Conformational Studies of Chelated Ethylenediamines by Nuclear Magnetic Resonance Tr is-( Et hy lened iamine) Nickel(I I) Ion in Aq ueous Solution Floyd F.-L. Hol and Charles N . Reilley Department of Chemistry, University of North Carolina, Chapel Hill, N . C . 27514
Proton magnetic resonance spectra of tris-(ethylenediamine)nickel(ll) chloride in aqueous solution were recorded at 100 MHz. Two well resolved signals from the ethylenic protons were observed at room temperature, but collapsed at 101 O C . The temperature dependence of the chemical shift is discussed in terms of conformational equilibrium of individual coordinated rings and the exchange brought about by racemization of configuration about the asymmetric central metal io:. , !t 32 O C , the conformer D-kkk is favored over D-k k k by 900 cal/mole in aqueous solution. The recemization rate at the metal ion is estimated to be 5.5 X lo3 sec-l at 101 O C and the free energyof activation at this temperature is found to be 15.7 kcal/mole. CONFORMATIONAL ANALYSIS of the coordinated ligand molecule is an important subject in metal chelate chemistry and has implications pertinent to the general subject of sterochemistry of purely organic structures ( I , 2). The chelated ethylenediamine ring in [ C ~ ( e n ) ~ ]was ~ + reported to have a gauche configuration by Quagliano and Mizushima ( 3 ) on the basis of infrared studies. This puckered ring structure received further support from subsequent X-ray data (4). The coordinated ring can exist in two conformations designated as k and k‘ in (I) by Corey and Bailar ( I ) . In D-[Co(en),I3++,the d
In the absence of other coupling, a chelated ethylenediamine ring in the gauche form should give an AA’BB’ spin system from the ethylene protons. For the species [Co(en)J3+,only a broad single signal was observed in H20, D20, DZS04,or in trifluoroacetic acid, even over a range of temperatures. This had been rationalized to be caused either by the lack of resolution (because of numerous peaks with relatively small chemical shift differences) or by the mobility of the chelated ring exchanging at a moderate rate among different conformations. Coupling to 59C0, which could be removed by heteronuclear double resonance, has not been reported as a source of this broadening. In our previous study of nickel(I1) complexes of N-alkylethylenediamines (9), the strong contact interaction ( I O , 1 1 ) in the paramagnetic complex was demonstrated to be of considerable utility in conformational analysis in nickel(I1) chelates because of the strong dependency of the contact shift on the dihedral angle formed between Ni-N and C-H in the complex which contains the moiety Ni-N-C-H. Application of this technique to the study of tris-(ethy1enediamine)nickel (11) ion seemed particularly worthwhile in view of conflicting data in the literature concerning the ring conformation. The gauche form was established from X-ray data (12) while the cis planar form was adopted from infrared evidence (5). EXPERIMENTAL
b
C
k’
k
(1)
three C-C bonds are parallel to the C s axis of symmetry of the complex ion in the D-kkk conformation and are oblique in the D-k’k’k’ form. The energy difference between the two forms was estimated ( I ) on interactions of nonbonded atoms to be roughly 1.8 kcal/mole in favor of the D-kkk. However, little direct experimental evidence has been obtained concerning the conformational equilibria in aqueous solution. Several attempts employing NMR techniques (5-8) have been reported. Present address, Research Center, Hercules Inc., Wilmington, Del. 19899 (1) E. J. Corey and J. C. Bailar, Jr., J. Amer. Chern. SOC.,81, 2620 (1959). (2) A. M. Sargeson, in “Transition Metal Chemistry,” R. L. Carlin, Ed., Vol. 3, Marcel Dekker, Inc., New York, N. Y., 1966, p 303. (3) J. V. Quagliano and S. Mizushima, J . Amer. Chem. SOC., 75, 6084 (1953). (4) K. Nakatsu, M. Shiro, Y.Saito, and H. Kuroya, Bull. Chem. Sac. Jup., 30, 158 (1957). (5) D. B. Powell and N. Sheppard, J. Chem. SOC.,1959,791. (6) H. Yoneda and Y. Morimoto, Bull. Chem. SOC.Jup., 39, 2180 (1966). (7) W. L. Jolly, A. D. Harris, and T. S . Briggs, Znorg. Chem., 4, 1064 (1965). (8) B. M. Fung, J. Amer. Chem. SOC.,89,5788 (1967).
