Infrared and laser Raman spectra of metal hexaisothiocyanate ions

Spectmchimioa Acta.Vol. 26A.pp. 322to 320. PergwnonPress1070.Printedin NorthernIreland

Infrared and laser Raman spectra of metal hexaisothiocyanate ions R. J. H. CLARKand A. D. J. GOODWIN William Ramsay and Ralph Forster Laboratories, University College Gower Street, London WC.1 (Received27 Januav 1909) Abstract-The infrared and laser Raman spectra of a series of metal hexaisothiocyanate ions have been recorded over the range 2200-70 cm-l. The following ions have been studied: M(NCS)sS- where M = V, Cr, Fe or MO, M(NCS),” where M = Mn or Ni and Mo(NCS),a-. Assignments for the various fundamentals are made, and the MN stretching frequencies sre shown to be dependent on both the oxidation state and the electron configurationof the metal atom. The various molybdenum salts gave the most satisfactory Raman spectra, both in the solid state end in aqueous solution, but the colours of some of the other salts were not well suited to 6328 A excitation. The M(NCS),“- ions appear to approach 0, symmetry quite closely, there being little or no bending of the M-N-C bonds.

INTRODUCTION METALthiocyanate and metal isothiocyanate complexes have been the subject of many spectral investigations, mostly from the point of view of characterising the internal vibrational frequencies of the thiocyanate groups in the complexes [l-5]. More recently, attention has been directed to the characterisation of the skeletal vibrational frequencies, and in particular the metal-sulphur (MS) and metalnitrogen (MN) stretching frequencies in the complexes [6-81. The present investigation is concerned with the study of the infrared and (for the first time) the Raman spectra of a series of metal hexaisothiocyanate complex anions over the range 220070 cm-l. The ions studied are the following: M(NCS),3- where M = V, Cr, Fe or MO, M(NCS),4- where M = Mn or Ni and Mo(NCS),~-. For some of the complex ions (notably the molybdenum ones) nearly complete spectral data could be obtained, but others (e.g. Fe(II1) and Mo(IV) salts) absorb the He/Ne (6328 A) Raman exciting line and thus do not yield satisfactory Raman spectra. The crystal structure of the complex K,Mo(NCS),CH,CO,H*H,O has recently been determined by X-ray diffraction methods [9]. Although the anion occupies s, very low symmetry site (C,) in the crystal, the only parameters that differ signif% cantly from those of a complex ion of 0, symmetry are four of the MNC bond angles, (which vary from 163 to 169”), and six of the twelve NMN angles (which range from [l] M. M. CHAMBERLAIN and J. C. BAILAR,J. Am. Chem. Sot. 81, 6412 (1959). [2] [3] [S] [5] [S]

P. C. H. MITCHELL and R. J. P. WILLIAMS, J. Chem. Sot. 1912 (1960). J. LEWIS, R. S. NYFIOLMand P. W. SMITE,J. Chern. Sot. 4690 (1961). A. Trraco and C. PECILE, Nature 191, 66 (1961). C. PECILE,Inerg. Chena.5,210 (1966). D. FORSTER and D. M. L. GOODQ~, Iwrg. Chewa. 4,715 (1965). [7] R. J. H. CLARK and C. S. WILLIAMS,Spectrochbn. Aota 23, 1081 (1966). [S] D. FORSTER and W. D. HORROCKS, Inorg. Chem. 6,339 (1967). [9] J. R. KNOX and K. ERIKS, Iw_wg. Chem. 7, 84 (1968). 323

324

R. J. H. CLARKand A. D. J. GOODWIN

88.2 to 94-O”). The complex involves N- rather than S-bonded thiocyanate groups. Preliminary X-ray diffraction studies on the complexes K,Ni(NCS),*4H,O and K,Cr(NCS),4H,O have been claimed [lo] to indicate S-bonded thiocyanate groups; however, these conclusions are almost certainly incorrect as they are in complete disagreement with those reached from studies of the infrared and optical spectra of the complexes. In addition, the complexes K,Mo(NCS),.4H,O and K&r(NCS)B*4H,0 have been shown to be isomorphous [3,9]. Thus all complexes included in the present studies involve N-bonded thiocyanate groups; evidence on this point is given in the discussion section. EXPERIMENTAL

