PARTIAL QUADRUPOLE SPLITTINGS OF SOME ORGANOPHOSPHORUS LIGANDS

Sh. Sh. Bashkirov, A. S. Khramov, and I. Ya. Kuramshin UDC 539.1+53~.2+541.49+541.6

In recent years much work has appeared on the interpretation of the quadrupole split- ting (AE) in the MSssbauer spectra of organotin and organoiron compounds based on the point- charge models, as exemplified by [1-3].

With the point, charge model the electric field gradient (EFG) at the ~dssbauer nucleus (57Fe or *1"Sn) is considered an additive quantity, i.e., the net EFG(q) is treated as the sum of the contributions from each ligand (qLi) [i]. Since AE ~ q, the additivity principle can also be extended to the quadrupole splitting, which can be viewed as the sum of partial quadrupole splittings (PQS) of the ligands.

We have attempted to delineate the applicability of the point-charge model to complexes of organophosphorus compounds with tin halides by Calculating the PQS's of the organophos- phorus ligands in the complexes L2Sn(CH3)nCI4_n (I) (n = 0, i, 2; L = [(CH~)2N]3PO, (CH3)3PO, (CH3) 2CIPO, (CH3)2(CH30)PO, (C~Hg)sPO) and examining possible correlations of the PQS's with the stereochemistries of the complexes.

We have reported the ~dssbauer spectra of compounds 2, 4, 5, 7-9, and 12 (Table I) earlier [4, 5]. We derived the M~ssbauer parameters by mathematical treatment on an M-220 computer using minimization with respect to X2; in all cases X 2 lay within the limits of the number of experimental points.

We calculated the PQS's by the procedure described in [i, 2], using the data of [3] for [ (CH~) 2N] 3PO.

Table 2 summarizes the resulting PQS's of the organophosphorus ligands; the theoretical (AEcalc) and experimental (AEexpt) quadrupole split~ings are compared in Table i.

When calculating the PQS's of the organophosphorus ligands we diverged from other workers [1-3] in taking the PQS of chlorine as 0.01 mm/sec, thus allowing for any possible intramolecular interaction of C1 with the organophosphorus ligands. This gave better agree- ment between AEcalc and AEexpt.

Our experimental results (Table i) yield 1.85 mm/sec for the PQS of the CH3 group, where- as Bancroft et al. got 1.03 mm/sec [3]. This sizeable discrepancy originates from the dif- ferent molecular structures assumed for the complexes. When calculating the PQS, we sup- posed, on the basis of the results of [6, i0], that the (CHs)nSnCl~_n fragment in complexes (I) lies in the equatorial plane of the L2Sn(CH3)nCI~-n octahedron, whereas Bancroft et al. [3] considered that the CH3 groups occupy the axial positions.

The structure of L2Sn(CHs)CI3 complexes is contentious [i0]. However, the linear cor- relation of ~ with AEexpt for the series of L2Sn(CH~)nCI~-n (n = 0, i, 2), and our calcula- tion of AEcalc support the axial positioning of L in complexes 4, 7, and i0.

Comparison of the two pairs of complexes 1 and 2, and 12 and 13 reveals a correlation of the PQS's with stereochemistry. Both 3sCl NQR and IR indicate that complexes 1 and 12 have the cis structure, while compounds 2 and 13 are the trans isomers. When the ligand is (C4H,)3P0 AEcalc and AEexpt of both compounds (12 and 13) are identical within experimental error. When the ligand is (CH3) 3PO AEexpt and AEcalc show a substantial discrepancy for the complex [(CH3)sP0]2SnCI~ but complete agreement for [(CH3) 3P0]a(CHs)2SnCI2. This discrepancy apparently stems from the lack of complete consistency with experiment of the theory of addi- tive EFG's, in which the EFG is negative for octahedral cis complexes. Thus, the EFG has been shown experimentally to be positive throughout an entire series of cis isomers [9]. Assumption of the trans structure for [(CH,)3P0]=SnCI~ gives complete agreement between AEexpt and AEcalc (0.51 mm/sec and +0.52 mm/sec, respectively).

V. I. Ul'yanov-Lenin Kazan State University. Translated from Izvestiya Vysshikh Uchebn- ykh Zavedenii, Fizika, No. 6, pp. 145-147, June, 1978. Original article submitted September 9, 1977.

0038-5697/78/2106-0823507.50 �9 1978 Plenum Publishing Corporation 823

TABLE I. ~6ssbauer Parameters and Calculated AEcalc of LaSn- (CHs)nCl4-n Complexes (isomer shifts (6) relative to BaSnO3 at room temperature; accuracy of 6, AEexpt , s and F= is • mm/sec)

N o . L

1 (CH3) 3PO 2 (CH3) 3PO 3 (CHs) 2 (CH30) PO

(CH~):~(CHsO) PO (CH~) 2 (CH.~O) PO (CHs)2CIPO

(CHs) 2ClPO (CH3) 2CIPO [(CH3) 2N]3PO [ (CH~) eN]3PO

[ (CHs) eNI~PO

(C4H9) 3PO (C4H9) 3PO

7 8

9 I0" I17 12 13

AEexpt AEealc r t ; r=

--0,26 I ,0,99; 0,99

4-4,20 !l,F2; 1,18 tram 0,98; 0,98 +0;56

--0,28

4-2,40 1,25; 1,2.0 +4,24 ~t,27::1,27 tram 1,24:1,.15 +0,4~

eL --0,2(', +2,26 1,06; 1.2~ +4.10 0,98; '0,98 +0,52 0,89:0,90 +2,36 +4.20 0,95; 0.95

- 0, "22 0,89; 0,89 !. 14 0,83; 0,83

S tereo- chemistry

[6, 8]

eis

t ram tram +

ci$

rrans

trans tram +

eis

tram

trans

tram

tram trans

cis

�9 tram

*From [7]. %~ = 1.32; AEexpt = 4.28; AEcalc = 4.36; s = 1.27; Fa = 1.06 [3]; 6 = 1.54; &Eexpt = 4.45 [7].

