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Exafs and X-ray diffraction studies of the hydration structure of stereochemically active Sn(II) ions in aqueous solution
24/31
CHCMICALPIIYSICS LIITERS
EXAFS AND X-RAY DiFFRACTION OF STEREOCHEMICALLY
STUDIES OF THE HYDRATION
Dcccmbcr
I%?.
STRUCTURE
ACTIVE Sn(ll) IONS IN AQUEOUS SOLUTION
L~rndcd Y-~J> .tbsorptwn Ik ~Iru~Iurc nicd~uxmcnls IIJVCbeen mddr on d 3 XISn(CIOJ)2 aqueous solullon and on Lr)\I.dhnL’SnO ~\II.I~)sc, 01 IIIISLYAIS d.11~rcbcal the bydrdllon s1wcIurc ot Sri(H) 1011sand conliim 111atthe Sn(ll) lo~lr poursdrc ~IL’ro~iicnIIc.III~ JL[IW In dqucous ,olulion Tlic rcwlr, rlrc LompdrCd \\ltb rhox obramcd b) an X-my diffrdLLlOll \rud!
I. Introduction
particular X-ray absorbing atom in the absence of solvent strucutre [3,4] _ In the present study, we de-
The HOMO electrons of Sn(flj, correspondmg to 5s III rhe ground s~;lte of the free ion, are usually dlstorted to form a non-spherlcal charge dlstrlbution around this element m sohds (they are “stereochemially active”) Structural studies of many Sn(ll) compounds have revealed the key role of the actlve Sn(ll) lone pairs m complex crystal structures [II. The Sn(ll) lone pairs are also thought to be active III solutlon. To our knowledge. however, few structural mvcstgatlons have been performed on the correspond~ng Sn(ll) complex ions m solution. A previous X-my dlffractlon study of a concentrated Sn(C104)2 aque-
scribe EXAFS measuremenfs on the Sn K edge of a 3 hl SII(CIO~)~ aqueous solution to determine the hydration structure of the Sn(II) ions in aqueous solu-
oussolutlon [2] met wlthsome dlfficultlesin elucidatmg the complete hydration structure of Sn(II) ions
smce peaks due to the hydrated SnZ+ Ions, anions and water structures overlapped III the radml dlstrlbutlon curve Recently, the EXAFS (extended X-ray absorption IIIICstructure) techmque has been developed as a tool for determining the local structure of a
tion. Crystalhne SnO was used as a structural standard In addltlon, the abovementloned X-ray scattermg data were re-analysed m the bght of the results derived front the present EXAFS study.
2. Experimental The solutton of tm(Il) perchlorate was prepared under a N, atmosphere by the reaction ofacopper(ll) perchlorate solution with tin metal and acidified shghtly with perchloric actd to prevent hydrolysis. Crystalhne black SnO was of reagent grade. The solution sample was contained in a Kapton sample cell, while fme powder of the crystalline SnO sample was mixed with epoxy that cured into a sample of the desired thickness.
525
0 009-2614/81/0000-0000/S
02.75 0 1982 North-Holland
Volume 93, number 6
CHCkflCAL
PHYSLCS LLTlCRS
EXAFSmeasurements were performed at the Standard Synchrotron Radtation Laboratory (Wiggler hne). The X-my absorption was measured III the vlcin-
ity of the Sn K edge(27,l key). The t~nsmitted radlatlon intensities were registered at 356 photon energies from 28.1 to 30.9 keV with gas ronlzation detcctors. The sohd sample was measured at hqurd-nitrogen temperature (77 K) and the solution sample at room temperaturt (298 K). Experunental details are dccrtbed elsewhere [S].
