NMROF 0''
IN
13C1 SOLUTIONSOF Go2+
1: 1 complex as rnentioned previously, in contrast to the 2-aminoethyl phosphate chelate system (the difference between successive stability constants, 0.39 in log K unit), Such a low stability of the 1:2 complex may be
121 ascribed to the two dominating factors: the low basicity of pyridyl nitrogen and the steric effect of the pyridine ring, which may generate a strong repulsive force against the second ligand.
Nuclear Magnetic Resonance of Oxygen-17 and Chlorine35 in Aqueous Hjydrochloric Acid Solutions of Cobalt(I1). I.
Line Shifts
and Relative Abundances of Solution Species'" by A. H. Zeltmann, N. A. Matwiyoff,lb and L. 0. Morgan'c University of California, Los Alamos Scientific Laboratory, Los Alamos, New Mexico (Received May 24, 1.967)
87644
The nmr spectra of oxygen-17 and chlorine-35 were observed for cobalt(I1) chloride solutions a t hydrochloric acid concentrations from 0.4 to 16 m. Downfield line shifts for both nuclear species were interpreted in terms of complexes present. Relative abundances of complex species were correlated by semi-thermodynamic equilibrium constants, and values of AH and A S were obtained for each equilibrium from temperature dependences of line shifts. Complex species found to be important were Co(Hz0)eZ+,CoC1(Hz0)6+,CoC12(H20)z,CoC1,(HzO)-, and CoC142-. The last one predominates a t the highest hydrochloric acid concentrations. Relative abundances of the three four-coordinate complexes were found to be compatible with spectrophotometric observations a t 619, 658, and 684 mF. Isotropic spin-exchange coupling constants in the complexes and the corresponding fractional s characters of the unpaired electrons in the ligand atoms were found to be: octahedral chlorine-35, 2.53 X lo6 cps, 0.0019; tetrahedral chlorine-35, 6.89 X lo8 cps, 0.0052; octahedral oxygen-17, 1.34 X lo7 cps, 0.0101; and tetrahedral oxygen-17, 3.26 X 107 cps, 0.0242. Data reported elsewhere by Horrocks and Hutchinson on proton shifts in aqueous cobalt(I1) thiocyanate solutions were reinterpreted, taking into account the trend to increasing tetrahedral coordination with increasing thiocyanate substitution in the complex species, to demonstrate that the assumption of constant Fermi contact interaction for ligand nuclei in a given symmetry is valid. For protons a small pseudo-contact term improves the data fit somewhat. The pseudo-contact term is unimportant for oxygen17 and chlorine-35 interactions, where Fermi contact terms are large.
Introduction The nmr spectmm of oxygen-17 in aqueous cobalt(I1) perchlorate solutions has been investigated by several grOUPs,2-4 and that of chhine-35 in aqueous cobalt(I1) chloride solutiorks at hydrochloric acid concentrations as high as 9.3 M 5also has been investigated. I n both cases, there is appreciable line broadening attributable to the paramagnetic species, and large downfield line shifts are observ.ed. In hydrochloric acid solutions, a very large increase in shift occurs in the intermediate concentration range, which appears to be associated
with the color change from pink to blue. Chesnuts attributed the shifts in low concentration of hydrochloric (1) (a) Work performed under the auspices of the U. S. Atomic Energy Commission. (b) Department of Chemistry, Pennsylvania state University, University Park,Pa. 16802. Visiting StaffMember. ( c ) Department of Chemistry, The University of Texas, Austin, Texas 78712. Consultant. (2) J. A. Jackson, J. F. Lemons, and H. Taube, J . Chem. Phys., 32, 553 (lQ60)* (3) T*J. Swift and R. E. Connick, ibidv 37, 307 (1962). (4) F. Klanberg, J. P. Hunt, and H. V. Dodgen. Inorg. Chem., 2 , 139 (1963). ( 5 ) D. B. Chesnut, J . Chem. Phys., 33, 1234 (1960).
