Laser Raman spectrum and vibrational assignment of the tetracyanonickelate ion

Spectrochimica Acta, Vol. 24A, pp. 973 to 980. Pergamon Press 1968. Printed in Northern Ireland

Laser Raman spect~zm and vibrational assignment of the tetracyanonickelate ion* D. JONES, I. J.

HYAMS~

and E. R. LIPPINCOTT

Department of Chemistry, University of Maryland College Park, Maryland (Received 25 October 1967) laser excited Raman spectrum of the tetracyanonickelate(II) ion in two crystal systems and in aqueous solution is presented. Factor group analysis on the two crystal systems yielded two sets of selection rules which aided in the assignment of the frequencies for the tetracyanonickclate (II) ion. Abstract--The

INTRODUCTION TEE infrared spectra of the t e t r a c y a n o n i c k e l a t e ( I I ) ion in a crystal lattice has been i n t e r p r e t e d b y MCCULLOUOH et al. [2] using t h e m e t h o d of site a n d f a c t o r group analysis [1]. F o r t h e i r assignment of t h e ion t h e y used combination bands observed in crystals to p r e d i c t t h e m a j o r i t y of the f u n d a m e n t a l s . The m e t h o d of site a n d f a c t o r group analysis works well for f u n d a m e n t a l s , h o w e v e r t h e r e is considerable d o u b t t h a t t h e v i b r a t i o n c o m b i n a t i o n s p e c t r u m o f solids can be c o m p l e t e l y interp r e t e d b y t h e f a c t o r group selection rules [3]. This is a result o f out-of-phase n o r m a l modes within a unit cell becoming active in c o m b i n a t i o n with o t h e r modes having a corresponding phase difference. This p a p e r presents the R a m a n spectra of the ion in t h e crystalline a n d solution state. OBSERVATIONS Observations of t h e crystalline spectra o f t h e t e t r a c y a n o n i c k e l a t e ( I I ) ion d e m o n s t r a t e t h a t the d e g e n e r a t e frequencies are split o n l y slightly a n d t h a t multiplet s t r u c t u r e is absent. F o r b o t h t h e B a a n d N a salts, t h e i n f r a r e d spectra strongly suggests t h a t t h e w a t e r molecules are h y d r o g e n bonded. I n potassium t e t r a c y a n o nickelate the internal f u n d a m e n t a l s of the w a t e r molecule show f a c t o r group splitting. EXPERIMENTAL S o d i u m t e t r a c y a n o n i c k e l a t e ( I I ) was p r e p a r e d b y a m e t h o d described in t h e l i t e r a t u r e [4]. * This work has been supported in part by a Materials Science Program from the Advanced Research Projects Agency, Department of Defense and a Public Health Research Grant to the University of Maryland. Present address: Department of Chemistry, Bowdoin College, Brunswick, Maine. [I] H. WINSTO~ and R. S. ~ F O R D , J. Chem. Phys. 17, 607 (1949). [2] R. L. McCuLLOUO~, L. H. JO~ES, and G. A. CROSBY,Spectrochim. Acta 16, 929 (1960). [3] S. S. MITRA and P. J. GIELISSE, Progress in Infrared Spectroscopy, Vol. 2, p. 93. Plenum Press (1964). [4] W. C. FERZ~:T,IUS and J. J. BURBANOE, Inorganic Synthesis, Vol. 2, p. 227. McGraw-Hill (1946). 973

