Metal nitrate and thiocyanate complexes with 2,6-lutidine n-oxide

/. inor~.,,mlcl. Chem.. 1972,Vol. 34, pp. 3139-3152. PergamonPress. Printedin Great Britain

METAL

NITRATE WITH

AND THIOCYANATE COMPLEXES 2,6-LUTIDINE N-OXIDE*

N. M. KARAYANNIS,+ C. M. M I K U L S K I , L. L. PYTLEWSKI and M. M. LABES.$ Department of Chemistry, Drexel University, Philadelphia, Pa. 19104 (Received 17 January 1972)

Abstract-Complexes formed by interaction of excess 2,6-1utidine N-oxide (LNO) with various metal nitrates and thiocyanates were prepared and characterized by means of spectral, magnetic and conductance studies, as follows: [M(LNO)n](NOa)3 (M = Cr, Fe), involving distorted octahedral complex cations and ionic nitrate; [M(LNO)~(ONO2)(O~NO)] (M = Mn, Co, Ni, Zn), pentaco-ordinated neutral complexes, containing one mono- and one bi-dentate nitrato groups; [Cu(LN O)2(ON O~)~], planar, and [Cd(LNO)4(ONO~)~], hexaco-ordinated, with monodentate nitrato ligands; [(LNO)(SCN)2Mg(LNO)2Mg(NCS)~(LNO)], dimeric, pentaco-ordinated, LNO-bridged, with N-bonded thiocyanate; [(NCS)2Hg(LNO)2Hg(SCN)2], dimeric, tetrahedral, LNO-bridged, with S-bonded thiocyanate; [(LNO)2(SCN)M(NCS)2M(NCS)(LNO)2] (M = Co, NiL pentaco-ordinated dimers with terminal isothiocyanato as well as bridging NCS groups; and [Cu(LNO)2(NCS)2]x, hexaco-ordinated polymeric complex with bridging NCS ligands. [Fe(LNO)6](NO3):~ is the first example of a compound characterized by the FeHiO6 moiety and exhibiting spin-free-spin-paired equilibria; the polymeric [Cu(LNO)2(NCS)2]x exhibits a subnormal magnetic moment. The influence of the steric effects of LNO on the types of complexes formed is discussed. INTRODUCTION

PYRIDINE N-oxide (PNO) and 3- or 4- substituted derivatives do not introduce

steric effects during c.o-ordination to metal ions, usually allowing the attainment of the co-ordination numbers favoured by each metal ion[l]. Thus, with 3d metal perchlorates, these ligands form cationic complexes of the types [ML6] "+ (M = V :~+, Cr~+, Mn '+, Fe '+, Fe 3+, Co '+, Ni '-'~, Zn 2+) and [CuL4] 2~ ; [CuL6] "+ has also been reported for PNO and its 4-methyl-derivative [1-6]. In the cases of 2- or 2,6- substituted pyridine N-oxides, steric factors become important, as demonstrated by the stabilization of complexes of the following types: [Co(2-picNO)5](C104)2 [7]; [Ni(2-picNO)4](C104)2 [8]; (2-picNO = 2-picoline N-oxide); * Presented in part at the 162nd National Meeting of the American Chemical Society, Washington, D.C., Sept. 12-17, 1971 ; see Abstracts No. IN OR 87. t Present address: Amoco Chemicals Corporation, N aperville, Illinois 60540. $ Present address: Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122. 1. R. G. Garvey, J. H. Nelson and R. O. Ragsdale, Coord. C h e m . Rev. 3. 375 (1968), and references therein. 2. R. Whyman, W. E. Hatfield and J. S. Paschal, Inorg. Chim. Acta. 1, 113 (1967). 3. R. L. Carlin, J. A m . chem. Soc. 83, 3773 (1961). 4. D . H . Herlocker, R. S. Drago and V. Imhof Meek, lnorg. C h e m . 5, 2009 (1966). 5. N. M. Karayannis, M. J. Strocko, C. M. Mikulski, E. E. Bradshaw, L. L. Pytlewski and M. M. Labes, J. inorg, nucl. C h e m . 32, 3962 (1970). 6. D.W. Herlocker, Ph.D. Thesis, University of Illinois, Urbana, Ill., 1966. 7. W. Byers, A. B. P. Lever and R. V. Parish, Inorg. C h e m . 7, 1835 (1968). 8. J. H. Nelson and R. O. Ragsdale, lnorg. C h i m . A e t a 2,439 (1968). 3139