Volume 71, Number 1
January 1.968
122
A. H.ZCLTMANN, N. A.
n/IATWIYOFF, AND
L. 0. MORGAN
acid to the presence of a single octahedral chloro coof H2017 from WO17 and of the technlques of the balt(I1) species, undergoing rapid exchange with magnetic measurements have been given previously. l 6 chloride ions, and the enhanced shifts in concentrated Results solutions to a single tetrahedral species present in low Downfield shifts of both 017and CISswere observed in abundance. Spectroscopists, working with concenall hydrochloric acid solutions of cobalt (11) chloride, trated hydrochloric acid solutions of cobalt(I1) salts, relative to line-center positions in solutions of equivalent have assumed the predominant species to be CoC142-.6 hydrochloric acid solutions in the absence of added Cotton, Goodgame, and Goodgame7 compared the paramagnetic salts. In the majority of cases, exchange spectra of cobalt(I1) in concentrated hydrochloric acid between coordinated species, C1- or H20, and the corresolutions with those of known CoC142- salts in organic sponding entity in bulk solution was found to be rapid, solvents and concluded that the tetrahedral species presso that3 ent is principally CoCIB(H20) -. The present work was undertaken to determine the AWN = p?iAwcO(x) (1) distribution of species over the entire range of acid in which p~ is the number of ligands AT complexed relaconcentrations and to establish the presence or absence tive to total N present and A W C ~ ( Nis) given by17 of water molecules in the coordination sphere of the highest chloro complex. Although the value of the first formation constant has been reported by a number of investigators,*-ls the results vary over a wide range. A N is the spin exchange coupling constant (in ergs), Best values for K1 lie in the region 0.2~0.7,but each is geffis the effective g factor observed for the paramag-, subject to some qualification, which creates doubt netic species, Y N is the magnetogyric ratio for the nuclei, as to its validity. Nevertheless, it seems probable that the monochlorocobalt(I1) ion, most likely C O C ~ ( H ~ O ) ~ +and , w is the resonant frequency of the unshifted nuclei. TAwN is constant with change in temperature, if there is formed with a stability constant from 0.1 to 1. Reis no change in species and exchange is rapid enough sults of most investigations suggest the formation of to give complete averaging. The latter was found to higher complexes, but no stability constants are rebe true in all except oxygen-17 shifts a t the lowest hyported. In most instances, experimental results suggest drochloric acid concentrations at 27’. Corrections that only a single species is observed in addition to were made using comprehensive line-broadening data to monochlorocobalt(I1) and that it is anionic and tetrahebe reported in a latter communication. dral. Observed shift values are listed in Table I, colamns From previous results of magnetic resonance measure10 and 12, for a series of solutions in which hydrochloric ments of oxygen-17 and chlorine-35 line shifts in cobaltacid concentration was varied from 0.424 to 15.69 m. (11) solutions, it appeared that determination of both, in Data are given in terms of A w ’ c , , ( ~ ) / w ~(K = 0’’ or the same solutions, might permit an accounting of the CISs), where several species to be made. With some judicious assumptions, that has proved to be the case, and the reAW’C~(N)= AWN/PN‘ (3) sults of those measurements are reported in the followand ing sections. pN‘ = moles of Co(II)/moles of N
Experimental Section Reagent grade cobaltous chloride hexahydrate was used without further purification. The salt was analyzed by means of displacement of hydrogen ions from a cation-exchange resin. The resultant acid solution was titrated with standard sodium hydroxide solution and the cobalt(I1) equivalence determined. The salt was found to have the composition COCIZ .5.98H20. Spectrographic analysis gave the following maximum abundances of transition metal impurities : Ni, O . O l ~ o; Fe, 0.003%; Cu, 0.000570; Cr, 0.001%; and Mn, 0.005yo. I n most cases, hydrochloric acid was added by passing gaseous hydrogen chloride (Matheson) into the solution, but in a few instances Baker’s Analyzed hydrochloric acid solution was added. I n each solution the OI7 was 1.7% or more. Descriptions of the preparation The Journal of Physical Chemistry
(4)
(6) T. Dreisch and W. Trommer, Z . Physik. Chem., B37, 37 (1937), and references cited therein. (7) F. A. Cotton, D. M. L. Goodgame, and M. Goodgame, J . Am. Chem. SOC.,83,4690 (1961). (8) P. Job, Compt. Rend., 196, 181 (1933); 198, 827 (1934). (9) D. F. C. Morris and E. L. Short, Electrochim. Acta, 7 , 385 (1962). (10) J. M. Smithson and R. J. P. Williams, J . Chem. SOC., 457 (1958). (11) M. W. Lister and P. Roaenblum, Can. J . Chem., 38, 1827 (1960). (12) B. Tremillon, Bull. Soc. Chim. France, 1483 (1958). (13) S, Tribalat and J. M. Caldero, Compt. Rend., 255, 925 (1962). (14) R. A.Robinson and J. B. Brown, Trans. Roy. SOC.New Zealand, 7 7 , l(1948). (15) R. H. Herber and J. W. Irvine, J . Am. Chem. SOC., 80, 5622 (1958). (16) A. H.Zeltmann and L. 0. Morgan, J . Phys. Chem., 70, 2807 (1966). (17) N . Bloenibergen, J . Chem. Phys., 2 7 , 595 (1957).
