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Resonance Raman and Electronic Spectra of Mixed Valence Compounds with Ni(II) - - -X-Ni (rV) and Ni(II)—X-Pt(IY) Chains (X = C1 or Br)

G. C. Papavassiliou and D. Layek Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation 48, Vassileos Constantinou Av., Athens 501/1, Greece Z. Naturforsch. 37 b, 1406-1409 (1982); received July 7, 1982 Resonance Raman Spectra, Electronic Spectra, Mixed Valence Compounds

The resonance Raman and electronic spectra of compounds Ni(en)2X3, Ni(dapn)2X3 etc. and of compounds [Ni(en)2Pt(en)2X2](C104)4 (where X = Cl or Br) show that these are mixed valence compounds with a metal halide chain (en = 1,2-diaminoethane, dapn = 1,2-diaminopropane).

Resonance Raman spectra of mixed valence com-pounds with a M(II )—X-M(IV) chain (where M = Pt or Pd and X = Cl, Br or I) were reported in a number of papers [1-22], The spectra have been observed in a poly crystalline [1-16] or in single-crystal form [17-22], [9]. The main feature of the spectra is the strong resonance enhancement of the — X - M ( I V ) - X - - symmetric stretching vibration in the chain. Resonance Raman spectroscopy was found to be an analytical tool to search compounds with a metal-halide chain [1-23].

The electronic spectra of this kind of solids show strong size effects [21-25]. These compounds are insulators or semiconductors. The gap frequencev (which is twice the activation energy) found by optical measurements [21, 25] is in a good agree-ment with that obtained from electrical conductivity measurements [26, 27]. In [Pd(en)2Pd(en)2Br2] (C104)4 for example the corresponding values of the gap frequency are 0.967 [21] and 0.98 eV [26].

In ref. [28] was reported that the compound Xi(en)2Cl3 (en = 1,2-diaminoethane) is a low-spin d7 Ni(III) complex while the compound Ni(dapn)2Cl3 (dapn = pn = 1,2-diaminopropane) is a Ni(II)-Ni(IV) mixed valence complex. In this paper we report the resonance Raman and electronic spectra of Xi(en)2Cl3, Ni(dapn)2Cl3, Ni(en)2Br3 (see ref. [28, 29]) and show that these compounds are mixed valence compounds with a Ni(II)---X-Ni(IV) chain (X = Cl or Br). Moreover, we report the preparation and the spectra of some new compounds with Ni(II )—X-Xi(IV) and Ni(II )—X-Pt(IV) chains.

The pure nickel compounds were prepared as in ref. [29]. One should use anhydrous forms of starting materials, i.e. Ni(dapn)2Cl2 etc. It was found that Xi(dapn)2X3 are precipitated easier than Xi(en)2X3. The following method gave crystalline Xi(dapn)2X3 precipitates. A saturated solution of Xi(dapn)2X2 in methanol-ethanol 1:5 mixture was oxidised b}̂ passing N2-X2 gas mixture. Dark blue (X = Cl) or dark red (X = Br) solution* was ob-tained which precipitated small crystalls after a few hours. The precipitate Ni(dapn)2X3 was filtrated, washed with methanol-ethanol (1:1) mixture, then with pure ethanol, carbon tetrachloride and air dried. Small crystalls with a copper (X = C1) or copper-gold (X = Br) luster were stable in dry atmosphere. Elemental analysis for Xi(dapn)2Br3, which is a new compound, gave the following results.

Analysis: CsHwNiBrzNi Calcd C 16.12 H 4.53 Ni 13.14. Found C 15.08 H 4.64 Ni 13.31,

C 16.30 H 4.40.

Using concentrated solutions of Ni(en)2X2 or Ni(dapn)2X2 in methanol and passing N2-X2 gas one can obtain thin shining deposits on the vessel wall or on the quartz-cell wall suitable for optical measurements. The same method was applied for preparation of Ni(en)2Cl3 and Ni(en)2Br3. Starting from Ni(C104)2 the compound Xi(dapn)2(C104)2 was prepared. Solution of this compound in methanol gave shining crystals of Xi(dapn)2X(C104)2 in a few hours after passing X2-X2 gas mixture.