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[M(LNO)4](CIO4)2 (M = Fe, Co, Ni, Cu, Zn; LNO = 2,6-1utidine N-oxide (2,6-dimethylpyridine N-oxide)) and [Mn(LNO)4(CIO4)](C104) [9]. It was of interest to us to study the nature and stereochemistries of the complexes formed by interaction of metal nitrates and thiocyanates with an excess of a sterically hindered aromatic amine oxide, such as LNO. Only a limited number of 2:1 complexes of this ligand with salts of these anions (namely Co(NOz)2, Co(NCS)2 and Zn(NO3)2) had been previously reported[10, 1 I]. Non-sterically hindered aromatic amine oxide form two types of complexes with divalent 3d metal nitrates, i.e. [ML~(NO3)~] and [MLr](NO3)2 or [CuL4](NO3)z [12, 13], while 2-substituted pyridine N-oxides reportedly yield [NiL2(NO3)~] and [NiL4(NO3)2] complexes, involving, respectively, bi- and mono-dentate nitrato groups [8]. Metal thiocyanate complexes with pyridine N-oxides have been studied to a lesser extent. Some illustrative metal thiocyanate complexes are: [CoL~(NCS)2] ( L = LNO, 2,4,6-collidine N-oxide)[10], [Fe(PNO)3(NCS)3][14], [Cu(PNO)2(NCS)z] [ 15], [Hg(PNO)(SCN)2] [ 16] and VO(NCS)2. nPNO (n -- 4, 5) [ 17]. The present paper describes our synthetic and characterization studies of metal nitrate and thilocyanate complexes with LNO. EXPERIMENTAL Synthetic procedures. J. T. Baker-grade L N O and the purest commercially available metal salts, triethyl orthoformate and solvents were utilized. The metal nitrate complexes were prepared by the following procedure: The hydrated metal salt is dissolved in a 9 : 1 mixture of triethyl orthoformate and acetone, the resulting solution is warmed, under stirring, to 50-60°C, and an excess of LNO (ligand to salt molar ratio, ca. 7: 1) is then added. Stirring at the above temperature is continued for 10-15 rain, and the solution is subsequently allowed to cool to ca. 40°C. At this point a large excess of ligroine (b.p. 63-75°C) is added and the reaction mixture is stirred for 20-30 min at room temperature. During the latter period, gradual formation of a copious precipitate occurs in all cases studied. As demonstrated by analytical data, complexes of the following types are obtained by this procedure: M(LNO)6(NO~)3 (M = Cr, Fe), Cd(LNO)4(NO3)z and M(LNO)z(NOD2 (M = Mn, Co, Ni, Cu, Zn). Complexes of the type M(LNO),~(NCS)2 (M = Mg, Co, Ni, Cu) and Hg(LNO)(SCN)2 are immediately precipitated, when excess LNO is added to a warm (50°C) solution of the salt in triethyl orthoformateethanol (1 : 1). The Ni(NCS)z complex is initially obtained as a light green hydrated product, which is completely dehydrated during desiccation over P205 to yield Ni(LNO)2(NCS)~. The new complexes which are generally stable in the atmosphere, were filtered, washed with ligroine or ether and dried in an evacuated desiccator over P205. Analytical results (Schwarzkopf Microanalytical Laboratory, Woodside, N.Y.) are shown in Table 1. Spectral and magnetic measurements. I.R. spectra (Tables 2 and 3, Figs. 1-3), electronic spectra (Table 4) and magnetic measurements (Table 4) were obtained by methods described elsewhere [5, 9]. The i.r. spectra of the new complexes do not exhibit co-ordinated water bands. Conductance measurements. The conductivities of 10 -3 M nitromethane solutions of the complexes

9. N. M. Karayannis, L. L. Pytlewski and M. M. Labes, lnorg. Chim. Acta 3, 415 (1969). 10. H. N. Ramaswamy and H. B. Jonassen, lnorg. Chem. 4, 1595 (1965). 1 I. M. S. Novakovskii, V. N. Voinova, N. S. Pivnenko and N. F. Kozarinova, Zh. neorg. Khim. 11, 1738 (1966). 12. R. L. Carlin and M. J.Baker, J. chem. Soc. 5008 (1964). 13. N . M . Karayannis, S. D. Sonsino, C. M. Mikulski, M. J. Strocko, L. L. Pytlewski and M. M. Labes, lnorg. Chim. A cta 4, 141 (1970). 14. S.A. Cotton andJ. F. Gibson, J. chem. Soc. (A) 2105 (1970). 15. S.J. Gruber, C. M. Harris, E. Kokot, S. L. Lenzer, T. N. Lockyer and E. Sinn, Austr. J. Chem. 20, 2403 (1967). 16. A.J. Pappas, J. F. Villa and H. B. Powell, lnorg. Chem. g, 550 (1969). 17. H. Brhland and E. Niemann, Z. anorg, allg. Chem. 373, 217 (1970).

