(SCN) ~ 697 427 Pt(SCN) ~ 695 427 Au(SCN)r

p(C-N) 2072 2065 2089 2100 2119 2117 2109 2100 2127

Sulfite

2

*(S-0)

F(S-S)

995 997 1010 1030

446 424 415 412

Na S0 RCo-S0 RCo-OS0 2

27 27 27

3

3

+

2

+

NA S 0 RCo-OS 0 + RCo-SS0 + CH -SS0 ~ 2

2

3

2

2

3

3

38

3

° R - (NH ) ; Y = As(C H ) . Raman spectrum. 3

6

6

5

3

b

as a measure of the bond order M — X ; in general, the higher the frequency, the greater the bond order. I n the nitro complexes, for example, the symmetric and antisymmetric — N 0 stretching, the —NO2 wagging, and the M — N stretching bands all show the same trends (26), giving increased bond order along the sequence N i ( I I ) , Co (III), Pt(II). The N 0 stretch ing frequencies are considerably different in the nitrito and nitro complexes, and further, the nitrito lack the wagging vibration, v as shown below. 2

2

6j


20.

Linkage Isomerism

FRASER

301

(

) M+

N+

0

0+

The infrared spectra of thiocyanates have been used for some time to de termine M — S or M — N bonding and, thus, the class behavior of the metal ions; in a number of cases the assignments have been confirmed by x-ray analysis. The C — S stretching absorption occurs around 830 c m . in nitrogen-bonded complexes, considerably higher than in the sulfur-bonded (ca. 700 c m . ) , reflecting the greater double bond character of C — S in M — N = C = S than in M — S — C = N . Unfortunately, a number of organic ligands (particularly those with a pyridine system) also absorb in the region, and recent work suggests (36) that the N — C — S bending vibra tion may be more useful in structure assignment; one band is found near 475 c m . in iV-thiocyanato complexes, whereas there are three or four bands in the spectra of sulfur-bonded complexes, with the most intense lying near 420 c m . Presumably, the greater number of peaks in the latter is caused by the nonlinear M — S — C N structure. The absorption spectrum of ( N H ) C o O S 0 has not yet been re corded in the literature, but it is likely to be similar to the spectrum of Cu(S0 ) ~ . The values listed for the absorption frequencies under R C o O S 0 + in Table I I I are those for the copper(II) complex (27). Sulfite ion possesses C symmetry, and this is retained in the sulfur-bonded com plexes—i.e., coordination through oxygen should lower the symmetry to C and so remove the degeneracy of the vz (asymmetric) stretching of the S 0 group. Accordingly, the spectrum of the S-sulfitopentaamminecobart(III) ion should (and does) show only two S—O stretching peaks while that of the O-sulfitopentaamminecobalt(III) ion should show three. A pure sample of the sulfur-bonded thiosulfatopentaamminecobalt(III) ion has not yet been obtained. The usual method of preparation yields a mixture of the isomers, with the oxygen-bonded in nine-fold excess. Dif ferential electron transfer experiments have shown that the ratio can be decreased to 2:1, and the assignment of structure has been based on these experiments and the changes observed in the infrared spectrum at 1000 and 420 cm." The rates of isomerization have been determined for complexes i n solution by following changes in the visible absorption spectra (2) and i n the solid state by following the disappearance of infrared absorption bands or by dissolving samples of the complex at various times i n water and measuring the visible spectra. Some of the specific rates are listed i n Table I V ; in general, rates of isomerization are greater in solution than i n the solid state (1) and appear to be independent of the nature of the anion


- 1

-1

- 1

- 1

3

3

5

2

+

2

2

2

3l)

3

1

f


2

302

WERNER CENTENNIAL

Table IV.

Metal

Anion

Co(III)

N0 CI

Specific Rates of Isomerization

State


Rh(III)

CI

Ir(III)

CI CI CI

Pt(IV)

CI

Specific Rate, 10 k, seer 6

- O N O - > -NO2 25 solid solid 25 50 soln. 25 soln. 25 25 solid soln. 25 solid 50 soln. 25 50 50 solid soln. 25 - N 0 - > -ONO 50 solid

3

CI

N0

Temp., °C.