974

D. JO~ES, I. J. HYAMS and E. R. LIPPII~OOTT

B a r i u m / p o t a s s i u m t e t r a c y a n o n i c k e l a t e ( I I ) was p r e p a r e d in t h e s a m e m a n n e r as t h e s o d i u m salt w i t h t h e s u b s t i t u t i o n of b a r i u m / p o t a s s i u m c y a n i d e for t h e s o d i u m cyanide. T h e i n f r a r e d s p e c t r a f r o m 4000 to 300 e m -1 were o b t a i n e d as mulls on a P e r k i n E l m e r m o d e 621 s p e c t r o p h o t o m e t e r c a l i b r a t e d b y s t a n d a r d t e c h n i q u e s a g a i n s t w a t e r v a p o u r , C02, a n d p o l y s t y r e n e . B e l o w 300 c m -~ m e a s u r e m e n t s were m a d e b y m e a n s of a F o u r i e r t r a n s f o r m s p e c t r o m e t e r m a d e b y t h e R e s e a r c h a n d I n d u s t r i a l I n s t r u m e n t s Co., c a l i b r a t e d a g a i n s t w a t e r v a p o u r . R e c o r d e d frequencies for b o t h instrum e n t s are a c c u r a t e to ± 2 cm -~. Table 1. The symmetry of the normal vibrations of the tetracyanonickelate (II) ion Representation Ale (R)

Number of vibrations 2

Approximate description of vibrational mode* 721

72(c-~

?22

PiNI--C)

A2g (I.A.) Bla (R)

1 2

v3 ?24

6(~,_c_x) V(c-N)

B2g (R)

2

Y5 ?26 ?27

V(Ni-C) (~{~l-c-s) ~(C-NI-C)

Eu (I.R.)

4

Aeu (I.R.) B2,, (I.A.)

Eg (R)

2 2

1

?2s

72(0-~

Y9 UlO

~)(Ni--C) ~{Ni-C-N)

?2ii

(5(o-Ni-o)

r12

=(Ni-O-N)

7213

"n'(C-Ni-C}

7214

7r(Yi--O--N)

7215

17(C--Ni-C)

hs

lr(~'-c-~

* The symbols ?2,(~and ~rare respectively, stretching, in-plane bending and out-of-plane bending vibrations. R ~ a m a n active, I.R.--Infrared active, I.A.--Inactive. R a m a n s p e c t r a were o b t a i n e d on a C a r y 81 s p e c t r o p h o t o m e t e r using t h e 6328/~ line of a single m o d e H e / N e laser w i t h a n o u t p u t o f ~-~30 m W to i r r a d i a t e t h e sample. Solution s p e c t r a were o b t a i n e d in q u a r t z cells of 8-10 ~1 c a p a c i t y . F o r t h e s p e c t r a of solids whole cluster of c r y s t a l s were r e m o v e d a n d used. T h e t w e n t y - o n e n o r m a l v i b r a t i o n s are g r o u p e d into 9 r e p r e s e n t a t i o n s as s h o w n in T a b l e 1. CRYSTAL SYMMETRY

X - r a y studies h a v e s h o w n t h a t Na2[Ni(CN)a]3H20 has t h e triclinic s y m m e t r y o f t h e space g r o u p C,I(pt) [5], while Ba[Ni(CN)4]4H20 has t h e monoclinie s y m m e t r y of t h e space g r o u p C2he(C2/c) [6]. F o r t h e p o t a s s i u m t e t r a c y a n o n i e k e l a t e t h e r e is d i s a g r e e m e n t in t h e l i t e r a t u r e for b o t h its s t r u c t u r e a n d w a t e r of crystallization content. BRASSEVR [7] claims it to be K2[Ni(CN)4]3H20 a n d triclinie while ROGERS [8] [5] [6] [7] [8]

I-I. BRASSEUR and A. DERAssE~FOSSE, Mere. Soc. Roy. Sei. Liege 2 4, 397 (1941). H. B}~ASSELTRand A. I)ERASS~'OSSE, Bull. Soc. France Mineral 61, 129 (1938). H. BRASSEtm and A. DERASsE~TFOSSE, Z. Krist. A97, 239 (1937). M. W. ROGERS, J. Am. Chem. Soc. 69, 1506 (1947).