Preparation of the wnvplex~ K,Cr(NCS),.4H,O. This complex was prepared by treating anhydrous chromic chloride with potassium thiocyanate [ll]. The anhydrous material was obtained by heating the tetrahydrate at 130’/1 mm Hg for four hours in a drying pistol. The analytical data for this salt, and for the following ones, are listed in Table 1. Table 1. Analyti~l data, (c&ulated percentagesare given in parentheses) Complex

(EtsPWsVWW, (EVhVW%, KsWNCS), (EtAWNW, (Et,N)$WNW,

(Et,N),Ni(NCS), K.$o(NCS),-4H,O (Et,N),Mo(NCS), (NH,),Mo(NCS),*4HsO (NH1)8M~(NCS),.&0.HC1 (NH4)8M~(NCS)B-H,0-CH,C0,H (pyH);Mo(NCS)s (pyH)&fo(NCS),

Colour yellow yellow pm@e pm@e colourleea pele green yellow cream yellow yellow yellow pale yellow bleck

%C

%H

%N

38*5(38*1) 46*2(46-0) 13q13.8) 46.2(45-6) 49*4(49-P) 48*2(49-2) 11.4(11*4) 43*2(43.1) 12.0(11*9) 13-6(13-O) l&7(16*7) 36*6(3&S) 29.5(31+3)

6*2(6*4) 7.6(7-6) 0*2(0-O) 7*6(7*6) 8*6(8*7) 8.2(8*7) l+i(1*3) 7.1(7.2) 3.q 3.6) 3.g(2.8) 3*4(3-l) 2.7(2*7) 2.O(2.0)

10-6(11-l)

l&0( 16.2) 14*9(16.1) 13.2(13*3) 14q16*1)

18.4(18*4) 18*2(18*5)

(Et,N),Cr(NCS),. This complex was prepared by adding a solution of Et,NCl in ethanol to a solution of K,Cr(NCS),*4H,O in ethanol. The precipitate was recrystall&d from methanol. K,Mo(NCS),.4H,O. 10.8 g (O-025 mole) KaMoCl, and 14.6 g (O-015 mole) KNCS were heated together in 50 ml of distilled water under nitrogen at ~80” until the red colour of the MoCI,~- ion was supplanted by the yellow colour of the Mo(NCS),~- ion. The hot solution was filtered under nitrogen and left overnight in a stoppered flask in a refrigerator. The resulting yellow crystals of the required compound were filtered off, washed with ice-cold distilled water under nitrogen, and dried. The yield was 10.4 g (65%). The anhydrous compound was obtained by heating the tetrahydrate at 140”/1 mm Hg for six hours. [lo] Z. V. ZVONKOVA, Zh. Fiz. Khim. 81, 2074 (1957). [ll] W. G. PALMER,Experinwntal Imga& C&n&try. Cambridge University Prees (1964).