NMROF
0 1 7 IN
HC1 SOLUTIONS OF Co2+
123
Table I : Oxygen-17 and Chlorine-35 Line Shifts in Aqueous HCl Solutions a t 27 f 1' *')/ucI*') ( A ' O C ~ ( O ~ ~ ) / O ~ ~(~A'UCO(CI )
-MolalityHC1
0.424 0.932 1.99 3.41 3.89 5.79 6.66 7.88 8.21 8.97 9.70 10.20 11.00 12.00 12.30 12.68 13.55 14.26 14.66 15.69 a
---x CoClz
ai
a1
UO
(21
uz
0,109 0.117 0.123 0.122 0.115 0.139 0.123 0.133 0.139 0.157 0.138 0.168 0.136 0.152 0.134 0.139 0.157 0.150 0.151 0.160
0.491 0.975 2.39 5.64 7.22 18.8 27.7 44.8 51.3 71.7 93.9 114 151 213 234 264 344 422 470 618
0.973 0.951 0.898 0.814 0.798 0.672 0.615 0.541 0,520 0,467 0.425 0.397 0.355 0.308 0,295 0.279 0.246 0,222 0,209 0.180
0.922 0,852 0.689 0.457 0.390 0.159 0.096 0.040 0.029 0.011 0,004 0,001
0.078 0.148 0.310 0.534 0.595 0,752 0.727 0.565 0.490 0,279 0.136 0,072 0.023 0.005 0.003 0,002
...
Resonant frequency = 8.000 Mc/sec.
...
... ...
... ...
... ...
...
...
...
...
...
' Resonant frequency
The relation between p ~ and ' p~ is p;U = px'nam
(5)
and am is the Fraction of total cobalt(I1) in species m, which contains n coordinated groups containing nucleus N. That brealkdown is done to facilitate reduction of data in terms of' species and coordination number. For example, in cob,alt(11) perchlorate solution the principal species is Co(HzO)e2+,A W C o ( O I ? ) / W O l r = 0.101, a0 = 1-00, and n = 6. Tlhen, A W ~ ~ ( O ~ ~ ) = / W0.0169. O ~ ~ Behavior of T A o ' c ~ ( N ) / w N as functions of temperature for several hydrochloric acid concentrations is shown in Figures 1 and 2. At the lowest concentrations, the shifts for 0 1 7 at room temperature and below were not at their saturation (rapid averaging) values. The 27" values, listed in Table I, were corrected for incomplete exchanges3 Shiifts for C136were in the limiting region for all solutions at 27". Measured extinction coefficients were obtained at 684, 658, and 619 mp for a large number of cobalt(I1) chloride solutions in the hydrochloric acid concentration range covered b,y the magnetic resonance measurements. Spectra were observed using a Gary Model 14 visibleultraviolet specltrsmeter, the cell compartment of which was kept at 26 f 0.2". Cobalt(I1) concentrationsfor those measurements were in the range 0.001-0.02 m; 1-cm cells were used for the measurement a t the lower concentrations of cobalt(I1) and 1-mm cells for those a t the higher concentrations. The spectra of standard aqueous solutions of methylene blue(pH 2.0) were used to obtain the relative path lengths. The data are shown as individual points in Figure 3. The system conformed
88
...