Laser Raman spectrum and vibrational assignment of the tetracyanonickelate ion 975 claims a monoclinic s t r u c t u r e w i t h one molecule of w a t e r of crystallization. X - r a y studies here indicate it to be triclinic. T h e w a t e r o f crystallization d e t e r m i n a t i o n was m a d e b y the loss of weight m e t h o d , a n d t h e results show it to contain o n l y one molecule. T h e infrared s p e c t r u m also indicates t h a t it p r o b a b l y o n l y has one molecule o f w a t e r of crystallization. All of these c o m p o u n d s contain four molecules in t h e u n i t cell. I n the case o f Na~[Ni(CN)4]3H20 crystals, the t e t r a c y a n o n i c k e l a t e ( I I ) ions, t h e w a t e r molecules, a n d the sodium a t o m s are located in general positions o f site s y m m e t r y C 1. I n t h e case of Ba[Ni(CN)4]4H20 crystals, the w a t e r molecules are located in general positions of site s y m m e t r y C1, b u t t h e t e t r a e y a n o n i c k e l a t e ( I I ) ions a n d t h e b a r i u m a t o m s are located on sites o f s y m m e t r y C~. Figures 1 a n d 2 relate the r e p r e s e n t a t i o n o f t h e indicated molecular group, site group a n d t h e f a c t o r group of the crystal. Using t h e a r g u m e n t of McCvLLOUGH [2], it is readily seen from Fig. 1 t h a t for t h e triclinic crystals all representations o f t h e molecular group for the t e t r a c y a n o n i c k e late(II) ion are correlated to an i.r. a n d R a m a n active r e p r e s e n t a t i o n o f the f a c t o r Molecular group

D4h

Site group C1

A~g(R) - A2g BIg(R) - B2g(R) - Eg(R)

Factor group Ct - - A g

(R)

A(T)(R)

Alu

A2u(T ) - - - -

B2u --Au(T)

n2u

Eu(T) Fig. 1. Correlation chart for the tetracyanonickelate(II) ion for the triclinie system Na~Ni(CN)43H~O and K2Ni(CN)4H~0. R--Raman active; T--Infrared active. Molecular group

D~ Alo(R ) -

A2o

Bla(R) B~g(R) A~(R) Alu A2u(T ) -

B2u Eu(T)

Site group

Factor group C2h

Ct -

Ag(R) Ag(R)

-

1

Bg(R)

-

Au(T )

Au(T) B,(T)

Fig. 2. Correlation chart for the tetracyanonickelate(II) ion for the monoelinic system Ba(Cl~)44H~O.

976

D. J o ~ s ,

I. J. H Y ~ s

a n d E. R. LIPPINCOTT

group, hence all its fundamentals can be active in the triclinic crystals. Similarly Fig. 2 shows t h a t only u representations of the molecular group for the tetracyanonickelate(II) ion m a y be infrared active in the monoclinic crystal Ba[Ni(CN~)]4H~O. Hence only fundamentals of u symmetry will be allowed in the infrared of the monoclinic crystal and vice versa for the R a m a n effect. All fundamental modes will be allowed in infrared and R a m a n for the triclinic crystal. Although s y m m e t r y theory tells us if a line is allowed, it cannot assure us t h a t it will be strong enough to be observed. ASSIGNMENT OF FREQUENCIES

Table 2 summarizes the observed frequencies, their representation and assignment. Figure 3 shows the R a m a n spectra of solid barium and potassium tetracyanonickelate.

The C - - N stretching frequencies The selection rules predict for the isolated ion of D4a s y m m e t r y an Alg, Big, and an E~ which can be associated with the C--N stretch. From the consideration of the different selection rules which operate for a monoclinic and trielinic crystal there should be observed in the R a m a n spectrum two bands for the barium salt, and three for the K and Na salts. Their infrared should show one band and three bands respectively. This is in fact what was observed as is shown in Fig. 4. The lowest line at 2120 cm -1 clearly is of u s y m m e t r y and is assigned to ~s(E~). In the infrared it is split into two as a consequence of site splitting. Polarization data for the aqueous solution of the tetracyanonickelate(II) ion show t h a t the higher frequency at 2145 is of Alg symmetry and is assigned to vl. The remaining unpolarized band at 2132 cm -1 is thus assigned to ~4(Blg) of the carbon-nitrogen stretch.