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
Ethylenediamine, analytical reagent grade from Aldrich Chemicals, was redistilled over sodium metal. The anhydrous salt of nickel chloride was purchased from Alfa Inorganics and deuterium oxide from Columbia Organic Chemicals. Solutions of ethylenediamine and anhydrous NiCln were prepared with from 1 :1 to 3: 1 1igand:metal ion ratios by direct weighing into DzOfollowed by thorough stirring. The total nickel ion concentration for all solutions is 0.40 molar. Some solutions containing the 3 : l ligand to metal ratios were also prepared by dissolving solid [Ni(en)J (C104)~, prepared according to Pavkovic and Meek ( I 3 ) , in D20. The proton NMR spectra were taken at 100 MHz with a Varian HA-100 spectrometer equipped with a V-4333 variable temperature probe and a V-6040 temperature controller, operating on HR mode. A Hewlett-Packard Model 200 AB audio frequency oscillator was employed to achieve variable modulation frequency. Calibration of spectra was obtained with a 140 ppm modulation side band of the residual HDO in the DzO solvent. Chemical shifts were measured against an internal reference, the methyl resonance of the sodium salt of 3-(trimethylsilyl)-l-propanesulfonic acid (TMS*). Usually 4 scans, both down and upfield sweeps, were recorded (9) F. F.-L. Ho and C. N. Reilley, ANAL.CHEM., 41, 1835 (1969). (10) D. R. Eaton and W. D. Phillips, Advun. Magn. Resonance, 1, 103 (1965). (11) E. De Boer and H. Van Willigen, Prog. NMR Spectrosc., 2, 111 (1967). (12) L. N. Swink and M. Atoji, Acta Cryst., 13, 639 (1960). (13) S. F. Pavkovic and D. W. Meek, Znorg. Chem., 4, 20 (1965).
II
b
Figure 2. Spectra of trischloride in aqueous solution (ethylenediamine)nickel(II)
~
; / L ' ;(39'V
at different temperatures
I
1 I d "I/' I
I
A
'LA,
I
I
I
-150
-100
5d
I I
I
-50
0
d,
PPM
Figure 1. Contact shift spectra of 0.40M NiClz solutions containing various amounts of ethylenediamine
I
The ratio of [en]:[NiCIt]is ( a ) 1:l; (b)1.5:l; (c) 2:l; ( d ) 2.5:l; and ( e ) 3:l at 32 "C. Chemical shifts are ppm downfield from the internal reference TMS*
for each spectrum at the prescribed temperature. The probe temperature was calibrated with a copper-constantan thermocouple. RESULTS
The NMR spectra of the ethylenic protons of the coordinated ethylenediamines in aqueous nickel(I1) salt solution at the ambient probe temperature is shown in Figure 1. The ratio of concentration of the ligand molecule to that of the central metal ion was varied from 1 : 1to 3 : 1. For the 1 :1 complex, a predominant single sharp peak was recorded at -93.3 ppm. An equally pronounced sharp peak at -91.2 ppm was observed for the 2 :1 complex. With the ligand to metal ion ratio at 3:1, two equally intense peaks were obtained at -70.4 ppm and - 107.5 ppm, respectively, with the center at -88.9 ppm. The latter peaks are much broader than those obtained for lower ligand to metal ion ratios. The one at -107.5 ppm is considerably broader, with half-height width approximately 30 ppm, than that at -70.4 ppm. This solution was studied at various temperatures, and spectra obtained are shown in Figure 2. The sharp signals at the right of the spectra are, respectively, the residual HDO in the DzO solvent and the internal reference (smaller sharp peak at the extreme right). The sharp peak at the far left is the 140 ppm modulation side band of the H D O signal. The chemical shift data are tabulated in Table I. Solutions of [Ni(en),] (C104)%dissolved in D 2 0 gave exactly the same spectra as shown in Figure 2. However, because the solubility of the perchlorate salt is relatively lower, the spectral data were extracted from solutions prepared by mixing nickel chloride and ethylenediamine.
