Inorg. Phys. Theor. 859

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859

Downloaded by University College London on 13 January 2011 Published on 01 January 1971 on http://pubs.rsc.org | doi:10.1039/J19710000859

Inorg. Phys. Theor.

Iron(iii) Complexes of Triphenylphosphine and Triphenylarsine Oxides ; their Electron Paramagnetic Resonance and Vibrational Spectra By S. A. Cotton and J. F. Gibson,' Inorganic Chemistry Research Laboratories, imperial College, London S.W.7 E.p.r. spectra are reported for the complexes FeL4(CI0,), and FeL,X, (L = Ph,PO, X = CI. Br, N03,NCS; L = Ph,AsO, X = CI, Br, NO3). The spin-Hamiltonian parameters D and h deduced from these measurements are related t o the structures of the complexes; the vibrational spectra are also discussed. Results imply that the perchlorate complexes contain planar FeL,3+ ions, whilst the chloride and bromide complexes possess the structure trans. [FeL4Xz]+ (FeX,)-. The nitrate and thiocyanate complexes appear to be monomeric, pentaco-ordinate systems.

INa previous publication 1 we described the complexes formed between ferric salts and pyridine N-oxide. A logical extension of this work lay in the examination of triphenylphosphine (and arsine) oxides, where the steric effects of the ligands would be expected to be much greater. Many complexes of these ligands with first-row transition metals have been prepared in the last ten many of which have been pseudotetrahedral ML,X, systems; the manganese complexes of this type have recently been examined by e.p.r.6 A number of 4 : 1 complexes are known, however, and there has been some controversy over whether the anions in these complexes are co-ordinated. Mn(Ph,MeAsO),(ClO,), has one anion co-ordinated 7 whilst recent work on the triphenylphosphine and arsine oxide complexes of manganese@) and nickel@) led to similar conclusions. The complexes FeL,(C1O4), and FeL,Cl, have been previously described; 2,599 conductivity results for the S. A. Cottonand J. F. Gibson, J . Chem. Soc. ( A ) , 1970,2106. F. A. Cotton and E. Bannister, J. Chem. SOC.,1960, 1873; 1878. F. A. Cotton and D. M. L. Goodgame, J. Chem. SOC.,1961, 2298. D. M. L. Goodgame, M. Goodgame, and F. A. Cotton, Inovg. Chem., 1962, 1, 239. D. J. Phillips and S. Y. Tyree, J . Amer. Chem. Soc., 1961, 83, 1806.

arsine oxide complexes indicated that the chloride complex probably possessed the structure [FeL,CI,I FeC1,- whilst the perchlorate complex exhibited ionpairing in solution. +-

EXPERIMENTAL

Physical measurements were carried out as described previously,l whilst FeBr, and Fe(NCS), solutions were prepared by the normal method.1 2,2-Dimethoxypropane was added to all the metal-salt solutions as a dehydrating agent. Fe(Ph,PO),(ClO,),.-This complex was prepared from hot ethanolic solutions of hydrated ferric perchlorate (0.42 g) and triphenylphosphine oxide (1-00 g) ; a pale yellow solid slowly precipitated which was washed with alcohol and ether. The complex Fe(Ph,AsO),(ClO,), was prepared in an identical way. Fe(Ph3PO),C13.-Hydrated ferric chloride (0.54 g ) in hot ethanol was treated with triphenylphosphine oxide (1.10 g ) 13 R. D. Dowsing, J. F. Gibson, D. M. L. Goodgame, M. Goodgame, and P. J. Hayward, J . Chem. SOC.( A ) , 1969, 1242. J. Lewis, R. S. Nyholm, and G. A. Rodley, Nature, 1965, 207, 72. * D. M. L. Goodgame, M. Goodgame, and P. J. Hayward, J. Chem. SOC.( A ) , 1970, 1352. M. J. Frazer, W. Gerrard, and R. Twaits, J . Inorg. Nuclear Chem., 1963, 25, 637.