...
...
0,002 0,010 0.015 0.080 0.148 0.273 0.306 0.335 0.285 0.223 0.134 0.061 0.048 0.035 0.017 0,010 0.007 0.003
...
...
U1
...
... ... ...
...
...
0.007 0,021 0.070 0.094 0.159 0.195 0.199 0.177 0.132 0.119 0.103 0.074 0.056 0.048 0.032
0.002 0,008 0.051 0,082 0.215 0.380 0.504 0.666 0.802 0.830 0.860 0.909 0.935 0.945 0.965
= 5.000 Mc/sec.
Exptla
(1003)" (993)" (968)o (930)" (896)' 849 812 726 694 538 398 300 199 99.6 90.0 75.0 44.9 33.6 31.8 27.0
104--Calod
999 987 959 919 908 861 835 728 678 505 420 317 195 104.4 87.7 70.0 43.0 29.6 24.4 15.0
-x
Exptlb
3.7 6.1 11.2 21.2 26.7 52.9 75.1 146 168 245 328 348 392 428 435 438 447 447 453 456
104--. Calod
3.1 6.6 14.1 25.9 29.8 54.9 77.3 136 162 245 315 357 401 431 436 441 450 454 455 458
Corrected for incomplete averaging.
to Beer's law throughout the range represented. Above 15 m HCl, the spectrum does not change and E. at a given wavelength is constant to the limit of solubility of hydrochloric acid and lithium chloride. Each experimental point represents a separate sample, for which cobalt (11) and hydrochloric acid analyses were carried out. Analysis of Shift Data. A qualitative indication of the nature of changes in the cobalt(I1) hydrochloric acid solutions may be obtained by casual examination of the tabulated and plotted results. In Figure 1, the rapid rise with temperature at low HC1 of normalized oxygen17 shifts in the low temperature region is the result of increasing exchange rate for HzO. I n cobalt(1.I) perchlorate aqueous solution, values become constant at about 35". Cobalt(I1) chloride in hydrochloric acid solutions produces oxygen-17 shifts, which decrease at higher temperatures because of increasing chloro complex formation; the mean HzO coordination number a decreases. The opposite effect is seen in Figure 2 for chlorine-35 shifts as f i for C1- increases. Tabulated shifts at 27" show the effects of increasing hydrochloric acid concentration, as a decrease in oxygen-17 shift values is offset by increase in those for chlorine-35. At the highest concentration, the observed oxygen-17 shift is less than 3y0of the values observed a t very low concentrations of hydrochloric acid. It is apparent that very little exchanging HzOremains in cobalt(I1) complexes at that point. I n the 5-9 rn hydrochloric acid range, the relative decrease in oxygen-17 shifts is considerably larger than the corresponding increase in chlorine-35, suggesting a decrease in total coVolume 72,Number 1
January 1968
A. H. ZELTMANN,N. A. MATWIYOFF, AND L. 0. MORGAN
124
26
28
30
32
I / T x IO3
34
36
38
4.0
(“Kl-’
Figure 1. Temperature dependence of 8-Mc/sec 01’ shifts in cobalt (11) solutions: (A) Co(C104)2 in 0.017 m HCIOa, po’ = 0.00203; (B) CoClZ in 5.73 m HC1, po’ = 0.002032; ( C ) CoClz in 7.8 mHC1, PO‘ = 0.002401; ( D ) CoClain 11.3 m HC1, PO’ = 0.003854; and (E) CoClt in 13.8 m HC1, PO’ = 0.004493.
Figure 3. eSvg as a function of HC1 molality; the circles represent experimental data. The solid lines were = calculated using the following equations: 1 3 . 8 ~ 1 6 7 . 8 ~ ~ 372.9aa; eeja 72.1~ l 8 O . l ~4-5 5 5 . 7 ~ ;and 8684 = 7 6 . 2 ~ 2 0 3 . 7 ~ 585.7oca.