3. analysis
of EXAFS
datn
and results Of234567
A standard analysrs was used to subtractthe slowly varying absorption background, convert the EXAFS to k-space, and Fourier t~nsfo~ to r-space [4,5]. The resultmg EXAFS data, x(k), multlphed by the wave vector, k, are plotted versus k in frg. 1 for the solutron sample. Fig. 2 shows the corresponding
r (E\) Tlg 1 Ke~f pdns (sol&l line) md the mdfnrtudss of ths Fourier tr~nsforn~s of the EXAI Son tbf Sn Ii c&e m (.I) crystatbnc SnO at 77 K and (b) tltr 3 hf Sn(CIO~)z .~queou> solution PI 298 fi The wmdow used for the translkm IS h = 3 3-l I 6 A-‘. gausslzln broadcncd by 0 5 8-t
Fourier lransforms. 0(r),forthectysrallme SnOand the solutron sample. Accordmg to the reported structure of SnO at 77 K [6], there are four equldrstant Sn-0 bonds (2 224 A) in the first-neighbor shell of the Sn atom. In the more distant shells up to 4 A, there are twelve Sn atoms, four bemg at 3.54,3.69 and 3.80 8. In the Fourier transform of the crystalhne SnO (fig. 7-a), the first peak near 1.75 a can be assigned to four Sn-0 bonds. The difference between the location ofche peak and the known near-neighbor spacing is due to the phase shift The more drstant
peaks are due to the more distant neighborsfor whtch Interference effects are severe due to their close spacing 171.
k&7 Fig I. The EXAFS oscdkon kx(k) on the Sn K cdgc m the 3 hf Sn(ClO& aqueous soIution as a fu~~~~on of photoelecWon momentum k.
The geometries of oxygen-coordmatcd Sn(il) xons vartes from a regular trrgonal pyr3nl~ with three short Sn-0 bonds to a regular square pyramtd wtth four Sn-0 bonds (2.05-2.35 A), with many intermediates between these two types [8 j . In addltlon, there are some weak Sn-0 mlenctions (2.70-3.2 A) on the active lone-pairs side, whch complete a drstotted oct~edron around the Sn(II) ion. It has often been discussed whether secondary weak urtefaccrufrs
may be regardedas chemtal bondsor occurdue to crystalpackmg In compounds. This questlon may bc answered m the present study smcc no effect of the crystal packmg is expected III solution. tn order to analyse the EXAFS spectrum in a quantltatrve manner, we applied a least-squares fitting procedure to the peak in pspace usmg the spectrum of the crystalhne SnCI as a standard. The radral dntance, r, the change in root-mean-square devmtlon, Au, relativeto the W-0 standardat 77 K, and the numberof nearnei~bots, N, are ai~owe~to varym the refmement procedure as described m detad elsewhere [5]. Based on the above consideration, we tested two kmds of models for the prommcnt peak(A) one type of Sn-OH;! bond without any weak secondary bonds and (B) two different types of SnOMz mte~ctio~. The refmements were carried out m different r-space regons m order to check the con-
Volume 93. number 6
2413 1 December 1982
CHCMICAL PHYSICS LETTERS
4. Analysis of X-ray scattering data and the resutts Details of the X-ray scattertng measurements and
data redu~&~on are ~onta~edin ref. [2], Figs.4a and
Sn-OH2 r(A) Au(A) N
-
2 ‘I
-
005
4b show the reduced mtensities, $k), multlpIied by k (ii this case, k = 4~ sin@/?L,B the Bragg angle and X the wavelength), and the dtfferential form, D(r) - 47r&o, of the radral d~st~bution function for the 33 M Sn(ClO& aqueous solution. In fig. 4b (dots), the first peak at 1A5 A ISdue to the Cl-O bonds m the parc~omte group. The 2.3 A peak is ascribed to the Sn-OH7 bonds within hydrated Sn2’ rons. The O-O inter&ions in the tetrahedral ClO; group cort-
I i
tributein part to the peak,As to the peakat 2.80 8, -
--
trlbution from the backgroundand from possible more distant nerghbors.Fmal parameter valuesare gtven UItable 1. The average%-OH, drstances are 2.21 a {model A) and 2 2.5 ,&(model 8). The number
previousauthors have stated that the weak SW-Hz0 interactions gave a predominant ~ntribution as in crystais. As mentioned m section 3, however, the EXAFS analysis has shown that such mteractions are of minor rmportance in the solution. Other contrtbuttons come from the ~temction between the 0 atoms
of
nearest-neighbor water molecules, N, m both cases is close ta four. These results agree with a general ylew of the st~reache~l~lly active Sn(II) WI [ 1,8f s
I
I
I
,
As seen in fig. 3, both models gave almost the same near-neighbor parr drstrtbutton functtonn,p(rf. Thus,
the secondary Sn-Hz0 interactions were not detected m the present EXAFS expenment. This may be mterpreted m such a way that there 1sno such weak mteraction or, If any, the correspondjng~(~) IS too broad 10 contributeto the EXAFS.WeWI say that the rn-
terimon between the acttve Sn(II) lone pans and water molecules ISnot sibilant m the solution.