The (Ni--C) stretching and bending of (Ni--C--N) region Again selection rules predict an Alg , Big, and an E~ which can be associated with the nickel-carbon stretch. For the bending of the (Ni--C--N) an A2~(va), B~g(ve) are expected. These should lie between 250-600 cm -1. An examination of the R a m a n spectra of the aqueous solution of the tetracyanonickelate(II) ion reveals three of the expected four fundamentals. Two strong bands are found near 300 and 410 em -1 and a weak one at 490 cm -1. The 410 em -~ is the only polarized line and is thus assigned to the AI~ of the (Ni--C) stretch. Again using the facility of the different selection rules which operate for the barium, potassium, and sodium salts, there should be observed for the former five fundamentals in the R a m a n and four in the infrared while for the latter nine fundamentals active both in the R a m a n and infrared. The R a m a n spectrum of BaNi(CN)44H~O shows five bands as predicted, these occur at 490 w, 460 w, 418 w, 390 w, and 302 s. As mentioned above the 418 cm -1 line is assigned to the Alg(V2) of the (Ni--C) stretch. The strong line at 302 cm -1 is assigned to vie, the out-of-plane degenerate vibration of the bending of the ( N i - - C - N) on the basis of it being split by site splitting in the case of the sodium and potassium salts. The vs(Blg ) of v(Ni--C) is expected to be lower t h a n v~(Alg)v(M--C), this is based on the following argument of McCu~ouGH [2]. I t has been demonstrated t h a t the metal-carbon interaction force constants for Ni(CO)4 are positive [9]. [9] L. H. J o ~ s , J. Chem. Phys. ~8, 1215 (1958).

Laser Raman

Table

spectrum

2. O b s e r v e d

and vibrational

vibrational

assignment

frequencies

of the tetracyanonickelate

for Na2Ni(CN)43H20,

ion

977

K2Ni(CN)4H20

and Ba Ni(CN)44H20

~K (era-1)

I.R.

~Na (era -1)

Raman

3645 3625 3570 3552 2672w 2617 w 2550 v w 2525 w

2208ww 2144 w 2136w 2126 s 2122 s 2102yew 2100 v v w 2087.5w 2083.5 w

2146 s 2135 sho 2127w

1619m 1597 s 1585 s 720m

555yew 534 (anh)

I.R.* 3595 3535 3440 3250 2683 2618 2563 2549 2535 2455 2429 2207 2149 2141 2132 2128 2095 2089 2087 2083 2080 1620

~Ba (cm -1)

Raman

I.R.*

Raman

R~Inan aqueous solution

3595 3540 3435 3240 2674 2617

1~H2o ~2o ~H2o vH2o

2547 2533 2458

2151 s 2143sho 2126w

2145 s 2133m

2147 s,P 2141 sho

2126 2120

2079 1620

710 w 673 617 552 543 (anh)

710 680 616 563

468

468

448vw 433 421

448 430 421

VH20 ~H~O ;~H20

490 v v w

491 w

460 w w 444vw

410 sh 405m 385m 335vr

336vvw 303m 312 sho 250vv~

180 b r d

107vw

182m

178 w

176 m b r d

150 m

156 m 131 w b r d 95 m

156 m b r d

74 m 67 m

* F r o m R e L [2].

m in m m m

417 w 391 w

411 m P

303 s

299 S

386 301m 307 sho

99 96 75 65 57

U8 ~1o

185 sho 17Om 136mbrd 101m

63.5 w

105 96 74 67

~9

]P12

417m 401m 385m

~1 ~4 v8 ~s

}'{C13_N) ~(018_N) '~(C18_N)

2085

485vw

445vw 415m 408m

Assigmnent

m m m m

166 v v w 156 v v w

86 81 61 47

vvw vvw w w'

~'1o

~2 ~5 ~Hzo

978

D. JoN~s, I. J. HYAMS and E. R. LIPPINCOTT

/ ,J

2160

~ I

2140

2120

(b)

(b)~ 510

430

)I I

350

270

190

I

110

3O

cm-I

Fig. 3. R a m a n spectra of (a) KsNi(CN)4H20 and (b) BaNi(CN)a4H~O.

R

Basalt

Fi II

IR.

K salt

I.R.

,,if

I

,,

,,

,U,,,

[i

,

,

,

i,,.,

,I J ,

I

J

,J

,

,, I l l

,

I,

Na salt

I.R.

