NOTES
1814 The linear relationship of Figure 1 makes it apparent that the P values are proportional to EI, and therefore, we must have
-3.8
I
I
I
I
I
I
-3.0
If, then, we have an absolute value for any one of the K's of the alcohols or water, we are in a position to establish a quantitative scale of K's for the entire series of compounds. As indicated above, the values for the alcohols are not known with any reasonable degree of certainty, and the values for water are widely disparate. We are prompted, therefore, to select arbitrarily one of the values for H 3 0 + upon which to construct the scale. The value decided upon is that of Hammett and D e y r ~ p ,-3.43 ~ for pKBHt of H30+, since this was obtained by what appears to be a valid experimental procedure and it is, incidentally, the median (and nearly the mean) value of the five quoted above. From eq 2 it is seen that the P K B Hof~ the alcohol can be calculated from
+P
PKBHt(R0H) = PKBH+(HOH)
= -3.43
fP
(3) Table I1 gives the new values for PKBH"for various alcohols as calculated from eq 3.
+ for Various Alcohols Table 11: Calculated ~ K B HValues
Alcohol
HOH MeOH
EtOH n-PrOH n-BuOH i-PrOH
t-BuOH C-CGH~~OII CeHbCHtOH ClCHzCHzOH ClaCCH20H FsCCH20H a
-3.43Q -2.17 -1.93 -1.91 -1.90 -1.72 -1.49 -1.72 -2.32 -2.98 -4.20 -4.27
-3.44 -2.17 -1.92 -1.70 -1.68 -1.67 -1.49
..*
% -2.2
- 1.4 10.0
10.5
11.0
11.5 EI, eV.
12.0
12.5
Figure 2. A plot of the calculated ~ K B Hvalues + for various alcohols us. the ionization potentials, EI, of the alcohols.
The slope of the line, a, is found to be -0.72718 and therefore
~ K B H=+ +5.73 - 0.72731 (5) The p K m t values calculated from eq 5 are also included in Table 11. Calculated values are not shown for the last five alcohols listed because their corresponding ionization potentials either have not been experimentally determined or they do not represent removal of an electron from an oxygen lone pair (e.y., C6H;CH20H). Using eq 5 we can, however, calculate the ionization energy from the ~ K B H +The . values obtained are as follows: c-CGH1lOH, 10.2 eV; C6H&H20H, 11.1 eV; C1CH2CH20H, 12.0 eV; C13CCH20H,13.6 eV; F,CCH20H, 13.8 eV. In the future, should a different and better value for the P K B H - of H30+be obtained, this relationship is still valid and the only change would be a shift in the line of Figure 2 up or down, correspondingly. (18) ilctually, a least-squares treatment gives the slope as -0.691 with a correlation coefficient of 0.990. However, chemical intuition requires, in this case, placing greater emphasis on the series H , Me, Et, i-Pr, t-Bu and lesser weight on n-Pr and n-Bu.
... ...
Standard.
Pure Nuclear Quadrupole Resonance I n Figure 2 the new P K B H values ~ for the alcohols are plotted vs. the corresponding EI values from Table I. The excellent correlation would appear t o justify the assumptions and the procedures adopted in this analysis. It is interesting to note that our ~ K B Hvalue + for CHBOHz+,-2.17 is in close agreement with the Raman experimental4 value, -2.2, but it does not agree well with the solubility value516 of -2.5. The equation for the straight line of Figure 2 relating the ~ K B Hof+ROH2+ to the ionization potential (in eV) is given by PKsHt = 4-5.73 f aEI T h e Journal of Physical Chemistry
(4)
i n Hexaohlorostannates of Hydrated Divalent: Cations by Jack D. Graybea1,l Ruth J. ilIcKownj2 and Shen D. Ing'& Department of Chemistry, W e s t V i r g i n i a University, Morgantown, W e s t V i r g i n i a 26506, and the Department of Chemistry, V i r g i n i a Polytechnic Institute, Blacksburg, V i r g i n i a 24061 (Received September 22, 196.5)
The pure nqr frequencies of KzSnCla, (NH&3nCls and RbSnCl8 at 23' have been reported to be a t 15.065
NOTES 15.474, and 15.60 MHz, re~pectively.~The crystal structures of these compounds are cubic with the cations occupying the corner, face-centered, edge-centered, and body-centered positions and the S I ~ C anions ~ ~ ~ occupying alternate octant center position^.^ If AI(HzO)62+ cations were substituted for the univalent cations in this structure and if the electric field gradient (EFG) tensor components at the chlorine nuclei possessed an appreciable contribution from the cation charges, then there should be a noticeable difference between the nqr frequencies of these two type compounds. If the primary contribution to the chlorine EFG tensor is, however, from the electrons in the Sn-C1 bond then such a substitution should produce little change in the nqr frequencies. Of the compounds studied, only the detailed crystal structure of Ni(H20)6SnC16has been determined.6 It has a rhombohedral structure with CY = 96" 45'. Except for the slight skewing of the angles, the cation surroundings of the SnC162- anion is very similar to