TABLE 2. Partial Quadrupole Splittings (PQS) of Some Ligands

Ligand PQS

C1 CHs (CH3)3PO (CH3) 2 (CH30) PO (CH3)2ClPO [(CH3) 2N]3PO* (CMo)3PO

--0,01 --1,85 +0,12 +0;13 +0,095 +0,12 +0, 10

*From [13].

In summary, we can conclude that the point-charge model in general provides a satisfac- tory description of complexes (I). However, we believe that the discrepancies, together with our earlier results [5, 6], suggest that treatment of results derived from the point-charge model requires supplementary structural information on the particular coordination compounds, i.e., the point-charge model can be used only as a first approximation for analyzing the stereochemistry and electronic structure of complexes.

LITERATURE CITED

I. M. G. Clark, Mol. Phys., 20, 257 (1971). 2. G.M. Bancroft, Coord. Chem. Rev., ii, 247 (1973). 3. G.M. Bancroft, V. G. Kumar Das, and K. D. Butler, J. Chem. Soc., Dalton Trans., 1974,

2355.

824

4. Sh. Sh. Bashkirov, F. G. Vagizov, I. Ya. Kuramshin, A. A. Muratova, A. N. Pudovik, and A. S. Khramov, Zh. Strukt. Khim., 17, 178 (1976).

5. R. A. Manapov, I. Ya. Kuramshin, A. A. Muratova, and A. N. Pudovik, Zh. Obshch. Khim., 45, 1975 (1975).

6. A. N. Pudovik, I. Ya. Kuramshin, E. G. Yarkova, A. A. Muratova, A. A. Musina, and R. A. Manapov, gh. Obshch. Khim., 43, 1229 (1973).

7. V. S. Petrosyan, N. S. Yashina, S. G. Sakharov (Sacharov), and O. A. Reutov, J. Organo- met. Chem., 52, 33 (1973).

8 A. I. Andreeva, I. Ya. Kuramshin, A. A. Muratova, D. Ya. Osokin, I. A. Safln, and A. N. Pudovik, Izv. Akad. Nauk SSSR, Ser. Fiz., 3_~, 2590 (1975).

9. D. Cunningham, M. J, Frazer, and J. D. Donaldson, J. Chem. Soc., Dalton Trans., 1972, 1647.

I0. F. P. Mullins, Can. J. Chem., 49, 2719 (1971).

NATURE OF THE OPTICAL ABSORPTION BANDS IN THERMALLY TREATEDCRYSTALS

OF ALKALI-METAL BROMIDES

Yu. M. Annenkov, Yu. I. Galanov, and T. S. Frangul'yan UDC 535.343.2

We have established that the quenching of crystals of NaBr and NaBr + 0.5 M% CaBr~ from 600~ generates an optical absorption band with maximum at 4.76 eV. Nechaev and Selezneva have recently reported the same effect in KBr crystals [i]. They attributed the quenching- induced absorption to the excitation of anions next to isolated cation vacancies. Proof of the validity of this interpretation would provide a reliable experimental method for detect~ ing free cation vacanices, which in turn should markedly contribute to studies of the radia- tive generation of Frenkel defects in the cation lattice of alkali halide crystals. These observations and their probable interpretation [i] have provided the stimulus for a thorough study of the nature of the optically active defects generated in the thermal treatment of alkali-metal halides.

Here we report a study of the optical absorption spectrum of previously quenched crys- tals of NaBr, NaBr + 0.5 M% CaBr2, KBr, NaCI, and KCI. We first demonstrate that quenching generates additional absorption bands only in crystals of NaBr, NaBr + CaBr~, and KBr. At- tempts to detect the same effects in alkali metal chlorides were unsuccessful. We describe the principal features of the color centers generated in NaBr and KBr by thermal treatment below.

i. Intense absorption bands with maxima at 4.76 eV (NaBr) and 4.69 eV (KBr) appear when the quenched crystal is annealed on an easily oxidized metal as support (Cu, Fe). The support material does not effect the position of the absorption maxima. Figure la shows typical optical absorption spectra of crystals of KBr, NaBr, and NaBr + 0.5 M% CaBr~ after annealing in air in contact with copper plates at 600=C for 40 min followed by quenching to room temperature. When annealed on a support made of unoxidizable material (mica, quartz), KBr does not show the additional absorption band and sodium bromide gives only a weak band. Thus, easily oxidized metals in contact with the crystal act as catalysts of the process responsible for the formation of optically active defects on quenching.

2. We next examined the spatial distribution of the color centers in quenched crystals. Crystals of dimensions 1.6 • 5 • 15 mmwere clamped between copper plates and quenched from 600~ to room temperature. A specimen of thickness 0.6 mm was cut from the center of the parallepiped. We examined the distribution of the color centers with a quartz microspectro- photometer (resolution 20 ~m) in the direction perpendicular to the face of the specimen in contact with the copper plates. Our measurements on NaBr + 0.5 M% CaBra (Fig. 2) reveal that the color centers are concentrated in the surface layers. Figure 2 also shows the opti- cal absorption spectrum of the specimen at a distance of 50 ~m from the surface.

S. M. Kirov Tomsk Polytechnic Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 147-149, June, 1978. Original article submitted September 29, 1977.

0038-5697/78/2106-0825507.50 �9 1978 Plenum Publishing Corporation 825