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Table 4. Electronic spectra and magnetic moments (300°K) * of transition metal nitrate and thiocyanate complexes with LNO Electronic spectra

Complex

Medium

Maxima, nm (~)

[Cr(LNO)n](NO~)~

Nujol

[Mn(LNO)2(NOa)2]

CHsNO~(3 × 10-a M) Nujol

[Fe(LNO)d(NOa)3 [Co(LNO)2(NO3)2]

CHsNOz(5 × 10-3 M) Nujol CHaNO~(3 x 10-4M) Nujol CHaNO~(10 -2 M)

[Ni(LNO)2(NOa)2]

Nujol CHaNO~(10 -2 M)

[Cu(LNO)~(NOD2] [Co(LNO)2(NCS)z]2

Nujol CHaNO2(4 × I0 -~ M) Nujol

CH3NO2(4 × 10-3 and 5 × I0 -4 M)

[Ni(LNO)~(NCS)dz

Nujol CHaNO2(4× 10-aM)

[Cu(LNO)z(NCS)~]x

Nujol CHaNOz(2 × 10-3 M)

Magnetic properties 106X~ cor, cgsu

< 300 vs, 330 sh, 400 s,sh, 580 s 398 sh (151), 584 (80) < 300 vs, 330 s,sh, 492 s,sh, 602 w,sh < 400, 510 sh (20) < 300 vvs, 334 vs, 480 sh < 4 0 0 ( > 2500) 480 sh, 560 s, 602 sh, 850 m, 1100 vw, sh 529 (127), 740 (10), 1100 b (15) 425 s, sh, 547 s,sh, 765 m, 1080 w,b,sh < 400 ( > 85), 692 (10), 765 (6), 1195 (3"5) < 350 vvs, 604 s 604(35) 328 s, 490 s, 555 s, 704 m,sh, 830 m,sh 1040 w,sh 520 sh, 575 (100), 608 (212), 635 (225), 1200 b (65), 1380 (58), 1500 sh (46) 380 s, 455 s, 480 sh, 520 sh, 800 m,b < 4 0 0 ( > 200),598 sh(13), 622 (14), 683 sh (12), 1055 b (5) 870 s, 1080 sh, 1350 sh 840:(41), 1200 b (34)

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s, strong; m, medium; w, weak; b, broad; v, very; sh, shoulder. *Magnetic properties were determined in the solid state by the Faraday method. were measured at 25°(2, by using a Wayne Kerr Universal bridge and a cell calibrated with 0-0IN aqueous KCI. The Cr(III) and Fe(III) nitrate complexes behave as 1 : 1 electrolytes (Am values of 68 and 771"1-1 cm ~ mole -I, respectively). The rest of the new complexes show a behaviour intermediate between those of non- and 1 : l-electrolytes; Am values (in f~-i cm 2 mole-l) are, in most cases, closer to the non-electrolyte region, i.e. Cd(LNO),(NOa)~, 19; M(LNO)~(NOa)2: Mn, 18; Co, 16; Ni, 22; Cu, 32; Zn, 19; M(LNO)2(NCS)2: Mg, 36; Co, 18; Ni, 17; Cu, 22; [Hg(LNO)(SCN)2]2, 30. DISCUSSION

Infrared and conductance data T h e m a i n b a n d s i n t h e i.r. s p e c t r u m o f L N O

(liquid film) in the 1800-200

c m -1

Metal nitrate and thiocyanate complexes

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Fig. 3. I.R. spectra (2400-2000, 900-650 and 500-200 cm -1) of [Mg(LNOh(NCS)2]2, [Co(LNO)2(NCS)2]2, [Ni(LNOh(NCS)2]2, [Cu(LNO)2(NCS)z]~ and [Hg(LNO)(SCN)2]2. The strong, broad band at ca. 750 cm -1 is due to the polyethylene windows utilized.