that in the cubic structure of the AIzSnCla compounds with a n/I(H20)62+replacing the corner M + ions. Unit cell dimensions for the compounds of RIg2+, R/In2+, and Ni2+have been reported.6 All three were found to be hexagonal with slightly different a and c dimensions. This presents a slight conflict regarding the determined structure of Ni(H20)6SnC16). The Sn-C1 bond lengths determined for the RlzSnC16compounds* and for Ni(Hz0)6SnCls5all lie between 2.41 and 2.45 A, indicating little effect on this bond by the cation. Bonding of the dn-pn type between tin and chlorine atoms has been suggested by many authors. The low values of the nqr frequencies of chlorine in the group IVa tetrachlorides have been cited as evidence for this type of A determination of the chlorine nqr asymmetry parameters in the group IVa tetrahalidesg strongly suggests the presence of such type bonding. The structure of the SnC1e2- anion has O h symmetry and is geometrically more suited for a-bond formation between the tin atom and the surrounding chlorine atoms than is the structure of SnC14 where one has the less symmetrical Td group. If this were the only factor t o be considered then the nqr frequencies of the SnCI6*- chlorine atoms should be less than those in SnCI4. The longer Sn-C1 bond length in the SnC1e2- anion as compared to that in SnC14 (2.33) indicates that the Sn-CI bond in the former is probably more ionic and perhaps also possesses less a-bond character. Although there is this indication of less a-bond character in the SnCls2- anion, it certainly does not suggest the absence of x-bond character. The lower value for the resonance frequency in SnC162-, as compared to SnC14, could be due to more ionic character, more a-bond character, or some compensating combination of these. It is not possible from an experiment of the type re-
1815 ported to unambiguously characterize the amounts of both ionic character and n bonding. Haas and RIarram'O have suggested that a molecule, in which n bonding of a chlorine atom to another atom occurs, should show a decrease in a bonding with increased bending vibration activity, ie., an increase in temperature. This in turn should result in an increase of the nqr frequency with increasing temperature. The normal behavior of nqr frequencies was first discussed by Bayer" and consists of a decrease with increasing temperature. AIeasurement of the temperature coefficients of the nqr frequencies can provide further evidence regarding the nature of the bonding. Using the results of Haas and Rfarram,'O the temperature coefficient due to vibrational activity is estimated to be about +1.5 kHa deg-l. The normal Bayer coefficient is of the order of - 2 kHz deg-I.
Experimental Section All of the compounds studied were prepared using a published method for preparation of hexabromostannates.I2 The preparation of all compounds, except &?(H20)6SnCl6, consisted of adding 0.2 mol of reagent grade metal chloride in 100 ml of 2 N HC1 to 0.2 mol of SnC14in 50 ml of 2 N HCl, evaporation of the resulting solution on a hot plate until crystal formation began, cooling of the solution to room temperature, and recovery of the product by filtration. For the preparation of Ni(Hz0)6SnC16 a 2-3 molar excess of SnCI4 was found to be necessary to produce the product in good yield. All compounds mere dried in a vacuum desiccator over CaCl2. All of the compounds are very hygroscopic and are easily hydrolyzed. The composition of all compounds was ascertained by elemental analysis for tin and chlorine. All nqr frequencies were measured using a noisecontrolled super regenerative spectrometer.l 3 The method used for frequency measurement has been (1) (a) Department of Chemistry, Virginia Polytechnic Institute, Blacksburg, Va. 24061. (b) To whom correspondence should be addressed. (2) Department of Chemistry, West Virginia University, Morgantown, W. Va. 26506. (3) D. Nakamura, Bull. Chem. SOC.Jap., 18, 183 (1963). (4) G. Engel, 2. Kristallogr., A90, 341 (1935). ( 5 ) L. Pauling, ibid., 72, 482 (1930). (6) M. Giglio, H. Novales, and A. Arias, Naturwissenschajten,52, 182 (1965). (7) R . Livingston, J . Phys. Chem., 57, 496 (1953). (8) M . A. Whitehead and J. H. Jaffe, Theor. Chim. Acta, 1, 209 (1963). (9) J. D. Graybeal and P. J. Green, J . Phys. Chem., 73, 2948 (1969). (10) T. E. Haas and E. D. Marram, J Chem. Phys., 43, 3985 (1963). (11) H. Bayer, 2.Phys., 130, 227 (1951). (12) M.Gatierrez De Celis and J. A. Quinoa, Acta. Salmanticensia, Ser. Cienc., 1, 14 (1955). (13) J. D. Graybeal and R. P. Croston, Rev. Sci. Instrum., 38, 122 (1967). Volume 74, Number 8 April 16,1QYO
NOTES
1816
described e1~ewhere.l~All frequency measurements are accurate to i0.005MHz. The observed nqr frequencies a t 300°K are given in Table I.