+
+
+
+
+
restriction t o fewer species. The minimum number of steps among the appropriate species is represented by the equations
+ zCoCI(HZO),+ + HzO O )3Hz0 ~ CoCl(HzO)rj+ + C1- zC O C I ~ ( H ~+ CoClz(Hz0)z + C1- I_ CoCla(Hz0)- + Hz0 CoCL(Hz0)- + C1C0C142- + HzO CO(HZO),~+ C1-
IO
(6)
(7) (8)
(9)
Designating the cobalt(I1) species by the fractional abundances, am,with m specifying the number of coordinated ,C1-, the successive equilibrium constants are with a1 = water activity and ukcI = mean Clactivity
26
2s
30
32
i / r x io3
34
36
313
40
(OK)-’
Figure 2. Temperature dependence of 5-Mc/sec ClS6shifts in cobalt(I1) chloride solutions: (A) 13.8 m HCI, pel' = 0.01748; (B) 11.3 m HC1, ~ C I = ’ 0.01826; ( C ) 7.8 mHC1, pci’ = 0.01651; (D) 5.73 m HCl, pci’ = 0.01894, measured at 3 Mc/sec; (E) same solution as D measured a t 5 Mc/sec; (F) 2.0 m HCl, pol’ = 0.0445; and (G) 0.424 m HC1, pci’ = 0.1683.
ordination number. That decrease parallels the change in solution color from pink to blue and the increase in eav at 684, 658, and 619 mp. eav is the absorbance in a l-em cell divided by cobalt(I1) molarity. Quantitative treatment of the data was based on equilibria among the several proposed complex species. A number of combinations were tried, including dehydration of the various chloro species without gain of C1-, change in total coordination number a t later stages, and T h e Journal of Physical Chemistry
K1 = (a1/CYo)(a1/a*)
(10)
Kz
(11)
= (~Z/(Y (al3/ad I)
K3 =
(allad
(a3/ aZ)
(12)
K4 = ( ~ ‘ 0 1 3 ) (ai/a*) (13) wherein it is assumed that the ratio of activity coefficients for the cobalt(I1) complexes is constant. Thus, the equilibrium constants are semi-thermodynamic. That procedure is open to question where the charge type of the species changes and the nature of the medium varies significantly over the range of validity of the equation. Nevertheless, some such assumption is necessary and that one has proved to be reasonable in EL number of instances.18-20 It was assumed that ligand-shift values depend upon coordination number and configuration only. Thus, (18) J. Bjerrum, Kgl. Danske Videnskab. Selskab., Mat.-Fys. Medd., 22, No. 18 (1946). (19) R. NasBnen, Acta Chem. Scand., 4, 140,816 (1950). (20) G.A.Gamlen and D. 0. Jordan, J . Chem. Soc., 1435 (1953).
NMROF
0 1 7 IN
125
HC1 SOLUTIONSOF Co2+
with octahedral complexes in the low hydrochloric acid concentration region and tetrahedral species a t high concentrations, there are four shift parameters to be considered: AWoctCo(0I7), AuoctCo(Clas), AOtetCo(017), and AutetCo(C1u). Those parameters and the several K’s were varied to obtain best fit to experimental data. A CDC 6600 subroutine was written for a least-squares programz1to obtain the best set of shifts and constants and to calculate net shift values on the basis of those parameters. Stoichiometric and ionic-strength corrections were built into the program as described below. Calculated net, shift values are listed in Table I, columns 11 and 13, for the constants given in Table 11. Relative abund,ances of the species which correspond to the listed constants are presented in Table I and plotted in Figure 4.
HCI MOLALITY
Figure 4. Abundances of the indicated cobalt complexes as a function of HCl molality; CoCl*molality, 0.11-0.16.