@15
20
25 dh,
I
30
1
t
0
5
1 0
2
the 3.3 M Sn(CIO&
I
1
,
4
6
B
aqueous solution Expenmenral
t‘lg 3. Near-ne!gh~r Sn-0 pvr dut~tb~t~an functcons for model
and theoreti~l (--.
530
I
15
I
Fg, 4 (a)The rcdu~d ln~cnsl~l~s,r(k), mult~~~cd by k for and thcoretiul (--.modei corrcspondl~g D(+4nrzpo
A (--- ) and model B (- - -)
1
10
(dots)
A; - - -, model 8). (b) The functions. Expe~mental (dots) model A, - - -, model B).
Volume 93, number 6
CHLMICAL
N/31
PHYSICS LIX-TCRS
in the perchIo~te group and anionic hydration water molecules, and from ~tem~tlons between the water molecules tn the first and second hydratron shells
aroundtheSn2+ion,Thebroad around 3,6-4.8A ISdue marnly to the interactions between Sna+ ions and water molecules in the second hydration she& The Cl-H20 interactrons due to the anionic hydration also gtve nse to a contRbution to the peak. In order to obtam the structural parameters of the water coordination around the Snz+ ions, we performed quantitative analyses using a least-squares method applied to the reduced intensrties. The detatls of the procedure and the program are described else-
tural models for the solutton. (1) For the perchlorate ion, a tetrahedral geometry was assumed and only the Cl-O distance, rcr_o, was refmed. The root-mean-square devtations, IY~,_~ and o~__~, were tuted to the values calculated from spectroscop~c data [ 111 (2) The structure of the hydrated Sn2+ Ion up to the first hydratron shell was described by three in-
dependent parameters, the distance, ~,,_0Hrt1), root-mean-squaredeviation, usn_ ox2 (I), and the number of nearest.nei~bor water moic~uIes, Nsn-oHa( The contribution from the weak, Sn-l-f20 mterachons was checked by rntroducing pammeters ‘Sn-HsO(l’), uSn -HzO(r’), and&,-HqO(l’) (Iv = 3 was assumed as m crystals) unto the refinements. (3) The mteractions between the Sna+ Ion and water molecules m the second hydra~on shell were
taken into account usmg three independent parameters~rsn-H~O(li),o~n-H~~~~)~dNS~-Ir,o(lr). (4) TheCl-H20 interactions arismg from the amonic hydration were mcluded and their dtstance, root-mean-square deviatron and the number of interactions were allowed to vary. (5) TheH~O(l~H~O~l) mteractions beIon~n~ to
the two hydration sbeils of the Sn2+ ion were described by three independent parameters,
rH,O(I)-H~O(ll)~ oH;‘I(I)-H20(11), and N&~(r)_H,Otrr). The O(ClO;)-Hz0 interactrons due to the aniorue hydration were ~corporated in the above interactioti. (6) Beyond the abovementioned discrete structures, a contmuum distribution of electrons was introduced for each species, which was described by the usual parameters Q-,and uo.
I981
TabIe 2 Structure
pammeters obtwwd from the icxs~squarcs refincmentsfor X.ri?yswttcrmgd~ta for 3 3 Xl Sn(C104)2 solutron. The v~lucs tn pxcnthcscsarc
utttts forr
thcu standsrd dcvtattcms The
and II arc A
Sn-0H2(1)
Model A
Model B
2 34(‘) 0 06(l) 3 4(3)
2.31(Z) 0 060) 3 6(3) ZSili O.lO(6) 3 J J(l) 0 17(7)
Sn+O(l’)
4%1) 0.17(7)
where[Y,lO].Fromthe~~a~ur~s in the dist~butlon curve described above, we used the following strut.