Aqueous SOution 22~00

,, I[

,

2 0 I0 0

] 600

,,

,

,

ll+ll

4 IN 00

I 200

lllll

0I

Cm-I

Fig. 4. Correlation chart for frequencies of the tetracyanoniokelate ion.

Laser Raman spectrum and vibrational assignment of the tetracyanonickelate ion 979 Assuming t h a t the metal-carbon force constants for the tetracyanonickelate(II) ion are anlogous to the ( ~ - - C ) force constants for Ni(CO)4 and from the definition of F~lo and EB~ the following inequality can be derived RR ~/~s > 1 Since there is only one unassigned R a m a n line below v2, near 390 cm -1, it is assigned to vsTwo unassigned fundamentals are left, namely, vs(B2g) and va(A2g) and two observed R a m a n lines from which to choose. The A2~ is inactive according to D4h s y m m e t r y but is expected to be active in the crystal spectra. I n the solid R a m a n spectra both lines were observed (460, 489 cm -1) but in aqueous solution only one (490 em-1). Thus the 490 cm -1 band is assigned to ve and the 460 cm -1 to vaThus all the expected g modes of ~(lV[--C) and a,~r(Ni--C--N) are assigned. For the u modes of v(Ni--C) and bending of the (Ni--C--N) four fundamentals are expected, namely an E~(~9) for the stretch and an Eu(~19), A2u(,l~), and B~,(u14) for the bending of Ni--C--N). Three frequencies (433, 421), 543, and 448 cm -1 appear in the 400-600 cm -1 region of the spectra for both crystal systems. These bands are obtained in Na~[Ni(CN)4], Na~[Ni(CN)4]aD~O , and Nag.[Ni(CN)4]3H20 , though in the last case t h e y are overlapped by water peaks. Only two of these can belong to the E~ representation. There is little doubt t h a t the frequency pair (433, 421) belongs to the degenerate E , representation apparently split, by the lattice interactions, though the distinction as to which kind of motion of E~ s y m m e t r y is involved is not obvious. The other E , vibration is assigned to the 543 cm -1 band. The remaining band at 450 cm -1 for the triclinic and monoclinic crystal must be either the ~1~ fundamental of Ag~ or the ~14 fundamental of B2~. Since under the Table 3. The observed fundamental frequencies for the tetracyanonickelate(II) ion Present (cm-1) From Rcf. [2] v1 ~2 va u4

v5 ~6 ~'7

2149 417 468 2141 401 490 - -

2149 419 325 2141 405 488 - -

vs ~a

t2132~ ~2128J 543

/21321 /2128J 543

~1o

(421]

(421]

/4331

/433/

vii

- -

__

1,1z

448

448

~'13

- -

- -

P14

- -

- -

Y15

- -

vl"

301 307

54

280

980

D. JONES, I. J. I-IYAM,qand E. R. LIPPINCOTT

isolated ion approximation A2~ is infrared active and B2~ is not, it seems appropriate to assign the 450 cm -1 to the A2~ vibration, ~1~, as it should probably be more intense than ~14. The only remainiug internal vibrations of the tetracyanonickelate(II) ion to be dealt with are the bending of the (C--Ni--C). Two in-plane bending modes of symmetry B2u(~7), and E~(~ll ) and two out-of-plane of symmetry A2~(~13) and B~(~15 ) are expected. These should occur around or below 100 cm -1. No assignment is offered for these, since in this region the extremal modes are also expected, and a great deal of mixing and coupling will occur between both types of modes. The appearance of extremal modes is clearly indicated in the R a m a n spectrum of barium tetracyanonickelate where only one internal R a m a n mode is expected but in fact three lines are observed in its R a m a n spectrum between 40-200 cm -1. CO~CLVSZO~ The factor group method of analysis of the spectra obtained from the tetracyanonickelate ion in different crystalline environments, together with its aqueous R a m a n spectrum, aided effectively in making frequency assignments for all vibrations except the bendings of the (C--Ni--C). The values obtained agree reasonably well with those of MCCULLOUGH[2] (see Table 3), and therefore no attempt was made to recalculate his force field or potential energy distribution.