The mean ion activities of HCl have been used throughout this work in place of single chloride-ion activities which are unobtainable. Because we are primarily concerned with presenting equations which will readily permit the recovery of species concentrations over a wide range of hydrochloric acid concentrations, we are satisTable I1 : Equilibrium Parameters for Cobalt(I1) fied that the procedure is reasonable. Earlier attempts Species in Aqueous Hydrochloric Acid Solutions‘ to obtain thermodynamic constants were made by apAS,b proximating mean ionic activity coefficients from data on AH,^ entropy Equilibrium constant stand-in substances. However, truly suitable stand-in (300‘K) kcal/mole units salts for which adequate data are available are not to be 2 . 9 k1.0 +6.113.1 K I 0.17 rt 0.06 found. Standard deviations of the various parameters 2.1 z t 1 . 0 -5.6rt3.4 Kz (1.7i: 0.4) X resulting from the data-fitting process are much larger 11.3 rt 1.3 +26.2 k 4 . 4 K a (3.1 k 0 . 9 ) X 10-3 0.80k0.4 -6.7h1.4 K~ (8.8rt 1.7)x 10-3 than those for the parameters obtained by the procedure outlined previously. The added computational difa Based on (A( w4; (Awtetc,(cia6)/wcia6) = (115.6k 1.2) x stand-in activities are used makes that method much less 10-4; all a t 300’K. ’ I n calculations of this quantity, no attractive than the one used here. account was taken of the variation of activity coefficients with Horrocks and HutchinsonZ4have correctly suggested temperature. This quantity therefore includes a contribution that the validity of the isotropic shift method of deterdue to that effect. mining a for a ligand may be questioned for pmr shifts since pseudo-contact interactions become important when a large g-tensor anisotropy is accompanied by a Approximate values of the K’s were used t o calculate relatively small contact interaction coupling constant. fractions of each complex species. The approximate However, I7O and 35Clhyperfine coupling constants are ionic strength was then obtained by the relation large, 1.34 X lo7cps and 2.53 X lo6 cps, respectively, 1.1 = mHc1 -I- mcoc12(3Qo LYI ~ 4 ) (14) and this effect-does not invalidate the isotropic shift method as is shown in the calculation below. With The electron-proton coupling constants for the equa~ / p = 0.9972 0.01817~~ ~0.015mc0ci2 ~1 (15) torial and axial methanol OH protons of the complexes (HzO)Co(CH30H)52+ and C ~ C O ( C H ~ O Hhave ) ~ + been values of rl(HC1) were calculatedzzfor solutions up to 4 determined directly: AM^^ = 2.9 X lo5 cps, AMaxia1 = m HCI. For higher concentrations, those values were 6.0 X 106cps, andAMeq = 4.9 X 105 cps, and AMaXia1 = obtained from the data of Akerlof and TeareaZ3 I n each 3.3 X los cps, respectively.26~z6 For these complexes, instance, the activity of water, ul,was gotten by integration of the Gibbs-Duhem equation over the appropriate (21) This program is based on R. H. Moore and R. K. Zeigler, Los range of ?&. The activity values were then used to Alamos Scientific Laboratory Report LA-2367, Los Alamos, N. M., Oct 1959, available from the Office of Technical Services, U. 8. Dedetermine best values of the K’s by least-squares fit of partment of Commerce, Washington, D. C. shift data for both oxygen-17 and chlorine-35. Calcula(22) H. S. Harned and B. B. Owen. “The Phvsical Chemistrv of tions were then repeated to refine the activity coeffiElectrolyte Solutions,” 3rd ed, Reinhold Publishing Corp., New Ybrk, N. Y., 1958, p 469. cients. Free chJoride-ion concentrations were obtained (23) G. Akerlof and J. W. Teare, J . Am. Chem. SOC.,99, 1855 (1937). from
+ +
+
[Cl-I = mHCl
+
+
~ C ~ C ~ , ( % Q OQI
- a3 - 2
Chloride activities are given by uk = rk[C1-].
(24) W. Dew. Horrocks, Jr., and J. R. Hutchinson, J . Chem. Phgs.,
4
(16)
46, 1703 (1967). (25) Z. Lus and S. Meiboom, ibid., 40, 1058 (1964).