Dumber
Cl-0
o-o
C1-4~0
72t91
l-136(6) 0019 4 2 43s 0 ox 6
39(Z) 0 06(l) 8(l)
H~0(1)--1120(11)
Sn Cl Ii,0
?*91(6) 0 07(4) 8(l) d 7(i)
0 3 0 2 0
J(2) t(t) 2(2) E(6) L?(Z)
7,2(9) I 138(6) 0019 4 2 438 0 024 6 3.8(2) 0 OS(2) 8(t) 2.9(?) 0 l?(lO) 6(l) J 7(l) OA(7) 3 28(8) 0 l(2) 2 9505) 0.1(Z)
The k-region used in the re~nements was 0.5-15.0 A- I. The resulturg parameter valuesare ltstcd tn table 2. Theoretical values from the best fit are compared with the experunental ones m fig. 4a and 4b. The agreement between the two values rs satnfactory. The short Sn-H,O bond dtstance obtamed IS 2.34 A, conststent watt+ the experimental errors with the value obtained from the EXAFS. Thenumber of water molecules bonded to the Sn2+ ion IS three to four, also m good agreement wrth the EXAFS result. Previous authors concluded two to three Sn-Hz0 bonds. Our results, however, are consistent wrth a general trend of oxygen-~oord~ated Sn(II) tons [Sl. As cart beseen tn fig. 4, the results of the IWOModels
are not sigmficantlydrfferent. Tins is because in model B the lack of the weak Sn-H,O(I’) tnteractrons is compensated with smaller o and larger IV values for
Volun~c93
number
the H~~(l~H~O(i~) drffrxtion
tion on
study
Z-1/31 Dcwmbcr 1982
CHLiWCAL PHYSKS LEI-ERS
6
interactions
Thus, the X-ray
alone could not gave cktr
informa-
the NSF through Program
the weak Sn-H1O mtenctkon.
the Dwision of Matenak Research
and the NIH through the Biotechnology in the lhvision
coopention
Resources
fo Research Resources (ii of Energy)
with the Department
5. Conclusion References
A combmatlon of EXAFS and Y-ray dIffractIon stuches has revealed the Somplete hydration structure of the Sn(Il) 10ns in
aqueous solution. Three to four water nlotecules are strongly bonded to the Sn(ll) ran at a distance of 125-2 34 A. lls Sn(ll) lone pairs are stcreo~l~ei~~i~3lly 3ct1vc and the 11itcr3ct~on wtll water molecules IS not si~fll~~arit in aqueous solutron.
I 11J
A. Zubtcrd atd J I. ZurLernann. Pro~r Inoq! Chcm. 24 (19723) 25 [Z j C lohnnsson .md ft. Oh~a~r, Aeta Chcm Sund 17 (1973) 6J3 131 P.A Lre, P.H Cltrln. P Llsrnbcrecr .tnd B bl Kun. Rrv. Mod Ph>s 53 (1981) 769 141 TM
H~)cs,J.NonCr>st
Sohds31
(1978357
1.51J B. Boyce, T kf H.~ycsand J C hl~klefan Jr
, Phyr
Rev 8?3(1981)2876 16) J PJIIWIIC~JIXI
C
Dsnes.Acu
17 I T $1. tlwcs. P N Scn 4 (1976) 4357
The authors thank Dr. Georg Johzmsson
for prov-
rding X-ray scattermg data for the 3.3 M Sn(CIO,), aqueous soilut~on. The Swedish Natural Scrence Research Council IS ~IaI~iulty
acknowledged for
atd Some of the mater& incorporated tn tlus work were done at SSRL wh.rch is supported by
financial
537
ISI T Saaaguch~ and 0
Cryst
836 (198012763
S H Hunlcr. J Phys C9
L~ndqv~ dcr~ Crrst B38(19823
1441 191 G. Johatsson and \I S.mdslrom, Chcnw~ Scnpl~ 4 (1973) 195
[ ia] ti
0ht.tkt.T
[II I
YJRlJ~UChl
Jnd
hi
&tJI’&.