3

CI

1

Reference

0.80 0.19 3.5 3.2 7.7 7.5 96 33 4.4 67 8.4 5.6

1* 1* 6* 2 28

7.4

6°

3.3 8.6

8° 8°

a

5" 5 5 5 5 5°

.5

2

Co(III) Pd(II) 0

CI

-SCN-> -NCS 125 solid 150

—

Calculated from data in paper.

unless this is also the ambidentate ligand (28). The rates are first order, and over a fairly wide range they are not affected by acid concentration or by changes i n ionic strength. Isomerization of nitritopentaamminerhodium(III) is more rapid than might be expected (the reason for this is not known), and conversion to the nitro complex proceeds to completion. This contrasts with the corresponding cobalt(III) complex—i.e., in the solid state the two isomers are in equilibrium, and the specific rates for the forward and reverse processes (see below) measured (6) at a number of different temperatures suggest that earlier values (2) give (k\ + fc ) rather than fci alone. 2

(NH ) CoONO+ ^ ( N H ) C o N 0 ^ 3

6

2

3

5

2

The rate of isomerization of the sulfur-bonded dithiocyanatobis(triphenylarsine)palladium (II) takes place so rapidly in solution that a visible absorption spectrum cannot be obtained. In the solid state the rearrange ment is much slower (30 minutes at 156°C.) and is even slower when the solid is incorporated at low concentration in a potassium bromide disc, with a half-life of 130 minutes at 150°C. This decrease on going from the


20.

Linkage

FRASER

Table V.

Complex

(NH )5CoOS(V (NH )5CoS0 (NH ) CoOS 0 (NH ) CoSS0 + (NH ) CoON0 * (NH ) CoN0 +' (NH ) CoONO+' 3

3

3

3

3

2

6

3

5

3

6

3


5

+

2

+

3

2

+

2

6

303

Isomerism

Specific Rates of Electron Transfer

Reductant

Temp., °C.

Cr(II) Cr(II) Cr(II) Cr(II) Eu(II) Eu(II) Eu(II)

25 25 25 25 25 25 25

M'

1

Reference

secr

]

18 18.6 13.3 0.18 160 100 75

32 32 32 32

24 24 24

pure solid to the diluted solid does not occur with the nitrite complexesSpecific rates are listed i n Table I V . The sulfur-bonded thiocyanatopentacarbonylmanganese(I) rearranges to the nitrogen-bonded isomer i n five minutes when dissolved in acetonitrile at 25°C. Adding petroleum ether will precipitate this latter complex as a yellow powder; if this is suspended i n N u j o l at 25°C, conversion back to the sulfur-bonded complex takes place within five hours (44)Reactions A s yet, little has been done to study the reactions of the coordinated ambidentate ligands, partly because of the instability of one of the linkage isomers. The rates of electron transfer between chromium(II) and cobalt(III) through — O S S 0 and — S S 0 have been measured, as have those between europium(II) and cobalt(III) through — N 0 and — O N O . Specific rates are listed i n Table V . The activation parameters for the chromium(II) reduction of cobalt(III) through — S 0 and — O S 0 are similar. Two rate constants differing by a factor of 74 are obtained for the reduction through the thiosulfate group, and partial reduction of the mixture of linkage isomers followed by separation on an ion exchange column shows that the oxygen-bonded complex is reduced more rapidly than the sulfur-bonded complex. The cobalt (III) center is always re duced before the bridging ligand i n the europium(II) reductions of com plexes containing oxyanions of nitrogen, although the kinetics are more difficult to follow because the liberated nitrous acid oxidizes the europium more rapidly than does the remaining cobalt(III) complex. 2

3

2

3

3

Bridged Complexes Since an ambidentate ligand possesses at least two groups capable of coordinating to metal ions, polynuclear complexes may also exhibit linkage isomerism. The selenocyanate complex of cadmium , [(C H )4N]2 [Cd2(CNSe) ], contains the dimeric anion on the next page, 10

6


4

9

304

WERNER

SeCN

CENTENNIAL

NCSe

NCSe \ / ca cd / \ / \ SeCN SeCN NCSe \

/

and the nitro group acts (26) as an unsymmetric bridge i n di-/x-hydroxy/z-nitrobis[triamminecobalt (III) ], H

I Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch020

O H -0

(NH ) Co3

3

Co(NH ) 3

3

+3

N—O

I 0 but few pairs of linkage isomers are known. One example is (14) di-/xthiocyanatobis[chlorotripropylphosphineplatinum(II)], which exists i n a and j8 forms: (C H ) P 3

7

S—C—N

3

\ CI

/

Pt

/ \

7

CI /

Pt / \ N—C—S P(C H ) 3

(C H ) P 3

\

7

CI \ / Pt Pt / \ / \ CI S—C—N P(C H )

a; yellow 3

N—C—S

3

\

/

3

7

0; greenish-yellow 3

The isomers are obtained b y treating di-/x-chlorobis[chlorotripropylphosphineplatinum(II)] with two equivalents of potassium thiocyanate— i.e., i n cold acetone the a isomer forms; in boiling acetone, the 0 isomer. There do not appear to be any examples of bridged linkage isomers where the ambidentate ligand is not involved in the bridge. Literature

Cited

(1) Adell, B., Tholin, G., Acta Chem. Scand. 1, 624 (1947). (2) Adell, B., Z. Anorg. Chem. 279, 219 (1955.) (3) Ahrland, S., Chatt, J., Davies, N. R., Quart. Rev. 12, 265 (1958). (4) Basolo. F., Stone, B. D., Bergmann, J. G., Pearson, R. G., J. Am. Chem. Soc. 76, 3079 (1954). (5) Basolo, F., Hammaker, G. S., Inorg. Chem.1,1(1962).