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J. Chem. SOC.(A), 1971

860 in the same solvent. The yellow product separated from the cool solution. The complex Fe(Ph,AsO),Cl, was similarly prepared. compounds Fe(Ph,PO),Br, and Fe(Ph,AsO),Br,.-These were prepared as red solids by the method described above, froni an ethanolic solution containing FeBr, (ca. 0.6 g) and Ph,PO (1.10g) or Ph,AsO (1.30g). It was not possible to prepare the arsine oxide complex in a completely pure state ; recrystallization from ethanol did not improve the purity. Fe(Ph,PO) ,(NO,) and Fe(Ph,AsO) ,(NO,) ,.-These complexes were prepared from hot ethanol solutions of hydrated Downloaded by University College London on 13 January 2011 Published on 01 January 1971 on http://pubs.rsc.org | doi:10.1039/J19710000859

,

obtained for the perchlorate complex [40 ohm-l cm2 (10-3~]and 52 ohm-l cm2 (104~)]indicated that ionpairing is probably occurring in solution, as noted for the arsine oxide a n a l o g ~ e . ~ The A m values obtained for the chloride and bromide complexes were about half that expected for 1 : 1 electrolytes and showed only a small concentration dependence ; this may mean that the species present in solution is not that present in the solid state. Electrovtic Spectra.-Electronic spectra were recorded for the phosphine oxide complexes in the range 5000-

TABLE1 Analytical results for the complexes Found Complex Fe(Ph,PO),(C104)3 Fe(Ph,PO) 2Cl3 Fe(Ph3PO),Br3

E g g ;:$:& Fe(Ph,AsO) ,(C104), Fe(Ph,AsO) 2C13 Fe(Pk,’AsO)2Br3 Fe(Ph3As0)2(N03)3

C 59.4 59.9 50.9 54.3 59.9 52.7 53.4 44.9 48.7

(yo)

Calc. (%)

2

r

H

\

N

4.4 4.1 3-5 4.0 4.0 3.8 3.8 3.3

5-1 5-1

3-5

4.7

Halide 28.6

25.9

7

c

68-9 60.1 50.7 54.3 59-7 52.6 53.6 46.0 48.8

1

H 4.1 4.2 3.5 3.8 3-9

5.4

3.7 3.8 3.2 3.4

4.7

N

Halide 28.2

5.3

26.5

25,000 cm-l; the d-d bands observed were very weak and broad, and for this reason we refrain from tabulating our results. However, in the chloride and bromide complexes, bands were noted in the positions where the tetrahalogenoferrate ions are known to absorb,lJ1 suggesting that the ‘ ionic ’ formulation for these complexes may be correct. Infrared Spectra (2000-600 cm-l) .-The spectra of RESULTS A N D DISCUSSION the complexes showed the typical bands due to coThe complexes described here were prepared by the ordinated Ph,PO and Ph,AsO; v(P-0) and v(As-0) usual methods for such complexes and were satis- move to lower frequencies on co-ordination, as expected factorily characterised by elemental analyses. The (cf. ref. 8 ) . The perchlorate complexes gave the usual complex Fe(Ph,AsO),Br, could not be prepared in a bands due to unco-ordinated perchlorate groups; no completely pure state, but the results obtained were bands seen were ascribable to co-ordinated perchlorate sufficient to indicate its structure. The formation of groups.12J3 2 : 1 nitrate and thiocyanate complexes is distinctly In the nitrate complexes, however, there was no band unusual; varying the ratio of ligand to metal salt had at 1350 cm-l, implying there are no ionic nitrate groups no effect upon the stoicheiometry of the product obtained. present.14 Instead, v3 is split; the magnitude of this Reaction of the ligands with ferric chloride in ether splitting is greater than that seen in Fe(pyO),(NO,),,varying the molar ratio, always resulted in the form- H,O. Only one pair of lines ascribable to v3 was seen, ation of the 2 : 1 complex, in contradistinction to the implying that all the nitrate groups are monodentate, ca,se of pyridine N-oxide, where a complex Fe(pyO),Cl, for if one of these groups were bidentate two pairs of could also be obtained.1 No attempt was made t o lines would be expected14 for v,. In the thiocyanate prepare a ferric thiocyanate-triphenylarsine oxide complex, v(C-N) is noted as a strong band at 2060 cm-l, complex. with a shoulder a t 2100 cm-l. The oxyanion frequencies Conductance Measureme&.-Conductance measure- are listed in Table 2. ments on the phosphine oxide complexes indicated that F a r I.Y.and Raman Spectra.-The complexes are very the nitrate and thiocyanate complexes were effectively poor Raman scatterers, but rather more information was non-electrolytes; Am values of 3-7 ohm-l cm2 were obtained from the far i.r. spectra. obtained from t o 1O-4M-nitrobenzene solutions , (a) Phosphine oxide com@exes. No band may be compared with the Am values of ca. 30 0hrn-l cm2 1 2 A. E. Wickenden and R. A. Krause, Inorg. Chem., 1966, obtained from 1 : 1 electrolytes.l0 The Am values ferric nitrate (0-8 g) and Ph,PO (1-1 g) or Ph,AsO (1.2 g). Fe(Ph,PO),(NCS),.-This complex was prepared from acetone solutions of Fe(NCS), (ca. 0.5 g) and Ph,PO (1.1 g ) . The dark red solid which slowly formed was washed with a little benzene. Analytical results for the compounds are tabulated in Table 1.