8Utt
ChCin
49 f 1976) 701 hl hlxds, Y MJC~JUJ. T Yamquchl .md II Ohr&, tlull Chem !& JdpJn 51(1979) 25-U
SOL! JdpJn
CHCMICALPIIYSICS LIITERS
EXAFS AND X-RAY DiFFRACTION OF STEREOCHEMICALLY
STUDIES OF THE HYDRATION
Dcccmbcr
I%?.
STRUCTURE
ACTIVE Sn(ll) IONS IN AQUEOUS SOLUTION
L~rndcd Y-~J> .tbsorptwn Ik ~Iru~Iurc nicd~uxmcnls IIJVCbeen mddr on d 3 XISn(CIOJ)2 aqueous solullon and on Lr)\I.dhnL’SnO ~\II.I~)sc, 01 IIIISLYAIS d.11~rcbcal the bydrdllon s1wcIurc ot Sri(H) 1011sand conliim 111atthe Sn(ll) lo~lr poursdrc ~IL’ro~iicnIIc.III~ JL[IW In dqucous ,olulion Tlic rcwlr, rlrc LompdrCd \\ltb rhox obramcd b) an X-my diffrdLLlOll \rud!
I. Introduction
particular X-ray absorbing atom in the absence of solvent strucutre [3,4] _ In the present study, we de-
The HOMO electrons of Sn(flj, correspondmg to 5s III rhe ground s~;lte of the free ion, are usually dlstorted to form a non-spherlcal charge dlstrlbution around this element m sohds (they are “stereochemially active”) Structural studies of many Sn(ll) compounds have revealed the key role of the actlve Sn(ll) lone pairs m complex crystal structures [II. The Sn(ll) lone pairs are also thought to be active III solutlon. To our knowledge. however, few structural mvcstgatlons have been performed on the correspond~ng Sn(ll) complex ions m solution. A previous X-my dlffractlon study of a concentrated Sn(C104)2 aque-
scribe EXAFS measuremenfs on the Sn K edge of a 3 hl SII(CIO~)~ aqueous solution to determine the hydration structure of the Sn(II) ions in aqueous solu-
oussolutlon [2] met wlthsome dlfficultlesin elucidatmg the complete hydration structure of Sn(II) ions
smce peaks due to the hydrated SnZ+ Ions, anions and water structures overlapped III the radml dlstrlbutlon curve Recently, the EXAFS (extended X-ray absorption IIIICstructure) techmque has been developed as a tool for determining the local structure of a
tion. Crystalhne SnO was used as a structural standard In addltlon, the abovementloned X-ray scattermg data were re-analysed m the bght of the results derived front the present EXAFS study.
2. Experimental The solutton of tm(Il) perchlorate was prepared under a N, atmosphere by the reaction ofacopper(ll) perchlorate solution with tin metal and acidified shghtly with perchloric actd to prevent hydrolysis. Crystalhne black SnO was of reagent grade. The solution sample was contained in a Kapton sample cell, while fme powder of the crystalline SnO sample was mixed with epoxy that cured into a sample of the desired thickness.
525
0 009-2614/81/0000-0000/S
02.75 0 1982 North-Holland
Volume 93, number 6
CHCkflCAL
PHYSLCS LLTlCRS
EXAFSmeasurements were performed at the Standard Synchrotron Radtation Laboratory (Wiggler hne). The X-my absorption was measured III the vlcin-
ity of the Sn K edge(27,l key). The t~nsmitted radlatlon intensities were registered at 356 photon energies from 28.1 to 30.9 keV with gas ronlzation detcctors. The sohd sample was measured at hqurd-nitrogen temperature (77 K) and the solution sample at room temperaturt (298 K). Experunental details are dccrtbed elsewhere [S].