Infraredand laserspectraof metalhexaisothiocyanate ions

326

1.2 g of K,Mo(NCS),.4H,O in 50 ml water was added to a (Et,N),Mo(NCS),. solution of Et,NCl (0.6 g) in 10 ml water, the tetraethylammonium chloride being in excess. The pale yellow precipitate of the required compound was filtsred off and dried at O-1 mm Hg on a vacuum line. (pyH),Mo(NCS),. This complex was prepared in the same way as for the tetraethylammonium salt except that a solution of pyridine in dilute hydrochloric acid was used instead of tetraethylammonium chloride. (pyH)&Io(NCS)s. This complex was prepared by the method of SMITH [3]. Recrystallisation of the complex was not possible owing to its insolubility. (NH,),Mo(NCS),.4H,O, (NH&Mo(NCS)~*H,O-HCl and (NH,),Mo(NCS),~H,O. CH&!O,H. These compounds were prepared by electrolytic reduction of ammonium molybdate in the presence of ammonium thiocyanate [ll]. 9-7 g (O-10 mole) potassium thiocyanate, 105 g (0.05 mole) (Et,N),V(NCS),. tetraethylammonium bromide and 2.45 g (0.016 mole) anhydrousvanadium trichloride were heated under reflux in 100 ml of absolute ethanol under nitrogen until all the vanadium trichloride had dissolved. The solution was filtered to remove precipitated KC1 and KRr and then concentrated to ~40 ml by pumping off the solvent. The solution was then heated to ~60” to dissolve precipitated material and allowed to cool slowly. The required complex precipitated after several hours and was titered off under nitrogen and dried in vacua. The yield was ~45%. About 2 g of the product were then recrystallised from ca 20 ml of absolute ethanol containing 1 g of tetraethylammonium thiocyanate, filtered off under nitrogen and dried in vucuo. (Et,PH), V(NCS),. This complex was prepared as detailed above for the tetraethylammonium salt, except that a solution of triethylphosphine in 50% concentrated hydrochloric acid was used in place of tetraethylammonium bromide. (Et,N),Fe(NCS),. This complex was supplied by Dr. 0. C. Headley, formerly of this department. (Et,N),Ni(NCS), and (Et,N),Mn(NCS)6. These complexes were prepared from hydrated nickel(I1) chloride and hydrated manganese(I1) chloride respectively by a similar method to that used to prepare (Et,N),V(NCS),, except that precautions to exclude oxygen were unnecessary. (Et,N),Ni(NCS)B (4 g) was recrystallised from ethanol containing tetraethylammonium thiocyanate (O-5g). (Et,N),Mn(NCS), is extremely soluble in ethanol, and so 2 g complex and 1 g of tetraethylammonium thiocyanate were dissolved in 10 ml ethanol at ~40”. The solution was cooled to 0” and the resulting crystals of the required complex were filtered off and dried in vacua. Fe(I1) and Co(I1) salts. Attempts were made to prepare hexaisothiocyanate complexes of Fe(I1) and Co(I1) by routes similar to those used for the V(III), Ni(I1) and Mn(I1) complexes. However, even when the resulting complexes were recrystallised from ethanol containing a ten-fold excess of tetraethylammonium thiocyanate, the only complexes which could be isolated were of the type (Et,N),M(NCS), Bpectroscopic measurements

The infrared spectra of the complexes, in the form of nujol or hexachlorobutadiene mulls, were recorded on Perk&Elmer 225 (4000-200 cm-l) and Grubb-Parsons

326

R. J. H. &ARK

and A. D. J. GOODWIN

GM3 (200-70 cm-l) instruments. The Raman spectra were recorded using a Cary 81 spectrometer fitted with a He/Ne (6328 A) laser source. The complexes were in the form of finely divided solids or (in the case of one of the molybdenum complexes) as an aqueous solution. We are grateful to the University of London for making this instrument available. RESULTS AND DISCUSSION For isolated octahedral ions of the type M(NCS),“-, belonging to the point group On, the vibrational representation reduces as follows:

rvib = 3or9 + 3e, + 2t,, + %, + 3tz, + 3& The a,r, e, and tapmodes are Raman active only, the t,, modes are infrared active only, while the remaining modes are inactive. Approximate descriptions of the vibrational modes are given in Table 2, in terms of CN, CS and MN stretching vibrations, and NCS, MNC and NMN bending vibrations. A convenient separation is Table 2. Species of vibrational modes of complex ions of the type M(NCS)e+

Approximate description

%(R)

WN) VWS) v(Iw G(NCS) WNC) W-MN)

1 1 1

Total modes

3

e,(R)

&,(I)

1 1 1 1 1 3

2

t&R)

t(R)