* Reprint requests to Dr. G. C. Papavassiliou. 0340-5087/82/1100-1406/S 01.00/0

* Perhaps the dark solution contains a large portion of Xi(dapn)2X4.

G. C. Papavassiliou-D. Layek • Resonance Raman and Electronic Spectra of Mixed Valence Compounds 1407

[Ni(en)2Pt(en)2X2] (C104)4 compounds (X = C1 or Br) were prepared as following. To a solution of [Pt(en)2Cl2]Cl2 (0.2 g) and NaC104 • H 2 0 (0.35 g) in water (10 ml) a solution of Xi(en)2Cl2 (0.11 g) in Avater (1 ml) was added. Immediately a reddish crystalline compound was precipitated. The pre-cipitate was dissolved by warming the mixture at 95 °C and left for crystallization at room tempera-ture. After a few hours long needle-shaped crystals of [Ni(en)2Pt(en)2Cl2](C104)4 were collected, washed with cold water and air dried.

Analysis: CSH32NsCl6Oi6NiPt

Calcd

C 9.97 H 3.32 N 11.63 Cl 22.12 Ni 6.09 Pt 20.26, Found C 9.74 H 3.18 N 11.79 Cl 19.58 Ni 5.98 Pt 20.10.

The compound [Ni(en)2Pt(en)2Br2](C104)4 was prepared by using equivalent amounts of [Pt(en)2Br2]Br2 instead of [Pt(en)2Cl2]Cl2 and Ni(en)2Br2 instead of Ni(en)2Cl2.

.4nalysis: C^lh.NsCUBr2O^NiPt

Calcd Ni 5.58 Pt 18.55. Found Ni 5.54 Pt 18.50.

Raman spectra were recorded using a Jobin Yvon Ramanor model HG-2S spectrometer. Ex-citing radiation M as provided by a Spectra-Physics argon and a Spectra-Physics helium-neon laser. The electronic absorption spectra were recorded with a Cary 17 D spectrophotometer.

Fig. 1 shows the resonance Raman spectra of polycrystalline pellets of Ni(dapn)2Cl,3 (a) and Ni(en)2Cl3 (b) and the spectrum of [Ni(en)2Pt(en)2Cl2](C104)4 single crystal (c) observed with the wavevector (E) of exciting light parallel to the needle-axis (Z). One can see that there are no considerable differences between the spectra (a) and (b) of Fig. 1. This means that, in contrast to the authors of ref. [28] and in accordance to that of the authors of ref. [29], both compounds Ni(dapn)2Cl3 and Ni(en)2Cl3 are mixed valence com-pounds with a Ni(II)---Cl-Ni(IV) chain. The peak at 263 cm - 1 is attributed to the fundamental stretching vibration of the Ni(II)---Cl-Ni(IV) chain. The peaks at 527, 793, 1055 cm"1 are attributed to the first, second and third overtones respectively. The spectrum of [Ni(en)2Pt(en)2Cl2](C104)4 is similar to that of [Pt(en)2Pt(en)2Cl2](C104)4 [18] but as in

uL-u(cm_1)

Fig. 1. Resonance Raman spectra of Ni(dapn)2Cl3 (a), Ni(en)2Cl3 (b) [Ni(en)2Pt(en)2Cl2] (C104)4 (c) in poly-crystalline pellets (a,b) and in a single crystal with E//Z (c). Excitation = 454.5 nm.

the case of compounds with a Pd(II)---Cl-Pt(IV) chain [2, 18], the peaks occur at higher frequencies than those of [Pt(en)2Pt(en)2Cl2](C104)4. Fig. 2 shows the Raman spectrum of a polycrystalline pellet of Ni(dapn)2Br3 (a) and the spectra of single crystals of [Ni(en)2Pt(en)2Br2](C104)4 (b) and [Pt(en)2Pt(en)2Br2](Cl04)4 (c) observed with E j| Z. The spectrum of Fig. 2 (a) does not show any resonance enhancement. The reason is that the band in the electronic spectrum of Ni(dapn)2Br3

occurs at a very low frequency (see below) in com-parison to the excitation frequency (col = 15803 cm -1). Similar results were obtained for Ni(dapn)2Br(C104)2 with coL = 15803 cm-1 and for [Pd(en)2Pt(en)2Br2](C104)4 with wL = 22000 cm-1. The peaks in the spectrum of [Ni(en)2Pt(en)2Br2](C104)4 (Fig. 2b) occur at higher frequencies than those of [Pt(en)2Pt(en)2Br2](C104)4