3. analysis
of EXAFS
datn
and results Of234567
A standard analysrs was used to subtractthe slowly varying absorption background, convert the EXAFS to k-space, and Fourier t~nsfo~ to r-space [4,5]. The resultmg EXAFS data, x(k), multlphed by the wave vector, k, are plotted versus k in frg. 1 for the solutron sample. Fig. 2 shows the corresponding
r (E\) Tlg 1 Ke~f pdns (sol&l line) md the mdfnrtudss of ths Fourier tr~nsforn~s of the EXAI Son tbf Sn Ii c&e m (.I) crystatbnc SnO at 77 K and (b) tltr 3 hf Sn(CIO~)z .~queou> solution PI 298 fi The wmdow used for the translkm IS h = 3 3-l I 6 A-‘. gausslzln broadcncd by 0 5 8-t
Fourier lransforms. 0(r),forthectysrallme SnOand the solutron sample. Accordmg to the reported structure of SnO at 77 K [6], there are four equldrstant Sn-0 bonds (2 224 A) in the first-neighbor shell of the Sn atom. In the more distant shells up to 4 A, there are twelve Sn atoms, four bemg at 3.54,3.69 and 3.80 8. In the Fourier transform of the crystalhne SnO (fig. 7-a), the first peak near 1.75 a can be assigned to four Sn-0 bonds. The difference between the location ofche peak and the known near-neighbor spacing is due to the phase shift The more drstant
peaks are due to the more distant neighborsfor whtch Interference effects are severe due to their close spacing 171.
k&7 Fig I. The EXAFS oscdkon kx(k) on the Sn K cdgc m the 3 hf Sn(ClO& aqueous soIution as a fu~~~~on of photoelecWon momentum k.
The geometries of oxygen-coordmatcd Sn(il) xons vartes from a regular trrgonal pyr3nl~ with three short Sn-0 bonds to a regular square pyramtd wtth four Sn-0 bonds (2.05-2.35 A), with many intermediates between these two types [8 j . In addltlon, there are some weak Sn-0 mlenctions (2.70-3.2 A) on the active lone-pairs side, whch complete a drstotted oct~edron around the Sn(II) ion. It has often been discussed whether secondary weak urtefaccrufrs
may be regardedas chemtal bondsor occurdue to crystalpackmg In compounds. This questlon may bc answered m the present study smcc no effect of the crystal packmg is expected III solution. tn order to analyse the EXAFS spectrum in a quantltatrve manner, we applied a least-squares fitting procedure to the peak in pspace usmg the spectrum of the crystalhne SnCI as a standard. The radral dntance, r, the change in root-mean-square devmtlon, Au, relativeto the W-0 standardat 77 K, and the numberof nearnei~bots, N, are ai~owe~to varym the refmement procedure as described m detad elsewhere [5]. Based on the above consideration, we tested two kmds of models for the prommcnt peak(A) one type of Sn-OH;! bond without any weak secondary bonds and (B) two different types of SnOMz mte~ctio~. The refmements were carried out m different r-space regons m order to check the con-
Volume 93. number 6
2413 1 December 1982
CHCMICAL PHYSICS LETTERS
4. Analysis of X-ray scattering data and the resutts Details of the X-ray scattertng measurements and
data redu~&~on are ~onta~edin ref. [2], Figs.4a and
Sn-OH2 r(A) Au(A) N
-
2 ‘I
-
005
4b show the reduced mtensities, $k), multlpIied by k (ii this case, k = 4~ sin@/?L,B the Bragg angle and X the wavelength), and the dtfferential form, D(r) - 47r&o, of the radral d~st~bution function for the 33 M Sn(ClO& aqueous solution. In fig. 4b (dots), the first peak at 1A5 A ISdue to the Cl-O bonds m the parc~omte group. The 2.3 A peak is ascribed to the Sn-OH7 bonds within hydrated Sn2’ rons. The O-O inter&ions in the tetrahedral ClO; group cort-
I i
tributein part to the peak,As to the peakat 2.80 8, -
--
trlbution from the backgroundand from possible more distant nerghbors.Fmal parameter valuesare gtven UItable 1. The average%-OH, drstances are 2.21 a {model A) and 2 2.5 ,&(model 8). The number
previousauthors have stated that the weak SW-Hz0 interactions gave a predominant ~ntribution as in crystais. As mentioned m section 3, however, the EXAFS analysis has shown that such mteractions are of minor rmportance in the solution. Other contrtbuttons come from the ~temction between the 0 atoms
of
nearest-neighbor water molecules, N, m both cases is close ta four. These results agree with a general ylew of the st~reache~l~lly active Sn(II) WI [ 1,8f s
I
I
I
,
As seen in fig. 3, both models gave almost the same near-neighbor parr drstrtbutton functtonn,p(rf. Thus,
the secondary Sn-Hz0 interactions were not detected m the present EXAFS expenment. This may be mterpreted m such a way that there 1sno such weak mteraction or, If any, the correspondjng~(~) IS too broad 10 contributeto the EXAFS.WeWI say that the rn-
terimon between the acttve Sn(II) lone pans and water molecules ISnot sibilant m the solution.