~ o l e / it t e r
Figure 3. Concentrations of [Ni(en),,12+ us. total ligand concentration Nickel concentration is 0.40M DISCUSSION
The NMR spectra of [Ni(en),]*+ in aqueous solution, shown in Figure 1, reveal the coexistence of several species (n = 1, 2, 3). This is expected from the reported formation constants of the complexes and the protonation constants of en. Calculation of the concentration of each [Ni(en),12+ species at different total ligand concentration added (aqueous solution at p H = 7.6) was made using a set of constants at 30 "C reported by McIntyre et al. (14). Figure 3 summarizes the results, and close agreement with the data in Figure 1 is seen. The tris-(ethylenediamine)nickel(II) ion was reported to have the octahedral configuration both in solid state and in (14) G. H.McIntyre, Jr., B. P. Block, and W. C. Fernelius, J . Amer. Chern. SOC.,81, 529 (1959). ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
601
Table I. Contact Shift of Bi(en)JCl2 in Aqueous Solution Contact shift ( 6 ) ~ (6 x T ) x 10-8 Temp. (OK) 1st peak 2nd peak 1st peak 2nd peak Center 305.1 -67.7 - 104.8 -20.65 -31.98 -26.32 323.6 -64.9 -98.0 -20.99 -31.72 -26.35 332.6 -63.4 -94.7 -21.09 -31.50 -26.29 342.6 -62.5 -91.3 -21.39 -31.30 -26.33 355.1 -63.3 -84.8 -22.47 -30.12 -26.29 (- 70.0) (- 26.22) -26.22 374.9 In ppm down field from the diamagnetic tris-(ethylenediamine)zinc(I1) chloride in DsO which has a single peak at -2.70 ppm from TMS* * At this temperature, both peaks collapsed to give a single peak.
soluticm (13). In several recent N M R studies of paramagnetic-octahedral nickel(I1) complexes (IO,15, 16), the observed ligand proton shift was discussed in terms of contact interaction resulting from the delocalization of the unpaired electron spin from the metal ion to the protons of the ligand molecules. The shift is expressed by the Equation (17, 18):
where A i is the hyperfine electron spin-nuclear spin coupling constant (in gauss) which is related to the actual unpaired electron spin density found at the ith proton (10). The contact shift of the spectra in Figure 1 is seen to decrease when the ratio of ligand to metal ion increases. This is in good agreement with the postulate (10) that the gain of unpaired electron spin by a n additional ligand is partially a t the expense of unpaired electron spin on the other coordinated ligands. The N M R spectra of paramagnetic nickel(I1) complexes of ethylenediamine in aqueous solution was reported by Milner and Pratt (16) and by Zamaraev er al. (19). The first group of authors observed a single signal for the 1 :1 complex and discussed the mechanism of contact interaction in terms of spin polarization and direct delocalization. No account was given of the conformation of the chelated ethylenediamine ring. Zamaraev et al. observed the spectra for various ligand to metal ion ratios at a frequency of 31.8 MHz, all at ambient probe temperature. They observed a single signal for the 1 :1 and 2 :1 complexes and a two-line pattern for the 3 :1 complex; they reported that the center of all the spectra remains practically the same. The lack of accurate chemical shift measurements can probably be attributed to the relatively low frequency of their measuring instrument. The two resonance lines in the 3 :1 complex were obviously observed and were attributed to the absence of fast conformational transition for the tris complex because it was felt that the ligands were considerably more closely packed than those of the 1 :1 and 2 : l complexes. On the other hand, fast conformational transitions were assumed for the 1 : 1 and 2 : 1 complexes and led to the observed complete averaging of all C-N protons of the chelated ligands. From the observation of nonequivalent -CH2- protons in the 3 : 1 complex (with SI/& a O S ) , it was concluded that the coordinated ethylenediamine ring is (15) R. J. Fitzgerald and R. S. Drago, J. Amer. Chem. SOC.,90, 2523 (1968). (16) R. S. Milner and L. Pratt, Discuss. Faraday SOC.,34, 88 (1962). (17) H. M. McConnell and D. B. Chesnut, J. Chem. Phys., 28, 107 (1958). ( 1 8 ) R. H. Holm, A. Chakravorty, and G. 0. Dudek, J. Amer. Chem. SOC.,86,379 (1964). (19) K. I. Zamaraev, Yu. N. Molin, and G. I. Skubnevskaya, Zh. Strukt. Khim. 7, 798 (1966). 602
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
nonplanar and the deviation from planarity (0 = 114'; see Structure 11) was estimated from the above mentioned chem-