(Fig. 2 c). The optical absorption spectra (electronic spectra)

of nickel or nickel-platinum mixed valence com-pounds show strong size effects as in the case of pure platinum and palladium or platinum-palladium mixed valence compounds [16-25]. The optical absorption spectra of compounds with M — X - M

1408 G. C. Papavassiliou-D. Layek • Resonance Raman and Electronic Spectra of Mixed Valence Compounds

« (cm"1 )

Fig. 2. Raman spectrum of a polycrystalline pellet of Ni(dapn)2Br3 (a) and resonance Raman spectra of single crystals of [Ni(en)2Pt(en)2Br2] ( C l O ^ (b) and [Pt(en)2Pt(en)2Br2] (C104)4 (c) with E//Z. Excitation 632.8 nm.

chains are not superpositions of the spectra of their constituents [25]**. The band position (and -shape) in the electronic spectra of thin deposits on quartz plates depends on the size of the particles. Thin deposits ofNi(dapn)2Cl3 or Ni(en)2Cl3 are copper-red in colour and become blue after rubbing with a soft paper. The colour-changes is due to the decreasing of the particle-sizes after rubbing. Fig. 3 shows the optical absorption spectra of Ni(dapn)2Cl3 and Ni(en)2Cl3 thin deposits on quartz plates before and after rubbing. One can see that Ni(en)2Cl3 shows the same spectra as Ni(dapn)2Cl3. This is in agree-ment with the observation of the authors of ref. [29], but in contrast to the observation of the authors of ref. [28]. The spectra reported in Fig. 2 ref. [28] correspond to a yellow-greenish compound

** In ref. [25] was erroniouslv written that the spectra of these compounds are superpositions of the spec-tra of their constituents.

which is obtained sometimes after chlorination of Ni(en)2Cl2. The presence of water favours the forma-tion of the yellow-greenish product which may be a ligand deficient complex. Similar results were ob-tained after chlorination of Ni(tn)2Cl2 (tn = l,3-diaminopropane). Thin deposits of Ni(dapn)2Br3 are copper-gold in colour. Their reflectance spectra show a broad band with a maximum at ca. 1450 nm. The deposits become green after rubbing with a soft paper and show an absorption peak at 905-nm 11050 cm -1). The peaks in the electronic spectra of Ni(dapn)2Br3 and Ni(dapn)2Br(C104)2 occur at lower frequencies than those of pure Pd- or Pt-compounds. This means that Ni compounds are more conducting than Pd- and Pt-compounds. Actually, we found that the d.c.-conductivity of Ni(dapn)2Br3 and Ni(dapn)2Br(C104)2 (in pellets) is 10-50 times higher than that of [Pd(en)2Pd(en)2Br2](C104)4 [27], The peaks in the optical absorption spectra of Ni(II)—X-Pt(IV) occur at higher frequencies than those of Pt(II)—X-Pt(IV).

The luminescence spectra of Ni(II)—Cl-Pt(IV) show a weak broad band at 650 nm (15384 cm -1), while pure nickel compounds do not show anv luminescence spectrum. Similar results have been obtained for Pd(II)—Cl-Pt(IV) and pure palladium compounds [25], while Pt(II)—Cl-Pt(IV) com-pounds show a strong luminescence peak at 1100 nm (9091 cm - 1) [25, 30],

X (nm) 666 500 iOO 333

Fig. 3. Optical absorption spectra of Ni(dapn)2Cl3 (a,c) and Ni(en)2Cl3 (b,d) deposits on quartz cell, be-fore (a,b) and after (c,d) rubbing with a soft paper.

G. C. Papavassiliou-D. Layek • Resonance Raman and Electronic Spectra of Mixed Valence Compounds 1409

have the formula [Ni(L-L)2M(L-L)2Xa]Y4 (where M = Ni or Pt, L - L = 1,2-diamine, X = Cl or Br and Y=C1, Br or C104) with a Ni(II)—X-M(IV) chain.

The similarity in the spectra of nickel mixed valence compounds with those of Pt- and Pd-com-pounds shows that the compounds described above

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