@15
20
25 dh,
I
30
1
t
0
5
1 0
2
the 3.3 M Sn(CIO&
I
1
,
4
6
B
aqueous solution Expenmenral
t‘lg 3. Near-ne!gh~r Sn-0 pvr dut~tb~t~an functcons for model
and theoreti~l (--.
530
I
15
I
Fg, 4 (a)The rcdu~d ln~cnsl~l~s,r(k), mult~~~cd by k for and thcoretiul (--.modei corrcspondl~g D(+4nrzpo
A (--- ) and model B (- - -)
1
10
(dots)
A; - - -, model 8). (b) The functions. Expe~mental (dots) model A, - - -, model B).
Volume 93, number 6
CHLMICAL
N/31
PHYSICS LIX-TCRS
in the perchIo~te group and anionic hydration water molecules, and from ~tem~tlons between the water molecules tn the first and second hydratron shells
aroundtheSn2+ion,Thebroad around 3,6-4.8A ISdue marnly to the interactions between Sna+ ions and water molecules in the second hydration she& The Cl-H20 interactrons due to the anionic hydration also gtve nse to a contRbution to the peak. In order to obtam the structural parameters of the water coordination around the Snz+ ions, we performed quantitative analyses using a least-squares method applied to the reduced intensrties. The detatls of the procedure and the program are described else-
tural models for the solutton. (1) For the perchlorate ion, a tetrahedral geometry was assumed and only the Cl-O distance, rcr_o, was refmed. The root-mean-square devtations, IY~,_~ and o~__~, were tuted to the values calculated from spectroscop~c data [ 111 (2) The structure of the hydrated Sn2+ Ion up to the first hydratron shell was described by three in-
dependent parameters, the distance, ~,,_0Hrt1), root-mean-squaredeviation, usn_ ox2 (I), and the number of nearest.nei~bor water moic~uIes, Nsn-oHa( The contribution from the weak, Sn-l-f20 mterachons was checked by rntroducing pammeters ‘Sn-HsO(l’), uSn -HzO(r’), and&,-HqO(l’) (Iv = 3 was assumed as m crystals) unto the refinements. (3) The mteractions between the Sna+ Ion and water molecules m the second hydra~on shell were
taken into account usmg three independent parameters~rsn-H~O(li),o~n-H~~~~)~dNS~-Ir,o(lr). (4) TheCl-H20 interactions arismg from the amonic hydration were mcluded and their dtstance, root-mean-square deviatron and the number of interactions were allowed to vary. (5) TheH~O(l~H~O~l) mteractions beIon~n~ to
the two hydration sbeils of the Sn2+ ion were described by three independent parameters,
rH,O(I)-H~O(ll)~ oH;‘I(I)-H20(11), and N&~(r)_H,Otrr). The O(ClO;)-Hz0 interactrons due to the aniorue hydration were ~corporated in the above interactioti. (6) Beyond the abovementioned discrete structures, a contmuum distribution of electrons was introduced for each species, which was described by the usual parameters Q-,and uo.
I981
TabIe 2 Structure
pammeters obtwwd from the icxs~squarcs refincmentsfor X.ri?yswttcrmgd~ta for 3 3 Xl Sn(C104)2 solutron. The v~lucs tn pxcnthcscsarc
utttts forr
thcu standsrd dcvtattcms The
and II arc A
Sn-0H2(1)
Model A
Model B
2 34(‘) 0 06(l) 3 4(3)
2.31(Z) 0 060) 3 6(3) ZSili O.lO(6) 3 J J(l) 0 17(7)
Sn+O(l’)
4%1) 0.17(7)
where[Y,lO].Fromthe~~a~ur~s in the dist~butlon curve described above, we used the following strut.