20.

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Linkage Isomerism

305

(6) Beattie, I. R., Satehell, D. P. N., Trans. Faraday Soc. 52, 1590 (1956). (7) Bonino, G. B., Salvetti, O., Ricerca Sci. 29, 3627 (1956). (8) Burmeister, J. L., Basolo, F., Inorg. Chem. 3, 1587 (1964). (9) Burmeister, J. L., Al-Janabi, M. Y., Inorg. Chem. 4, 962 (1965.) (10) Burmeister, J. L., Williams, L. E., 151st National Meeting of the American Chemical Society, Pittsburgh, Pa, March 28-31, 1966. (11) Busch, D. H., J. Chem. Ed. 41, 83 (1964). (12) Bystrov, D. S., Sumarokova, T. N., Filimonov, V. N., Optics Spectroscopy 9, 239 (1962). (13) Caglioti, V., Sartoni, G., Scrocco, M., Atti Accad. Naz. Lincei, Rend. Classe Sci. Fis. Mat. Nat. 22, 266 (1957). (14) Chatt, J., Hart, F. A., J. Chem. Soc. 1961, 1416. (15) Collman, J. P., Sun, J., Inorg. Chem. 4, 1273 (1965). (16) Cotton, F. A., Francis, R., Horrocks, W. D., J. Phys. Chem. 64, 1534 (1960). (17) Gibbs, W., Genth, F. A., Am. J. Sci. 24, 86 (1857). (18) Griffith, W. P., Wilkinson, G., J. Chem. Soc. 1959, 2757. (19) Howarth, O. W., Richards, R. E., Venanzi, L. M., J. Chem. Soc. 1964, 3335. (20) Jørgensen, C. K., Inorg. Chem. 3, 1201 (1964). (21) Jørgensen, S. M., Z. Anorg. Chem. 5, 147 (1894). (22) Ibid. 19, 149 (1899). (23) Leden, I., Chatt, J., J. Chem. Soc. 1955, 2936. (24) Miller, R. G., Peters, D. E., Fraser, R. T. M., "Exchange Reactions," p. 203, I.A.E.A., Vienna, 1965. (25) Murmann, R. K., Taube, H., J. Am. Chem. Soc. 78, 4886 (1956). (26) Nakamoto, K., Fujita, J., Murata, H., J. Am. Chem. Soc. 80, 4817 (1958). (27) Newman, G., Powell, D. B., Spectrochim. Acta 19, 213 (1963). (28) Pearson, R. G., Henry, P. M., Bergmann,J.G., Basolo, F., J. Am. Chem. Soc. 76, 5920 (1954). (29) Pearson, R. G., J. Am. Chem. Soc. 85, 3533 (1963). (30) Penland, R. B., Lane, T. J., Quagliano, J. V., J. Am. Chem. Soc. 78, 887 (1956). (31) Penland, R. B., Mizushima, S., Curran, C., Quagliano, J. V., J. Am. Chem. Soc. 80, 527 (1958). (32) Peters, D. E., Fraser, R. T. M., J. Am. Chem. Soc. 87, 2758 (1965). (33) Ray, P. R., J. Indian Chem. Soc. 4, 64 (1927). (34) Ray, P. R., Maulik, S. N., Z. Anorg. Chem. 199, 353 (1931). (35) Rivest, R., Can. J. Chem. 40, 2234 (1962). (36) Sabatini, A., Bertini, I., Inorg. Chem. 4, 959 (1964). (37) Selbin,J.,Bull, W. E., Holmes, L. H., J. Inorg. Nucl. Chem. 16, 219 (1961). (38) Simon, A., Kunath, D., Ber. 94, 1980 (1961). (39) Stranks, D. R., private communication. (40) Tramer, A., "Theory and Structure of Complex Compounds," p. 225, The MacMillan Co., New York, 1964. (41) Turco, A., Pecile, C., Nature 191, 66 (1961). (42) Turco, A., Pecile, C., Nicolini, M., J. Chem. Soc. 1962, 3008. (43) Werner, A., Ber. 40, 765 (1907). (44) Wojcicki, A., Farona,M.F., "Proceedings of the 8th International Conference on Coordination Chemistry," p. 262, Springer-Verlag, Vienna, 1964. (45) Yamaguchi, A., Penland, R. B., Mizushima, S., Lane, T. J., Curran, C., Quagliano, J. V., J. Am. Chem. Soc. 80, 527 (1958). RECEIVED June 13, 1966.


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