4, 405.

E. G. Taylor and C. A. Kraus, J . Amer. Chem. SOC.,1947, 69, 1731. l1 A. P. GinsbergandM. B. Robin, Inorg. Ckem., 1963, 2, 817. 10

13

980. 14

H. Brintzinger and R. E. Hester, Inorg. Chem., 1966, 5, N. F. Curtis and IT.M. Curtis, Inorg. Chem., 1965, 4, 804.

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Inorg. Phys. Theor.

861

TABLE2 Oxyanion vibrational frequencies Fe(Ph,PO) (C104)3

,-Fe(Ph,AsO),(C104)s

V1 v2 Y3


V,

1090s 623m

1090s 625m

Fe(Ph3PO)2- Fe (Ph,AsO) 2(NOS)3 (NO,) s 1005m lOlOm 805ms 811ms 1523s, 1280s 1510s, 1290s Obscured Obscured

unequivocally assigned as v(M-0), in agreement with previous findings 8 for Mn(Ph,P0)42+ complexes, although as the earlier workers found, there are bands near 400 and 300 cm-l which are metal-sensitive. In the halide complexes, the bands at 378 cm-1 in the chloride and 290 cm-1 in the bromide are assigned as v3 of FeX4- [literature l5 values are 378 cm-l (X = C1) and 285 cm-l (X = Br)] whilst the bands at 288 cm-l (X = C1) and 220 cm-1 (X = Br) are assigned as vas (Fe-X) in a travts-[Fe(P$PO),X,]+ system. In the thiocyanate complex, the medium intensity band at 479 cm-1 is assigned as 6 (NCS), implying that the thiocyanate groups are N-bonded.16 In the i.r. spectra of the nitrate and thiocyanate complexes, two bands occur that may be assigned as v(Fe-ON0,) or v(Fe-NCS), at 320 and 280 cm-l (Fe-ONO,) and 318 and 280 cm-l (Fe-NCS). The lower of the two bands in the nitrate complex is coincident with a prominent Raman line. A square-basedpyramidal structure will have, formally, no higher symmetry than C2v, and thus three v(Fe-ONO,) bands (2A1+B1) would be expected in both the i.r. and Raman spectra, whilst in a trigonal bipyramidal (Da)structure, two v(Fe-ONO,) bands are allowed [vl (Al', Raman) and v5 (E', i.r. and Raman)]. On the basis of the available data, a definite choice between the two structures is not possible, although on the basis of the paucity of v(Fe-ONO,) bands, the D, possibility is preferred; it is also more compatible with e.p.r. results (q.v.). It has been shown1' that assignment of v(M-ONO,) vibrations can be rather difficult. Little information was obtained from the Raman spectra; in the case of the halide complexes, the characteristic bands due to v1 of FeX,- were observed at 336 cm-1 in the chloride complex and 206 cm-1 in the bromide [literature values15 are 330 (X = C1) and 200 cm-l (X = Br)]. (b) Arsine oxide complexes. The medium to strong band which appears in the i.r. spectra of all the complexes in the region 400435 cm-l is assigned as v(Fe-OAsPh,) ; a similar assignment has been made in Mn2+ and Ni2+ complexes8 and is consistent with the values found in other complexes of iron with oxygen-containingligands.1 The halide complexes have bands at 285 (X = Cl) and 218 cm-l (X = Br) assigned as v(Fe-X) for the [Fe(Ph,As0,)X2] ions whilst the tetrahalogenoferrate stretching bands occur in their usual positions in both +