region are[18, 19]: 1760vw, 1670w, 1553 m, 1490 s, 1443 s, 1409 m-s, 1380w, sh, 1361 s, 1245vs (VN-O), l150W, 1090m, 1026m, 975 m-s, 9 1 0 w - m , 844m (SN-O), 762 VS (TCH), 670 sh, 595 w, 555 s, 540m, 514 s, 492 m, 458 sh, 372 m-w, 300 w,b, 286 m, 267 m, 250 m, 240 m, 220 m, 210 w. Co-ordination of this ligand to metal ions through the N - O oxygen results in sizeable negative VN-Oand positive Yc~ shifts, as well as small 8N-O shifts[20] (Tables 2, 3). Although the i.r. spectrum of the ligand is quite rich, no problems arose in assigning characteristic polyanion vibrational bands and (tentative) metal-LNO and metal-polyanion absorptions. The only new compounds involving ionic nitrate exclusively are the Cr(III) and Fe(III) nitrate complexes. These compounds exhibit i.r. bands (Nujol mull spectra), typical of ionic (D3h) NOa; no absorptions attributable to coordinated nitrato groups are observed in the spectra of these complexes [21-24] (Table 2, 18. H. Shindo, Chem. Pharm. Bull., Tokyo 4, 460 (1956). 19. N. M, Karayannis, C. M. Mikulski, M. J. Strocko, L. L. Pytlewski and M. M. Labes, J. inorg, nucl. Chem. 33, 3185 (1971). 20. Y. Kakiuti, S. Kida and J. V. Quagliano, Spectrochim. A cta 19, 201 (1963). 21. C. C. Addison, N. Logan, C. S. Wallwork and C. D. Garner, Quart. Rev. 25, 289 (1971); S. P. Sinha, Z. Naturforsch. 20A, 1661 (1965); G. Topping, Spectrochim. Acta 21, 1743 (1965). 22. H. Brintzinger and R. E. Hester, lnorg. Chem. 5, 980 (1966); R. E. Hester and W. E. L. Grossman, lnorg. Chem. 5, 1308 (1966); B. Taravel, F. Fromage, P. Delorme and V. Lorenzelli, J. chim. Phys. Physico-Chim. Biol. 68, 715 (1971). 23. N. F. Curtis and Y. M. Curtis, lnorg. Chem. 4, 804 (1965). 24. A. B. P. Lever, E. Mantovani and B. S. Ramaswamy, Can.J. Chem. 49, 1957 (1971).

Metal nitrate and thiocyanate complexes

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Figs. 1, 2). In addition, these compounds do not exhibit VM-O(nitrato) bands [25]; they are obviously of the type [M(LNO)6](NO3).~(M--Cr, Fe), in the solid state. Their behaviour as 1 : 1 electrolytes in nitromethane is most probably due to the displacement of L N O by nitrate groups in solution [8, 13]. The rest of the metal nitrates studied yield neutral complexes, solely involving co-ordinated nitrato groups. The i.r. spectra of these compounds show bands characteristic of co-ordinated nitrate[21-24], and low frequency absorptions, attributable to u.~,-o (nitrato) modes [25] (Table 2, Figs. 1, 2). Distinction between mono- and bidentate co-ordinated nitrate, on the basis of i.r. data, is usually difficult. In fact, a bidentate nitrato group has the same local site symmetry (C.,,,) as that of a monodentate nitrato group, coordinated in such a manner, as to produce a linear M - O - N grouping[21,22]. In these cases, the splittings of the fundamental vibrational modes of the nitrate ion are of similar magnitudes, and no distinctions between these types of co-ordination are possible from examination of the energies of these absorptions[21,22]. Compounds involving co-ordinated monodentate nitrato ligands, but non-linear M - O - N groupings, with Cs local site symmetry for the polyanion, can be rather easily distinguished from bidentate nitrato complexes, on the basis of i.r. evidence [ 13,21,22]. However, the monoand bidentate nitrato groups in the complexes reported are apparently of about the same local site symmetry. In such cases, the most reliable i.r. criterion for distinguishing the type of co-ordinated nitrate is provided by the relative energies of various combination bands of the polyanion [23,24] and in particular those occurring in the 1800-1700 cm-' region [24]. Compounds with ionic nitrate exhibit a single band in this region; this absorption is due to combination of the v, symmetric stretch (A, in D3h) and the v4 doubly-degenerate in-plane bending mode (E' in D3,,) of the NO3- ion [24]. Examination of the spectra of [M(LNO)n](NO:0:~ (M =-Cr, Fe) in this region, show that only one i.r. combination band occurs (Table 2, Fig. 2). This evidence lends additional support to our formulation for these complexes. Compounds with C~v co-ordinated nitrate exhibit two bands at 1800-1700 cm -~ ; these are due to a splitting of v4 into two components (14, and B2 in C2,,) and their combination (allowed by symmetry) with /21 (A 1 in C2v). The splitting between the two (Vl + v4) bands is generally larger for bi- rather than monodentate nitrato complexes, owing to a greater distortion of bidentate NO3 groups from D3h symmetry [23,24]. Finally, complexes involving both mono- and bidentate coordinated nitrato groups, exhibit four bands at 1800-1700 cm-' [24]; and, in many cases, two sets of bands corresponding to the fundamental vibrations of each type of nitrate coordination [23]. The only new complexes clearly involving one type of coordinated nitrate, are those of Cu(II) and Cd(II). These compounds exhibit single bands corresponding to the fundamental modes of co-ordinated (C2v) nitrate, and two (/2, + L'4) bands (Table 2, Figs. 1, 2), showing a separation of 28-35 cm-'. This separation is roughly intermediate between those reported for bi- and monodentate nitrato complexes [23,24]. The overall evidence is suggestive of the presence of monodentate nitrate in these complexes. In fact, the low frequency i.r. and electronic 25. J. R. Ferraro and A. Walker, J. chem. Phys. 42, 1273 (1965); R. H. Nuttall and D. W. Taylor, Chem. Commun. 1417 (1968); J. I. Bullock and F. W. Parren, Chem. Commun. 157 (1969).