Table I : Nqr Frequencies of Hexachlorostannates of Divalent Cations
chlorine atom are increased. If the EFG tensor has any appreciable contribution from the cation charges then it should result in lower coupling constants for the divalent cation compounds. The fact that there is little difference in the nqr frequencies of the two series indicates that the EFG tensor is determined predominantly by the bonding electrons. The average temperature coefficients for ;\fg(HzO)~SnC16and Ca(HzO)SnCl6 are given in Table 111.
Table 111 : Temperature Coefficients for S q r Frequencies Of
h~g(HzO)eSncl6and Ca(HzO)f,SnCls Temp range, OK
300-188 300-240 188-113 240-77
The temperature dependence of the nqr frequencies of 34g(HzO)e,8nCleand Ca(HzO)6SnClewere studied from 77 to 300°K by use of constant temperature slushes. The samples were placed in a finger dewar with the sample immersed in the slush and the oscillator coil external to the finger. The results of these studies are given in Table 11. -~
~
Table I1 : Temperature Dependence of the Xqr Frequencies for h~g(Hz0)6snCl6and Ca(Hz0)6SnCle Temp. OK
Mg(Hz0)sSnCle freq, MHa
Ca(Hz0)eSnCL freq, MHz
300 278 240 188 147 113
15.836 15.835 15.823 15,819 15.822 15.827 15,801 15,838 15.804
15.904 15.903 15.894 15.896 15.902 15.909
77
15,928 15.900
Results and Discussion A comparison of the nqr frequencies of the M(HzO),SnC16 compounds studied with those of the univalent cation compounds shows that the latter are only slightly lower than the former. Comparing the structures of the two types of compounds and making an estimate of the ratio of the EFG tensor a t a chlorine nucleus due to the nearest-neighbor cations one finds that the contribution of the cation charges to the EFG tensor is about 4 times as large for the Mn8nC&type as for the ;\I(HzO)sSnCle. This is due to the fact that although the charge of the cation is larger in the latter type of compounds the interatomic distances between the cations and the The Journal of Physical Chemistry
---Temp ooeff, K H s deg-L--Mg(Hz0)eSnCk Ca(Hn0)eSnCle
0.15 0.17 -0.11 -0.21
It is apparent from the magnitude and sign of the coefficients that the normal inverse temperature behavior is not followed. The near zero magnitudes and the sign inversion suggest that the normal Bayer effect is being opposed by an effect of comparable magnitude and opposite signs. It was pointed out earlier that the presence of T bonding would provide just such a contribution. The presence of appreciable T character in the Sn-C1 bond is thus concluded from this study. The presence of multiple room temperature resonances for the Zn and Ni compounds indicates the occurrence of two different crystallographic environments of the chlorine atoms. This would be expected for compounds having the rhombohedral structure. The closeness of the members of the frequency pairs is further evidence that the cations contribute little t o the chlorine EFG tensor. The observation of two nqr frequencies of the Mg and Ca compounds at lower temperatures indicates that either there has been a phase change or else the resonances due to the two inequivalent crystallographic positions are so similar as to be unresolvable at higher temperatures. I n summary it can be said that the bonding in the SnC162- anion for compounds with hydrated divalent cations is comparable to that in compounds with univalent cations that is it is little effected by the nature of the cation. Furthermore, the nature of the temperature coefficients indicates that there is appreciable T character in the Sn-C1 bond of SnCY-. Acknowledgments. The authors wish to thank the National Science Foundation for a grant which supported this work. (14) J. D. Graybeal and R. J. McKnown, J. Phys. Chem., 73, 3156 (1969),