&&(I)

Degrees of vib. Mom

1 1 1 1 1 1

1 1 1

1 1 1

6 6 6 12 12 9

6

3

3

51

into the following: (A) internal vibrations of the thiocyanate groups and (B) vibrations of the MN, skeleton. The results, given in Table 3, are discussed in these terms. Although the infrared spectra of all the complexes could be studied in the 2200-70 cm-l range, the colours of many of them militated against the satisfactory recording of good Raman spectra using 6328 A excitation. Thus it did not prove possible to obtain a Raman spectrum of the Fe(II1) and Mo(IV) complexes, and those of the Cr(II1) and Ni(I1) complexes were weak. However, the ammonium salt of the Mo(NCS),a- ion is readily soluble in water, kinetically inert, stable in the absence of oxygen and only pale yellow. Good Raman spectra of this complex were obtained in aqueous solution. The Raman spectra of all the complexes are reported for the first time ; the infrared spectra of some of the salts, with the exception of that of the Mn(I1) complex, have been studied previously [l-5,12] in the region above 400 cm-l. However, the only infrared data on the V(NCS),a- ion relate to solvated forms of the ion [3, 131. Moreover, the present spectra and their assignments are more complete than those in the literature. Certain of the salts of Cr(III), Mo(III), Ni(I1) and [12] A. SABATINI md I. BERTINI,~JT~. Chem. 4, 959 (1965). [13] H. B~ELAND and P. MALITZKE,2. Anorg. A&em. Chem. 860,70

(1967).

Maxed

327

snd laser spectra of metal hexaisothiocyanateions

Table 3. Infrared and Raman spectra of the hexaisothiocyanatecomplexes Complex

v(CN)

WS)

*MN)

b(NCS)

S(MNC)

Other

bends

Tervalent salts

oWJ),VWW,

IR IR

2116 m 2066 va 2064 vs. br 2074 v8 2118 8 2098 v8 2068 va 2088 “B 2131 m 2087 B 2082 va 2068 vs 2070 y8 2066 w 21118 2084 sh 2072 w 2080 va 2107 8, p 2064 B 2099 m 2060 8 2060 w 2114m 2090 B 2066 vs 2060 vs, br 2080 vs

R R IR R R IR IR

2104 m 2080 B 2062 vs. br 2126 m 2106 B 2112vs,sh 2102 W, br

IR

2048 v8 2016 vs

R R IR IR IR IR IR IR R R

(Et,PH),V(NCS), K+(NCS)a Aqueous solution

WW),QWW,t K&o(NCS)&H,O

(NH,);Mo(NCS),*Q

(NH,),Mo(NCS),*H,OHCl (NH,),Mo(NCS),~H,OGH&O,H

PI

E IR IR R R

IR R* R* R

(Et,N),Mo(NCS)e

I: R R

(WH),Mo(NCS),

(Rt,N),WNCS),

486 w

136 w

832vwl 84Ovwl 82OvwT

267 m 230 wt 3368 340s 368s

486 s 486 B 474 s

101 m, br 98 m, br am.

838 w

LO.

484 w, br

137 w

183~

82Ovwl 817 m 816 m 816 m 837 m 831 mw

3618 296 s 296 8 294 8 239 8 217 8

481 478 477 477 482

98 m, br mm. n.m. mm. 138 w

118 w?

812m 840 m

838 m

2918 236 ma 216m 231 vw? 216 a 296 s n.o.

820 m 823vwf

833 m

s 8 8 8 w, br

183m

477 8

urn.

ll.0.

Il.0.

488 w

136 w

183 w

486 8 486 w

102 s 136 w

211w 182 w

294 8 270m, br

480 s 479s

106 m, br 98 m, br

160 s

Il.0.

167 8

486 w?

11.0.

196 B 210 m, br

473 B 472 vw?

236s

4718

96 m. br

302 s

484 8

99 m, br

838 m 836 m ll.0.