* All g values are effective; g values defined by geB = hy/j3H. The real g is taken t o be isotropic at 2.00 in this analysis. 16 J. S. Avery, C. D. Burbridge, and D. M. L. Goodgame, Spectrochim. Acta, 1968, 24A, 1721.

the i.r. and Raman spectra. The nitrate complex has i.r. bands at 300 and 270 cm-l assignable as v(Fe-ONO,) ; these occur at rather lower energy than those in the phosphine oxide analogue. This may indicate a weaker Fe-ONO, bond in the arsine oxide complexes; in agreement with this, the splitting of the nitrate ion v3 is smaller in the case of the arsine oxide complex. The low-energy vibrational spectra are listed in Table 3. TABLE3 Far i.r. and Raman spectra of the complexes (i.r. 550-200 cm-l, Raman 400-170 cm-l) 1.r.: 538sbr, 508s, 458m, 440sh, 418w, 394w, 302m, 290m, 261m R: 395m, 308m, 294m, 275w, 256s, 211sh, 196w 1.r.: 550s, 535sh, 486w, 428m, 414m, 345m, Fe(Ph,PO),(ClO,), 304w, 272w, 250sh, 238s R: 356w, 279w, 256w, 245w 1.r.: 540s. 520sh, 483m, 456w, 438m, 380sh, Fe(Ph,PO),(NO,), 340sh, 320s, 280s,br, 260sh, 230m R : 280m, 265s 1.r.: 542s, 526sh, 477w, 463sh, 448m, 414m, Fe(Ph,PO),Cl, 378s. 318sh, 300sh, 288s, 258m, 230m R: 336s, 261m, 244m, 18Ow 1.r.: 540s, 525sh, 470w, 445m, 416m, 320sh, Fe(Ph,PO),Br, 290s,br, 260m, 220s R: 206m Fe(Ph,PO),(NCS), 1.r.: 540s, 479m, 460m, 445m, 422m, 318s, 280s,br 1.r.: 474s, 464s, 45Om, 390w, 362s, 350% Ph,AsO 326m, 286w, 260m, 244m R: 355w, 325w, 260m, 237vs, 182m Fe(Ph,A~0),(C10~)~ 1.r. : 470% 460sh, 418m, 391w, 360s, 338m, 316m, 290w, 256m R: 323w, 262m, 242s, 200w,br, 184w Fe(Ph,AsO),(NO,),: 1.r.: 475s, 465s, 452sh, 434% 390w, 358s, 338m, 300s, 270s, 256m, 233m R : 318w, 304m, 264m, 242s, 204w, 180w,br Fe(Ph3AsO),C1, 1.r.: 474s, 462s, 450sh. 404m, 378s, 358sh, 340sh, 314m, 285s. 240m R: 337s, 315w, 270w, 245s, 226sh, 200~7, 185w Fe(Ph,AsO),Br, 1.r.: 472s, 462s, 452sh, 410m, 362s, 342m, 320sh, 290s, 234m, 218m R: 244m, 206s, 185w