(1) ical shift data. In contrast to these conclusions, our results suggest that fast conformational transitions occur in the 3 : l complex and that the appropriate deviation angle is 76' (see Section on Ring Puckering). In a previous report (9) on the studies of nickel(I1) complexes of N-alkylethylenediamines, we observed that fast conformational exchange occurs between the puckered conformations ( k and k ' ) of the five-membered ring. A single signal was observed for the methylene protons from the coordinated ethylenediamine and its symmetric tetra-N-methyl derivative. However, nonequivalent methylene protons were observed for unsymmetrically N-methyl substituted ethylenediamines, although conformational exchange is considered to be fast on N M R time scale in these cases also. The nonequivalency of the methylene protons was attributed to the population preference for the conformer of lower energy content-Le., the conformer in which the alkyl substituents occupy the quasi equatorial position. By studies of the temperature dependence of the contact shifts, conformational enthalpy differences among the fast exchanging conformations could be obtained. The spectra of tris-(ethylenediamine)nickel(II) ion in aqueous solution at various temperatures is also quite informative. It can be seen from the tabulated data in Table I that both peaks shifted upfield with increasing temperature, as can be expected from the Curie behavior in Equation 1. However, when the product of contact shift and absolute temperature is plotted against temperature, the trend shown in Figure 4 was observed. Expected horizontal lines were obtained from the center of the spectra of the tris complex (line 3) and from the tetraaquomonoethylenediamine complex (line 4). Apparent deviation from Curie behavior was observed for two signals, one with a positive deviation (points on line 1) and the other with a negative deviation (points on line 2 ) . These deviations may be explained (9) by a shift in the equilibrium constant for exchange between the two differently populated conformations shown in Structure I. The preference of population in our previous studies of 1:1 complexes came from the unsymmetric substitutions, e.g., the k
-’-’-’-’
1
v
3c
b‘ (-102)
Jbb’ - 104.8
4
4d -677
43 (- 10)
I
Y
+-observed-.
E
Figure 5. Contact shifts of methylene protons in a chelated ethylenediamine ring
e e c
-
$0
6, and 6 b are estimated shifts at probe ambient temperature at axial and equatorial, respectively, in a “frozen” conformation
h
cx
2
25
A
2
2c
I
3
I
320
,
1
340
,
1
1
360
31
T (‘IO
Figure 4. Temperature dependence of product
(aij X
T)
Points V and A are, respectively, data from the downfield and upfield signals of the tris-complex (Figure 2); line 1 and 2 are drawn using Equations 2 and 3 and values of -10.0 ppm for 6, and Expression 7 for A F ; Points 0 on line 3 are data from the center of the spectra of the tiis- complex; points 0 on the line 4 are data from the 1 :1 complex
form is preferred over the k ‘ if 1 = alkyl in Structure I. In the tris-(ethylenediamine)nickel(II) ion, with a given configuration about the asymmetric central metal ion (consider arbitrarily the D-configuration), the Dkkk conformer is preferred over the Dk’k ’ k ’ ( 1 ) because of the less severe nonbonding interaction between the atoms in the former. The k k’ exchange results in interconverting the -CH2- protons in the ring between two nonequivalent environments-namely, the equatorial and axial positions. The exchange process involves simply rotation about internal, single-bond axes of the metal-en ring ( I ) . The barrier of this process is low, in analogy with the cyclohexane systems, and the process should be fast on the NMR time scale at ambient temperature in aqueous solution. A single signal for all methylene protons of 1 :1 complexes of coordinated en and of tetra-N-methyl en shows complete averaging between two equally populated conformations. However, if the two conformations k and k’ are not equally populated, because of either the unsymmetrical substitution as in the 1:l complexes, or the nonbonded interactions as in the 3 :1 complexes, a multiline contact shift spectra is expected even when the k $ k’ interconversion is fast. With equal population of k and k ’ conformations, a residual time-averaged difference is to be expected, but its magnitude would be relatively small and is neglected in this work. The temperature dependent data is illustrated in Figure 4. The downfield signal (line 1) can be attributed to the C-Hb proton which is situated at the equatorial position in the most preferred k conformation, and possesses a dihedral angle with the Ni-N bond which is favorable for strong coupling with the unpaired electron spin at the nickel(I1) ion (9). As temperature is raised, the population of k ’ conformer is increased and the C-Ha proton would be found more frequently at the axial position, which has less coupling with the unpaired electron spin at the central metal ion. The result is a net shift upfield. This upfield shift brought about by changes of population is, of course, in addition to the upfield shift