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spectra of the Cu(II) complex (cf. Tables 2, 4; vide infra) suggest that Cu(II) is tetraco-ordinated in this compound. In the case of the Cd(II) complex, a hexaco-ordinated configuration is more likely than one with a higher-co-ordination, especially in view of the steric hindrance introduced by co-ordination of four L N O ligands, to the Cd 2+ ion. The above complexes are, therefore, formulated as [Cu(LNOh(ONO2h] and [Cd(LNO)4(ONO2)2]. The fact that Cu(NO3)~ does not form an N-oxide bridged, pentaco-ordinated dimer with L N O , as in the case of the corresponding P N O complex [26] is presumably due to the steric effects of the methyl substituents of the L N O ligand. The M(LNO)2(NO3h (M = Mn, Co, Ni, Zn) complexes exhibit 3-4 absorptions at 1800-1700 cm -~. Moreover, bands attributable to the fundamental vibrations of C2v nitrate are either split or broad in the spectra of these complexes (Table 2, Figs. 1, 2). This evidence is in favour of structures involving two types of co-ordinated nitrate [23, 24], and is further substantiated by the low frequency i.r. and the electronic spectra and magnetic moments of these compounds (cf. Tables 2, 4) which are suggestive of co-ordination number five for the central metal ions (vide infra). Possible structures for these complexes involve either one mono- and one bidentate nitrato group and a monomeric configuration or monodentate and bridging nitrato ligands and a bi- or poly-nuclear structure. Monomers involving both mono- and bidentate nitrate reportedly include several transition metal nitrate-amine complexes and tetranitratometalate anions[23, 24]. Nitrato bridging of the type M - O - N ( O ) - O - M has been reported only for anhydrous or partially hydrated transition metal nitrates [27] while bridging through the oxygen of a monodentate nitrato ligand (i.e. of the type M-O(NO2)-M) was recently established for the dimer [Cu(pyh(NO3h]. py (py = pyridine) [28]. The 2-picoline analogue of the latter complex, although exhibiting a similar arrangement of the ligand atoms around copper, is monomeric; dimerization is prevented by the blocking action of the methyl substituents in this case [28, 29]. A more severe blocking action of this type ought to be exerted by the L N O methyl groups in the complexes discussed. Hence, a monomeric structure of the type [M(LNO)z(ONO2)(OzNO)] (M = Mn, Co, Ni, Zn), involving mixed mono- and bidentate nitrato ligands, appears to be the most probable for these compounds. The fact that no combination band at 1800-1790 cm -1 is observed in their i.r. spectra, although by no means conclusive [24], is also in favour of the absence of bridging nitrate. The low frequency i.r. spectra of the new metal nitrate complexes (Table 2, Fig. 2) are generally in support of the structures proposed. Thus, for the [M(LNO)6](NO3h (M = Cr, Fe) complexes only VMo(LNO) bands were identified. The rest of these complexes exhibit both VMo(LNO) and VMo(nitrato) absorptions, with the exception of the Cd(NOa)2 complex; in the latter case UMo(nitrato) was not observed at wavenumbers down to 200 cm -1; this is probably due to the fact that Vcdo(nitrato) modes occur below 200cm-1125]. Vuo(LNO) occurs above 400 cm -1 in the tetraco-ordinated Cu(II) complex, at 385-350 cm -1 in the pentaco26. 27. 28. 29.