Ph3P0

E.9.r. Spectra.-Typical spectra are shown in Figures 1 and2. Fe(Ph3PO),(C10,),, at X-band (9.3 GHz), gave a spectrum typical of a high-spin FeIII ion in a tetragonally distorted environment, with D rather large; this excludes the other possibility of a tetrahedral structure. Apart from the typicalgl* = 6, gil = 2 lines, a split transition is noted above 6 kG at X-band. The spin-Hamiltonian was solved as described previously,l8 and the values of D = 0.84 cm-l, A = E/D = 0.005 give good agreement between observed and calculated transitions at both X-and Q-band (36 GHz). (A typical analysis is shown in Table 4 for one of the spectra shown in Figures 1 and 2. To avoid excessive 16 J. Lewis, R. S. Nyholm, and P. W. Smith, J. Chem. SOL, 1961, 4590. 1 7 J. I. Bullock and F. W. Parrett, Chem. Comm., 1969, 157. l* R. D. Dowsing and J. F. Gibson, J. Chem. Phys., 1969, 50, 294.

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J. Chem. SOC. (A), 1971 tabulation of data no other analyses are set out in detail, but the D and A values derived for all the compounds are given in Table 5). The arsine oxide analogue gave rather similar spectra, but with the high-field X-band line occurring at rather higher field, implying a higher D value; also, there is no detectable splitting in this line, so that A is taken as 0. At Q-band, the "gL = 6"

there are other, weaker, absorptions in higher field. Such spectra might be thought to be consistent also with a single species with a rather low D value (ca. 0.1 cm-l) but the Q-band spectrum shows that this is clearly not the case, since absorptions take place from zero field up to 14 kG, indicating that D > 0.25 cm-l. Again, at Q-band, the most prominent feature of the

TABLE4 E.p.r. spectra (in gauss) of the nitrate complexes Downloaded by University College London on 13 January 2011 Published on 01 January 1971 on http://pubs.rsc.org | doi:10.1039/J19710000859

Observed r

Fe(Ph,PO),(NO,),

Fe(Ph,AsO)

,

D

Calc. for 0.55 cm-l A = 0.04

=

D

Calc. for 0.62 cm-l A = 0-067 =

Calc. intensity

Transition

X-BAND

960s 1360s 3400m 3850m 4400w 6100w 7900w 9 lO0vw

910s 1580s 3350m 3700ni 5200w 7700w 97O O w

973 1355 3460 4057

898 1546 3683 3494

1-45 2-47 3.51 1-75

6153 8321

7438 9606

1-75 3.14

13,730

3-37

13,OOOw

Q-BAND

536 550

1.0i 1.52 690s 2.00 1-41 1300m 2.95 895 lOOOm 4743 3870m 2-12 4750m 4560w 437 1 0.56 6732 2.34 7022 7050s 2.87 7024 2-78 5744 6 1OOsh 2-55 6319 2-84 7180s 7232 2.43 10,lOOw 1.89 10,836 11,OOOm 10,528 10,950 2.54 13,315 13,000~ 12,333 2.50 12,600~ 12,364 12,714 3.35 14,095 1-85 14,800~ 15,370~ 0.25 16,108 16,130~ Differences between calculated and observed transitions sometimes arise because of the difficulty in defining the exact point a t which t o measure the field on a powder or frozen solution spectrum (&500 G), because of the large linewidths ( &100 G) and because of the limited accuracy of the calculation (i:100 G). Sometimes intense predicted lines are not seen: sometimes observed lines are not predicted; these effects are explained in ref. 18. Under the heading Transition are given the high-spin wave functions which contribute t o the transition; x, y and z refer to the magnetic field along these directions of the zero-field splitting tensor. 315s