predicted from Equation 1. The outcome is the positive deviation observed in the plot of (6 X T ) OS. temperature. This process explains also the opposite behavior of C-Ha proton, shown in line 2 of Figure 2. The degree of deviation of the product, (6 X T ) , us. T from the horizontal is an indication of the magnitude of the enthalpy difference between the two states and the free energy difference between the k and k’ conformations of a given coordinated ring can be found in the following way. The contact shift of methylene protons in the ring at the axial and equatorial positions with a giuen conformation is 6, and 6 0 , respectively, at room temperature as shown in Figure 5 . The observed shifts 6aal and 6 b b t are the result of the two conformers exchanging rapidly between two unequally populated states k and k ’ with a free energy difference, AF. The temperature dependence in Figure 4 (lines 1 and 2) can be depicted by following equations (9):
(3) where Kr (i = e or a) is the composite constant from Equation 1:
The value of AF in Equations 2 and 3 is temperature-dependent because of the entropy contribution: AF
=
AH
- TAS
(5)
The distance between Lines 1 and 2 in Figure 4, can be related to the free energy difference of the two conformations:
This can be visualized in the following way. If there were only very small free energy difference, Le., AF = 0 , then 6bb’ = 6aa,; this predicts that only a single resonance would be observed for the ethylenic protons. The fact that two resonances are observed indicates that AF is finite and that the separation of the observed peaks is a measure of AF. In order to calculate AF at a given temperature from the contact shift data in Table I, the value of the quantity ( K b Ka)is necessary. This quantity is the product of temperature and the contact shift difference between the equatorial and axial protons at that temperature. Because of the high interconversion rate of the k and k’ forms of the coordinated ethylenediamine ring and the use of aqueous media, the direct observation of ab and 6, for a “frozen” ring form is not plausiANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
603
Table 11. Relative Free Energy" Difference of Coordinated Ethylenediamines
Conformational change This workb Est'dc Dkkk Dkkk' 300 f 30 600 Dkkk -P Dkk'k' 600 f 60 1200 Dkkk + Dk'k'k' 900 f 90 1800 In units of cal/mole. 32 "C. Reference ( I ) . d From reference ( 2 1 ) ; discussed in the text. 4
From
( -)pnd
400 1200 1600
5
ble. Fortunately, 6 0 and 6, may be assessed in another way. 6,,!)/2 = ( 6 6 6,)/2; 6, can be In Figure 5, 6, = (6hbI calculated from the observed spectra and then 6 b can be calculated if 6, is known. The contact shift for the axial proton a t the probe ambient temperature of 32 "C is expected to lie further upfield than the observed high field signal of -67.7 ppm in Figures 2 and 5. A set of 6, values, in steps upfield from this peak, were arbitrarily chosen. For each 6,, a set of AF's were evaluated for the temperatures from 305.1 to 342.6 OK. A least-squares procedure is then utilized to fit the values of AF with temperature according t o Equation 5 and the standard deviation was obtained. Such calculations were repeated with other trial 6, values. With 6, values upfield from the observed high field signal, the standard deviation became smaller. With 6, = -10.0 i 5.0 ppm, the free energy change could be best expressed as:
+
AF = 360
+
- 0.2 T
(7)
With this expression and the chosen values of -10 ppm for 6, and -162 ppm for 6 b , Equations 2 and 3 may be used t o generate curves of ( 6 h b ' X 7') and (6,,~ X T ) us. T, which fit closely the experimental points. See Figure 4, lines 1 and 2. In addition t o the above procedure for selecting the 6, to be - 10.0 f 5.0 ppm, this value is close t o the observed shift of the axial protons in the ethylenic ring of nickel(I1) chelates of EDTA, CyDTA, and PDTA (20); in these complexes, the ethylenediamine ring has a fixed conformation due to the presence of the chelating acetate groups and averaging does not occur. In summary, the thermodynamic parameters for a n individual ring in conformational equilibrium k e k ' in the tris-(ethylenediamine)nickel(II) ion in aqueous solution are AF = 0.30 f 0.03 kcal/mole, A H -- 0.36 i= 0.03 kcal/mole, and