Bivalent S&e

W,N),MnWCS), (Et,N),Ni(NCS),

t

LO.

823vwT

94 m, br 164 me 182 mw 134 mw

Quadrivalent Selt (pyH)&o(NCS),

LO.

+Remanm eamrementa in aqueous eolution; B, strong; m, medium; meamred; R, Ramen measurements; IR, infrared measurements. t Very poor Raman spectrum.

190 m, br

w. weak; LO., not observed; am.,

not

Mn(I1) have been studied previously in the infrared 16, 12-j between 400 and 200 cm-l with results in good agreement with those presented here, except for the Mn(I1) complex. For the remaining salts, all infrared spectra below 400 cm-l are reported for the first time. (A) Internal vibrations of the thiocyanate groups CN stretching vibrations. The highest of the internal modes of vibration of the thiocyanate group is the CN stretching vibration, which occws near 2100 cm-l in all

R. J. H. CLARKand A. D. J.

I

3

I

I

2100

I

2060

GOODWIN

I

c

2020

CM-’

Fig. 1. Ramanspectrumof the complex (NH&do(NCS),~CH&O$3~H,O in water in the CN stretchingregion. the complexes studied. In most cases, only a single infrared active band is observed in this region, the t,, mode, but for some salts a more complex pattern is observed owing to site symmetry and correlation effects. There are small shifts in the t,, mode with change in the central metal atom, in the sense MO N V < Cr N Fe. It is of interest to establish the relative order of the aI@, e, and t,, CN stretching modes in these complexes. This was accomplished by studying the Raman spectrum of an aqueous solution of the ammonium Mo(II1) salt ; this consists of two bands in the 2100 cm-l region, one at 2107 cm-l with a depolarisation ratio (p) of 0.11, and the second at 2062 cm-l (p = O-78). Clearly the former is the a,, mode, and the latter the e, mode, and it is assumed that this relative order of frequencies is maintained in all the other hexaisothiocyanate salts. Indeed the order a,, > e, > t,, for the CN stretching modes of this salt is the same as that found [14] for the CO stretching vibrations of the neutral metal hexacarbonyls Cr(CO),, Mo(CO), and W(CO),, and speaks for a positive interaction constant between the CN groups. Further evidence that the alp mode lies above the e, mode is that in the solid state Reman spectrum of this Mo(II1) salt,, the lower frequency band only is split as a consequence of solid state effects. CS stretching frequencies. The CS stretching frequencies for all the complexes [14]

L. E. OBUEL, Iwg.

Chem. 1,26 (1962).