729 795 1003 3986 4468

line occurs at rather lower field than in the phosphine spectra is the strong geff= 2 line. Assuming the oxide complex, again implying a higher D value; num- g!ff = 2 line to come from the FeC1,- ion, (Et4N+FeC14gives a very strong, symmetrical, geff = 2 resonance at erical calculation gives D = 1.05 cm-l, A = 0. The spectra of the bromide complexes at X-band, TABLE5 show a gL = 6, gll= 2 type of signal as expected from Zero field splitting parameters D and A = E / D a trans-FeL,X, system with D > ca. 0.3 cm-l, but the D (cm-1) gee = 2 component includes a signal from the FeBr40.84 0.005 Fe(Ph,PO),(C10,)3 ion, which is rather weak at X-band but of comparable Fe(Ph,AsO), (C10,) , 0-000 1.05 [Fe(Ph,PO),Br,] [FeBr,] intensity to that from the hexaco-ordinate chromo1-20 0-000 [Fe(Ph,AsO),Br,] [FeBr,] 1.50 o*ooo phore at Q-band. D is estimated to be 1.2 cm-l for [Fe(Ph3PO),CI,][FeCl,] 0.63 0.010 [Fe(Ph,PO),BrJ+, partly from the observation of a very [Fe(Ph,AsO),Cl,] [FeCl,] 0.55 0.010 Fe(Ph,PO) ,( NCS), 0.07 0.100 weak line at 8 kG at X-band. No such definite estimate Fe(Ph,PO)!dNO,), 0-55 0.040 is possible for the arsine oxide complex, where D is Fe(Ph,AsO) .(NO,) , 0.62 0.067 thought to be ca. 1.5 cm-l. The chloride complexes gave more complicated both X - and Q-band), good correlation is obtained for spectra. At X-band, the strongest line is at geff= 2, both complexes of observed and calculated transitions; but another prominent line occurs near geff= 6, and the phosphine oxide complex has D = 0.63 cm-l,

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Inorg. Phys. Theor.

863

arsine oxide analogue has D = 0055 the chromophores tram-[Fe(Ph,RO),C12]+. One or two shoulders on the gE , = 2 line at


A = 0-01 and the cm-l, A = 0.01 for

0

5

lkGl

m

FIGURE 1 X-band spectra of A, Fe(Ph,PO),(ClO,),, B, Fe(Ph,PO),(NO,),, and C, Fe(Ph,PO),Cl,

FIGURE 2 Q-band spectra of A, Fe(Ph,PO),Br,, B, and C, Fe(Ph,PO),Cl, Fe(Ph,PO),(NO,)

,,

both X - and Q-band cannot be fitted with the calculated data which otherwise gives an excellent agreement between observed and calculated results for the [Fel o Cf. R. D. Dowsing, J. F. Gibson, D. M. L. Goodgame, M. Goodgame, and P. J. Hayward, Nature, 1968, 219, 1037; J. S. Griffith, Mo2. Phys., 1964, 8, 213.

(Ph,RO),ClJ chromophores. These lines probably arise from a very slight distortion of the FeC1,- ions-a D value of ca. 0.02 cm-l would be enough to account for this. The spectrum from the thiocyanate complex is typical of a d5 ion in an environment where D is small, since transitions are seen over the range 0-6 kG at X-band, with the strongest absorption contained in the 2 4 kG region. Unfortunately, it was not possible to obtain a Q-band spectrum in order to confirm the selected parameters of D = 0.07 cm-l, A = 0.100 but the X-band fit is quite good. The observation of such a signal implies that the complex is not dimeric, as would be possible if two thiocyanates acted as bridging ligands in a dimer viz. (Ph,PO) ,( SCN),Fe (NCS),Fe(NCS),(Ph,PO), and bridging thiocyanates may be excluded on the far i.r. data also. The A value obtained is closer to that expected for an ‘ axial ’ system (0.00) than for a ‘ rhombic ’ system (0.33) and implies that the complex has at least a three-fold axis of symmetry.19 A values of this order have been found in the, formally, D, I\’In(y-picoline),Cl, system.20 I n the latter case, divergence of A from the anticipated value of 0.0 was ascribed to slight non-planarity of the Mn(y-picoline), plane for steric reasons. I n the trigonal bipyramidal system (ca. DS) envisaged here, steric effects are expected to be smaller, but solid-state packing effects may introduce a little disorder. The X-band spectrum (Table 4) of Fe(Ph,PO),(NO,), has its most prominent lines near gl = 6 and gll = 2, indicating a system with A close to 0.0; the gl = 6 line is split, implying A is not quite 0.0, whilst a number of transitions are observed at higher fields. The Q-band spectrum has its strongest absorptions near 300 gauss and 7000 gauss; observation of the low-field line implies that, a t Q-band, hv-2D or 4D (the spacings of the Kramers doublets for A = 0.0). Numerical calculation gives a best agreement of observed and calculated data for D = 0.55 cm-l, A = 0.04. The arsine oxide analogue (Table 4) gave an X-band spectrum where the splitting in the gL = 6 line was more pronounced than in the case of the phosphine oxide analogue, implying that A was rather greater. The Q-band spectrum was rather similar to that obtained from the phosphine oxide complex, but there was a definite splitting in the line near 7 kG. Calculation gives D = 0.62 cm-l, A = 0.067 as the best result. These results are interpreted in terms of a monomeric, five-co-ordinate structure, similar to that proposed for the thiocyanate complex; here the slight divergence of A from 0.0 is explicable on the basis of the co-ordinate sphere not having quite D,, symmetry, because of steric interactions between the nitrate groups. +