S. Sdavni6ar and B. Matkovic, A cta Crystallogr. B25, 2046 (I 969). S. C. Wallwork andW. E. Addison, J. chem. Soc. 2925 (1965). A. F. Cameron, K. P. Forrest, D. W. Taylor and R. H. Nuttall, J. chem. Soc. (A) 2492 (1971. A. F. Cameron, R. H. Nuttall and D. W. Taylor, Chem. Commun. 865 (1970).

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ordinated Mn(lI), Co(II), Ni(II) and Zn(II) compounds and at 320cm -1 in [Cd(LNO)4(ONO2)2], which is hexaco-ordinated. These data are fully consistent with the co-ordination numbers assigned to these compounds [19, 30]. Tentative VM,)(nitrato) assignments are also in agreement with values previously reported [25]. The location and number of vcN, Vcs, 8Ncs and M-thiocyanato bands in the i.r. spectra of metal thiocyanate complexes are generally diagnostic of the mode of co-ordination of the N C S group and the stereochemistry of the complex [31-37]. vcN appears as a single band in the spectra of the Mg(II) and Hg(II) complexes (Table 3, Fig. 3); the presence of bridging N C S groups in these compounds is ruled out, since in bridging thiocyanate complexes v(:x occurs at higher frequencies [31]; the 3d metal complexes exhibit splittings of the uc,~ mode, suggestive of the presence of bridging NCS groups (vide infra). Mg(LNO)2(NCS)2 exhibits u(,~. at 2095, Vcs at 835, 8Ncs at 479 cm-~; these data are indicative of the presence of co-ordinated isothiocyanate (N-bonded) ligands. Two bands with the characteristics of VMgo(LNO) are observed; this is most probably due to the presence of both terminal and bridging L N O ligands [ 1, 16]. This complex is thus, formulated as [(LNO)(SCN)2Mg-(LNO)2-Mg(NCS)2(LNO)], i.e. a LNO-bridged pentacoordinated dimer. Tentative VMgoand vM~ assignments (Table 3) are in agreement with data for Mg(II) complexes with O- and N-ligands previously reported [38]. The Hg(II) complex, on the other hand, involves S-bonded thiocyanato ligands, as shown by the occurrence OfvcN at 2112, Vcs at 698 cm -L and 8Ncs as a triplet [33]. The bands tentatively assigned as VHgo and VHgs are again in agreement with analogous assignments appearing in the literature[30, 33]. This complex is formulated as [(NCS)2Hg(LNO)2Hg(SCN)d involving a dimeric, LNO-bridged configuration, and an essentially tetrahedral environment for Hg(II); a similar structural assignment was made for the P N O analogue [ 16]. The splitting of VcN in the 3d metal thiocyanate complexes, and the occurrence of the higher frequency band at 2170-2150 cm -~ is strongly in favour of the presence of bridging N C S groups in these compounds[31]. The Cu(II) complex is obviously polymeric, involving a distorted octahedral structure, as demonstrated by the occurrence of Vcuo and Vc.x at 330-320 cm -1 (Table 3, Fig. 3) and its electronic spectrum (vide infra). Vcux has been identified in this region in polymeric octahedral, NCS-bridged Cu(II) complexes [36], while Vc~,o(LNO) would be expected to occur at considerably higher wavenumbers, should the complex be tetra- or pentaco-ordinated[19]. NCS-bridging groups frequently give rise to splittings of VcN[34, 36]; their exclusive presence in the Cu(II) complex, which is 30. G. Schmauss and H. Specker, Z. anorg, allg. Chem. 363, 113 (1968); 364, 1 (1969). 31. P. C. H. Mitchell and R. J. P. Williams, J. chem. Soc. 1912 (1960); A. Turco and C. Pecile, Nature 191, 66 (1961). 32. D. Forster and D. M. L. Goodgame, lnorg. Chem. 4, 715 (1965). 33. A. Sabatini and I. Bertini, lnorg. Chem. 4, 959 (1965). 34. M. E. Farago and J. M. James, inorg. Chem. 4, 1706 (1965). 35. S, M. Nelson and T. M. Shepherd, J. inorg, nucl. Chem. 27, 2123 (1965). 36. R . J . H . Clark and C. S. Williams, Spectrochim. ,4 cta 22, 1081 (1966). 37. C. Pecile, lnorg. Chem. 5, 210 (1966). 38. R. E. Hester and R. A. Plane, lnorg. Chem. 3, 768 (1964); J. Reedijk and W. L. Groeneveld, RecL Tray. Chim. 87, 1079 (1968).