(26) 2. Lus, ibid., 41, 1748 (1964). Volume 78, Number 1
January 1068
A. H. ZELTMANN, N. A. MATWIYOFF, AND L. 0. MORGAN
126 the average A M values, from which the chemical shifts in the rapid-exchange limit can be calculated, are 3.5 X lo5 cps and 4.6 X lo5 cps, respectively. These values are nearly the same as the A M values measured for the unsubstituted complex Co(CH30H)s2+: 3.9 X lo5 cps, and 4.5 X 106 cps, respectively (the difference in the A Mvalues for this complex was attributed by Lue26 to experimental error and "salt effects"). These data, which are a result of direct measurement, demonstrate that the A M values for the mixed complexes are not significantly different from those of the unsubstituted complexes even for pmr measurements. The complex, (CH30H)5CoCl+,should be reasonably similar to the chloro-aquo complex of Co(II), which we studied. Equations 3 and 4 of ref 24 may be considered to define a coupling constant A,. Rewriting eq 3
If ratios of A , and A , are t o be meaningful, the coupling constants must be in the same units
-=
+ 1) x
-p"(S
V
(39Il2
+ g l l g l - 4919 (3 cos2 x - 1) r3
45kT
+
We may substitute for A v , / v in the previous equation to find the following definition of A,(")
where the individual terms refer to the complexes Co(HzO)6 +, Co (NCS) (HzO)5+, Co (NCS)z(HzO)2, and CO(NCS)~(H~O) -, respectively, with A v H " ~ = 4670 cps and A v H = ~ 22,390 ~ ~ cps. The fit may be improved by assuming a pseudo-contact interaction contribution to the proton shift equal to about 20% of AvH"~ for Co(NCS)(H2O)j+. The two parameters then are AvHoCt = 4530 cps and A v H = ~ 21,160 ~ ~ cps.
species shown in Figure 4 that attempts to recover specific species distributions of the four coordinate complexes by most available methods, such as spectrophotometry, would be quite difficult, if not impossible. Solid curves in Figures 3 are those calculated from the concentrations given in Table I and least-squares deviationestimation of the molar extinction parameter. The curves were calculated using the following equations
€658
+ + = 7 2 . 1 ~ ~+2 1 8 0 . 1 + ~ ~5 5 5 . 7 ~ ~ 4
€684
=
€019
+
(3911~ gllgl
- 4gLZ)(3 cos2 x - 1)
15
r*(Co-H)
Cobalt-hydrogen and cobalt-oxygen distances were estimated from data of BullenZ7for cobalt(I1) bisacetylacetone dihydrate. Therefore
Anticipating that A , ( 0 1 7 ) ~ 4 0 1 7 )
AM = A ,
--
~ 4 0 1 7 )
Ap(O17)
-
+ A,
N
is negligible relative to
A , = 1.34 X 10'
1-34 x 107 1.72 X lo5 5.772 2.75 X 2 sT(2.05 X
+ ( 2 ~ " )+ p('))AvbItet
AV = (6pc6)
Discussion It is apparent from the relative mole fractions of
Since AVP
A similar calculation may be made for 35Cl,indicating that the pseudo-contact interaction may be neglected in that case also. An alternate interpretation of the results for the cobalt(I1) thiocyanate systemz4 follows, assuming that temperature-dependence data would show the system to be in the fast-exchange region. Using four thiocyanate equilibria, analogous to eq 6-9 of this work, the proton data may be fitted quite well with the following two-parameter equation
CPS
= 1 3 . 8 ~ 167.8~~3 372.9~~4
and
+
+
7 6 . 2 ~ ~ 2 2 0 3 . 7 ~ ~5 8 5 . 7 ~ 4
The fit to experimental data is better than the experimental error over the entire range of hydrochloric acid concentration. If the procedure is reversed and the observed data are interpreted in terms of expected cobalt(I1) species, a single four-coordinate species suffices t o rationalize the experimental results. The spectrophotometric method is not sensitive enough t o the presence of the intermediate species t o serve the analysis of the system. I n the treatment of successive equilibria, Bjerrum, Na~anen,~g and Gamlen and Jordan20 eliminated water activity from their formulations. While this resulted in somewhat different equilibrium constants, when our data were treated in the same manner it did not lead to different interpretation. It was noted that the weighted variance of computed and experimental shifts was larger than in the treatment using water activity and (27) G. J. Bullen, Acta Cryst., 11, 703 (1959).