CONCLUSIONS

Despite the rather limited conclusions that could be drawn from the far i.r. and Raman spectra, a good deal 20 R. D. Dowsing, J. F. Gibson, M. Goodgame, and P. J. Hayward, J . Chem. SOC.( A ) , 1969, 187.

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J. Chem. SOC. (A), 1971 of useful stereochemical information has been obtained mainly from the e.p.r. spectra. The results imply clearly that the 4 : 1 perchlorate complexes are genuinely square-planar ; the D values of 0434 cm-l for the phosphine oxide complex and 1.05 cm-1 for the arsine oxide complex may be compared with 1.00 cm-l found 21 for the complex Fe[N(SiMe,)& where the environment of the iron atom is planar, three co-ordinate.22 Triphenylarsine oxide is known 23 to producehigherD,- values than triphenylphosphine oxide, and the higher value of D in the arsine oxide complex appears to reflect an increase in the strength of the inplane Fe-O bond. The e.p.r. spectra of the halide complexes are further evidence for their being considered to have the structure tram- [Fe (Ph,P 0)4XJ FeX,-, which appears to be so common1s24for iron complexes of the stoichiometry FeL,X,. As has been 1 0 1 increases found in a number of MnII systems6,*~20 in the order C1 < Br, but without depopulation experiments, the sign of D cannot be determined. In view of the magnitude of D in these complexes, such experi+-

D. C. Bradley, R. G. Copperthwaite, S. A. Cotton, and J. F. Gibson, unpublished results, 22 D. C. Bradley, M. B. Hursthouse, and P. F. Rodesiler, Chem. Comm., 1969, 14. 23 F. A. Cotton and D. M. L. Goodgame, J . Amer. Chem. Sot., 1960, 82, 6771. 21

ments would have to be carried out below 10 K. Gerloch et al. have suggested% that a tetragonal elongation corresponds to a negative sign of D; that is, the &5/2 state lies lowest in zero field, so that it is anticipated that this situation applies to the perchlorate complexes, at least. The results of studying the 2 : 1 nitrate and thiocyanate complexes by a number of techniques point to the complexes being monomeric, five-co-ordinate, entities. Whilst five-co-ordination is not unknown for iron(m), crystallographically characterised cases appear to be confined to square-based pyramidal species, such as Fe(NO)(S,CNEt,), 26 and FeCl(S,CNEt,), 27 where chelate ligands are involved. So far, however, we have been unable to grow suitable single crystals for X-ray study to investigate whether the proposed trigonal bipyramidal structure is correct. One of us (S. A. C.) is indebted to the S.R.C. for a studentship. [0/1586

Received, September 15th, 19701

24 M. A. Bennett, F. A. Cotton, and D. L. Weaver, Acta Cryst., 1967, 23, 581. 2s M. Gerloch, R. C. Slade, and J. Lewis, J . Chem. Soc. ( A ) , 1969, 1422. 26 M. Colapietro, A. Domenicano, L. Scaramuzza, A, Vaciago, and L. Zambonelli, Chem. Comm., 1967, 583. 27 R. L. Martin and A. H. White, Inorg. Chem., 1967, 6 712.