Dumber
Cl-0
o-o
C1-4~0
72t91
l-136(6) 0019 4 2 43s 0 ox 6
39(Z) 0 06(l) 8(l)
H~0(1)--1120(11)
Sn Cl Ii,0
?*91(6) 0 07(4) 8(l) d 7(i)
0 3 0 2 0
J(2) t(t) 2(2) E(6) L?(Z)
7,2(9) I 138(6) 0019 4 2 438 0 024 6 3.8(2) 0 OS(2) 8(t) 2.9(?) 0 l?(lO) 6(l) J 7(l) OA(7) 3 28(8) 0 l(2) 2 9505) 0.1(Z)
The k-region used in the re~nements was 0.5-15.0 A- I. The resulturg parameter valuesare ltstcd tn table 2. Theoretical values from the best fit are compared with the experunental ones m fig. 4a and 4b. The agreement between the two values rs satnfactory. The short Sn-H,O bond dtstance obtamed IS 2.34 A, conststent watt+ the experimental errors with the value obtained from the EXAFS. Thenumber of water molecules bonded to the Sn2+ ion IS three to four, also m good agreement wrth the EXAFS result. Previous authors concluded two to three Sn-Hz0 bonds. Our results, however, are consistent wrth a general trend of oxygen-~oord~ated Sn(II) tons [Sl. As cart beseen tn fig. 4, the results of the IWOModels
are not sigmficantlydrfferent. Tins is because in model B the lack of the weak Sn-H,O(I’) tnteractrons is compensated with smaller o and larger IV values for
Volun~c93
number
the H~~(l~H~O(i~) drffrxtion
tion on
study
Z-1/31 Dcwmbcr 1982
CHLiWCAL PHYSKS LEI-ERS
6
interactions
Thus, the X-ray
alone could not gave cktr
informa-
the NSF through Program
the weak Sn-H1O mtenctkon.
the Dwision of Matenak Research
and the NIH through the Biotechnology in the lhvision
coopention
Resources
fo Research Resources (ii of Energy)
with the Department
5. Conclusion References
A combmatlon of EXAFS and Y-ray dIffractIon stuches has revealed the Somplete hydration structure of the Sn(Il) 10ns in
aqueous solution. Three to four water nlotecules are strongly bonded to the Sn(ll) ran at a distance of 125-2 34 A. lls Sn(ll) lone pairs are stcreo~l~ei~~i~3lly 3ct1vc and the 11itcr3ct~on wtll water molecules IS not si~fll~~arit in aqueous solutron.
I 11J
A. Zubtcrd atd J I. ZurLernann. Pro~r Inoq! Chcm. 24 (19723) 25 [Z j C lohnnsson .md ft. Oh~a~r, Aeta Chcm Sund 17 (1973) 6J3 131 P.A Lre, P.H Cltrln. P Llsrnbcrecr .tnd B bl Kun. Rrv. Mod Ph>s 53 (1981) 769 141 TM
H~)cs,J.NonCr>st
Sohds31
(1978357
1.51J B. Boyce, T kf H.~ycsand J C hl~klefan Jr
, Phyr
Rev 8?3(1981)2876 16) J PJIIWIIC~JIXI
C
Dsnes.Acu
17 I T $1. tlwcs. P N Scn 4 (1976) 4357
The authors thank Dr. Georg Johzmsson
for prov-
rding X-ray scattermg data for the 3.3 M Sn(CIO,), aqueous soilut~on. The Swedish Natural Scrence Research Council IS ~IaI~iulty
acknowledged for
atd Some of the mater& incorporated tn tlus work were done at SSRL wh.rch is supported by
financial
537
ISI T Saaaguch~ and 0
Cryst
836 (198012763
S H Hunlcr. J Phys C9
L~ndqv~ dcr~ Crrst B38(19823
1441 191 G. Johatsson and \I S.mdslrom, Chcnw~ Scnpl~ 4 (1973) 195
[ ia] ti
0ht.tkt.T
[II I
YJRlJ~UChl
Jnd
hi
&tJI’&.
8Utt
ChCin
49 f 1976) 701 hl hlxds, Y MJC~JUJ. T Yamquchl .md II Ohr&, tlull Chem !& JdpJn 51(1979) 25-U
SOL! JdpJn