The JOUTnal of Physical Chemistry
NMROF
017IN
HC1 SOLUTIONS OF Co2+
127
the average deviation was greater than the experimental The constant reported here is believed to be the better error of an individual shift determination. of the two. Although the relative abundances of dichloro and The ratio of f a values for oxygen-17 and chlorine-35 trichloro complexes are low, their presence indicates in octahedral species is 5.3, while that in tetrahedral is the pathway bg which successive coordination of chloride 4.7. It is generally true that fractional s character inoccurs and confirms CoC12- as the dominant species in creases from fluorine to nitrogen in the second row of the concentrated hydrochloric acid solution. While there periodic table. I n CUF~(H,O)~, the f a for fluorine is are apparently significant differences between the spectra 0.0055 and that for oxygen is 0.011.31 It is also to be of C 0 c 1 ~ ~in- concentrated hydrochloric acid solutions expected that increased separation of s and p levels in and those in organic solvents7 the oscillator strengths chlorine, as compared to fluorine, decreases the s characof the former are not more than 8-10yo less than those ter of the bonding hybrid. Those two factors may be of the latter. The oscillator strength of ( % A S ) ~ C O C ~responsible ~ for the rather large observed ratios. in CHzClzis 5.38 X as compared to 5.30 X for the cobalt(I1) species in 16 rn HCI. That difference Table 111: Isotropic Spin-Exchange Coupling Constants for is presumably attributable to relatively strong ion-solOxygen-17 and Chlorine-35 in Cobalt(11) Complexes vent coupling in aqueous solution. Ligand shifts may be interpreted in terms of isotropic (Alh) x 10-7, ( A / N hyperfine coupling constants in the complex ions using Principal radian X 10-6, fs X eq 2. Static nnagnetic susceptibilities of octahedral and Nucleus Symmetry species used sec-1 cps 108 tetrahedral cobalt(I1) species were taken to be 5.00 and Clar Octahedral CoCl(H,O)E 1.59 2.63 1.9 4.80 Bohr mag;netons, respectively. Nuclear magnetoCl*s Tetrahedral CoC1424.33 6.89 5.2 gyric ratios are 3.628 X lo3 radians sec-' gauss-' for 017 Octahedral C O ( H , O ) ~ ~ + 8.40 13.4 10.1 oxygen-17 and 2.624 X lo3for chlorine-35. The several CoCl(HzO)6 017 Tetrahedral CoCl3(H20)- 20.5 32.6 24.2 coupling constants are listed in Table 111,together with the estimated fractional s character in the ligand atoms. The latter are based on (Az,/h)o1, = 4.61 X log sec-' Acknowledgments. We are indebted to Dr. W. Burton and (A38/h)~lsb = 4.46 X l o 9 sec-', calculated from Lewis and others for helpful comments during discusHartree functions,2*J9and 3O sions of the work reported here. We also wish to thank fs = ~ S A N / A , ~ (17) Patricia Stein who made many of the measurements, Dr. B. B. McInteer and Mr. R. M. Potter of the Los The ratio of coupling constants in tetrahedral and octaAlamos Scientific Laboratory for supplying the enhedral configurations is 2.49 for oxygen-17 and 2.80 for riched NO17,and Mr. s.V. Hooker of Pennsylvania State chlorine-35, as compared with 2.25 estimated from unUniversity for recording some of the electronic absorppaired electron occupancy of u antibonding orbitals in tion spectra. tetrahedral and octahedral cobalt(I1). Our value of (A/h) for oxygen-17 in octahedral configuration is (28) D.R. Hartree, W. Hartree, and B. Swirles, Phil. Trans. Roy. slightly lower than that reported by Swift and Connick,3 SOC.London, A238, 229 (1940). 1.48 X lo7 see-'. That value was obtained from the (29) D.R.Hartree and W. Hartree, Proc. Roy. SOC.(London), A156, 45 (1936). shift listed by Jackson, Lemons, and Taube.2 Our (30) F. Keffer, T. Oguchi, W. 0. Sullivan, and J. Yamashita, Phys. value for the coupling constant is based on much Em., 115, 1553 (1959). larger observed shifts from a variety of samples, and the (31) R. G.Shulman and B. J. Wyluda, J. Chem. Phys., 35, 1498 oxygen-17 concentration was considerably greater. (1961). +
+
Volume 7.8, Number 1 January 1968