Inframd and laser spectraof metal hexaisothiocyanateions

329

studied lie in the range 812-846 cm-l, this range being indicative of N- rather than S-bonded thiocyanate groups. In general, these vibrations give rise to one very weak band in the infrared spectrum of each complex and to one or two somewhat stronger bands in each Raman spectrum. No trends are evident in these vibrational frequencies. In the infrared, this band is frequently obscured by absorption associated with the tetraethylammonium ion. NCS bending frequencie.s. The NCS bending frequencies for the complexes lie in the narrow range 471-488 cm-l giving rise to weak Raman and strong infrared bands in the appropriate spectra. Only a single Raman band is observed in each spectrum in this region (the t,, mode) and likewise, only a single infrared band (the t,, mode), as expected from group theoretical considerations (Table 2). Both the actual frequencies of the NCS bending vibrations as well as the appearance of but a single infrared active band for each complex are indicative of N-bonded thiocyanate groups ; moreover they suggest that any deviation of the MNCS chains from linear must be slight. (B) Vibrations of the 1\1zN,skeleton As opposed to the various NCS vibrational frequencies, MN stretching frequencies. the MN stretching frequencies of the hexaisothiocyanate ions are very sensitive to the oxidation state and to the electron configuration of the metal atom. For instance, the infrared active MN stretching frequencies for the tervalent salts, V(336) Cr(361) and Fe(270), lie above those for the bivalent salts Mn(196) and Ni(236), and within each oxidation state, lie in the relative order of the ligand field stabilisation energies of the ions. The oxidation state dependence of the MN stretching frequencies (a general result for metal ligand stretching frequencies) [16] is most clearly demonstrated by a consideration of the results for the a6 ions, for which Fe(II1) (270) lies above Mn(I1) (196) and for the molybdenum salts, for which Mo(IV) (302) lies above Mo(II1) (291 cm-l). The two Raman active MN stretching modes (al, and e,) occur at lower frequencies than the infrared active t,, mode in all cases, but their relative order proved ditlicult to ascertain. The only evidence on this point was obtained from polarisation measurements on the Mo(NCS),~- ion in aqueous solution. The bands at 216 and 236 cm-l in the Raman spectrum of this ion have depolarisation ratios of O-77 and O-71 respectively, which suggests that the 236 cm-l band is totally symmetric. Thus the implication is that the order of the MoN stretching modes is t,, > a,, > e,, but it is emphasised that this point is not certainly established. By analogy with the results for the Mo(II1) salt, it is suggested that fer the V(II1) salt, the Raman active band at 267 cm-l corresponds to the a,, mode whereas the 230 cm-l band corresponds to the e, mode. If corresponding vibrational frequencies for the two complexes are compared, the greatest difference would be expected for the t,, mode, which involves the metal atom, rather than for the a,, and e, modes which do not. This is indeed the case, the difference being 45,28 and 13 cm-l respectively for the tlu, aI9 and e, modes. No bands could be observed in this region of the Reman spectrum of the complex [US] R. J. H.

hARK,

Sptwochim.

Aota 81, 965 (1965).

330

R. J.

H. CLARKand A. D. J. GOODFP~N

W4N)3WNCSh, Presumably because of the intensity of the colour of the complex ion. Only one band is observed in the Reman spectra of the Mn(I1) and Ni(I1) complexes in the 200 cm-l region for similar reasons. It seems likely that this band is associated with the a,, rather than with the e, MN stretching mode, as its frequency is close to that of the t,, mode in each case. MNC and NMN bending vibrations. Considerable difficulty is attached to the assignment of the low-lying bending vibrations of the M(NCS),“- ions, as these are likely to be extensively mixed. The only reliable guide is that provided by the force constant calculations on the Zn(NCS),2- ion by FOR~TERand HORROCKS[8] who found that the ZnNC bending frequencies (150 and 165 cm-l) were higher than the NZnN bending frequencies (calculated to lie <40 cm-l). The two types of bending mode were, however, considerably mixed. On the above basis therefore, we prefer to assign the 8(MNC) modes above the 8(NMN) modes. A further possible guide to the assignments is provided by the infrared active CMC bending modes of Cr(CO), (109cm-l) [16] and of Cr(CN),3- (124 cm-l) [17]. The spectra of the M(NCS),“- ions below 200 cm-l are characterised by a medium, broad band on the infrared at 98-105 cm-l and a weak-medium band in the Raman spectra at 134-138 cm-l. These bands are probably to be associated essentially with MNC bending vibrations. However, a further weak-medium band occurs in the Raman spectra of the complexes at 182-183 cm-l, for which we have been unable to propose a satisfactory assignment. It would not appear to be associated with the tetraethylammonium cation, as no corresponding band appears in the Raman spectrum of tetraethylammonium thiocyanate, and moreover the band is observed in the Raman spectrum of (pyH),Mo(NCS),. We therefore cannot advance certain assignments for this spectral region, even though lattice vibrations are unlikely to encroach into it owing to the large masses of the ions involved. Acbrwwledgement-A. ship.

D. J. G. thanks the Science Research Council for the award of a student-

[M] R.J. H.CLlLRK mdB.CROCum,Imwg. Chim.Acta 1, 12 (1967). [17] V. CAC~LIOTFI, G. SA~TORI and C. FURLANI,J. Imorg. NW?. Chem.

13,22(1960).