A S = 0.20 f 0.02 e. u. (32 "C). A comparison of the free energy change found in this work with the pertinent data reported in the literature is presented in Table 11. The free energy values in column 1 indicate the existence of appreciable amounts of D-kk ' k ' and D-k 'k ' k ' at room temperature in equilibrium with D-kkk' and D-kkk. These relative free energies are derived from the treatment of data, shown in Figure 4, by use of Equations 6 and 7. It should be emphasized that Equation 6 assumes that the free energy of the k + k' conversion for a n individual ethylenediamine ligand is independent of the conformation of the remaining two ligands; thus, the free energies are additive and the relative energies or D-kk'k' and D-k'k'k' are, respectiveiy, twice and thrice that of D-kkk'. The relative abun-
F. F.-L. Ho, S. Watkins, L. E. Erickson, D. Young, J. B. Terrill, and C. N. Reilley, to be published. (21) A. M. Sargeson, in "Chelating Agents and Metal Chelates," F. P. Dwyer and D. P. Mellor, Ed., Academic Press Inc., New York, N. Y . , 1964, p 183. (20)
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
dances of these species changes over the temperatures employed and theoretically the ratios of free energies of 1 :2:3 could be refined and the results would be of great interest. Unfortunately, the data are not sufficiently precise for this purpose; nevertheless, the ratio of 1 :2:3 is not wildly off as reasonable agreement of lines 1 and 2 with the experimental points is obtained. The estimated values in column 3 of Table I1 are those reported by Corey and Bailar ( I ) on the basis of calculations on nonbonded interactions using the hydrogen-hydrogen potential function of Mason and Kreevoy (22). Corey and Bailar emphasized that although their calculated energy difference is only a rough approximation, the order of stability should be valid. No experimentally measured value for the energy difference was available for the aqueous ethylenediamine complex. The solid X-ray data on many [M(en)3]n+ complexes revealed the presence of only one conformation in the solid state for each complex. Dwyer and coworkers, in their studies of the cobalt(II1) chelate with optically active (-)-propylenediamine (pn), reported values for the free energy change for the equilibrium D-[Co(en),-( -)-(pn)~,J a+ e L-[Co(en),-( -)-(pn)&,] and these are included in Table 11. The values for the (-)pn and en are expected to be different. The absolute configurations of (-)pn and the tendency of the €Ha to be equatorial in the chelated ring cause this ligand to take the k' conformation as its preferred state. Therefore, the L-stereoisomer is favored because the ligand C-C bonds in L-k'k'k' are parallel with the molecular C3 axis. Dwyer and coworkers estimated that the free energy difference between the L and D isomers increases by 0.5 kcal/mole for each molecule of (-)pn added to the complex. This value is slightly greater than that found in this study of en complexes. The difference perhaps arises from the fact that in the complexes of (-)pn, more intense nonbonded interactions occur when the -CHI group replaces a proton. The experimental free energy values for the cobalt(II1) complexes of (-)-propylenediamine reported by Dwyer et af. and for the nickel(I1) complex of ethylenediamine in this work exhibit the same trends as estimates presented earlier by Corey and Bailar. The experimental values are, however, consistently smaller. The difference is not unexpected in light of several recent publications concerning tris-(ethylenediamine) complexes. Gollogly and Hawkins (23) pointed out that the calculation of nonbonded interaction energies based on Mason and Kreevoy's formulation (a high-energy expression used by Corey and Bailar) ( I ) leads to high value. The use of lowenergy expression of Hill's (24) in some cases gives smaller energy of interaction is in closer agreement with experimental value (25). Strong hydrogen bonding solvents like water probably make the conformational energy difference smaller too. The N-H protons in k' conformation are in a better position to form hydrogen bonds (26). Also, it has been reported (26,27)from X-ray studies that the ethylenediamine in [Cr(en)d [CO(CN)G]. 6H?O exists in k'k'k' conformation. In this complex, the cyanide anion and the water of crystallization provide many -
(22) E. A, Mason and M. M. Kreevoy, J. Amer. Chem. SOC.,77, 5808 (1959). (23) J. R. Gollogly and C . J. Hawkins, Znorg. Chem., 8, 1168 (1969). (24) T. L. Hill, J . Chem. Phys., 16,399 (1948). (25) D. A. Buckinpham, L. G. Marzilli, and A. M. Sargeson, ' Znorg. Chem., 7 ,