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assigned a polymeric distorted octahedral structure of the type [Cu(LNO)2(NCS)2]x (x > 2), is also evidenced by the identification of Vc,s at 212 cm-' [36] and the overlap of Vcs with YCH[34] (Table 3, Fig. 3). The Co(II) and Ni(II) thiocyanate complexes appear to involve both isothiocyanato and bridging NCS ligands. Furthermore, their electronic spectra and magnetic moments are indicative of pentaco-ordination for the central metal ions (cf. Table 4; vide infra). A dimeric structure of the type [(LNO)2(SCN)M-(NCS)2M(NCS)(LNO)2] (M = Co, Ni) is, thus, most probable for these compounds. In fact two VMNand two Vcs bands are observed in their spectra; the higher frequency VMNand Vcs bands (Table 3, Fig. 3), which correspond to the terminal NCS groups, occur at wavenumbers intermediate between those reported for tetrahedral and for octahedral Co(II) and Ni(II) thiocyanate complexes[32, 36]. VMo(LNO), on the other hand, appears at ca. 380 cm -~, as is also the case with other pentacoordinated Co(II) and Ni(II) complexes with L N O or 2-picNO[19]. Finally, the lower frequency VcNis indicative of the presence of terminal isothiocyanato groups [31-37], while that of bridging NCS is further evidenced by the multiple 8NCS bands and vMSabsorptions (tentatively identified) at ca. 215 cm -1 [33,36]. Electronic spectra and magnetic m o m e n t s

[Cr(LNO)6](NOa)z has an electronic spectrum very similar to that reported for [Cr(LNO)6](C104)3[9] and a normal magnetic moment[39] (Table 4). This compound and its Fe(III) analogue are characterized by complex cations involving Oh MO6 moieties; however, the effective symmetry influencing the metal ion is lower than Oh, as demonstrated recently by Byers et al. for cationic transition metal complexes with aromatic amine oxides [7]. The magnetic moment of the Fe(III) complex (3.81 BM) is suggestive of a 6AIg-ZTzocrossover situation[40]. A ground state with three unpaired electrons (S = ]) is excluded by ligand-fiel theory in octahedral d 5 compounds [41], but spin free (S = ~)-spin-paired (S = ½) equifibria have been reported for a variety of tris(di-N,N-alkyldithiocarbamato) Fe(III) chelates, containing the FeSn moiety [40]. In these cases the ligand-field is of the same order as the mean-pairing energy ~-for the d 5 configuration [40]. LNO gives rise to ligand-fields of similar or higher magnitudes than those ofdi-N,N-alkyldithiocarbamates (e.g. Dq towards octahedral Cr(III) : LNO, 1686 [9], N-diethyldithiocarbamate 1560 cm -1142]); thus, spin-free-spin-paired equilibria, although quite unusual for Fe(III), should not be unexpected for [Fe(LNO)6] a+. Studies of the temperature dependence of the paramagnetism of [Fe(LNO)6](NOs)a would contribute to the understanding of the behaviour of this complex; work in this direction will be undertaken as soon as the proper instrumentation becomes available. The two Co(II) complexes have been previously reported and characterized 39. B. N. Figgis and J. Lewis, Progr. inorg. Chem. 6, 37 (1964), and references therein. 40. L. Cambi and L. Szeg6, Ber. deutsch, chem. Ges. 64, 2591 (1931); A. H. Ewald, R. L. Martin, I. (3. Ross and A. H. White, Proc. Roy. Soc. 280A, 235 (1964); A. H. White, R. Roper, E. Kokot, H. Waterman and R. L. Martin, A ustr. J. Chem. 17, 294 (1964); E. Kokot and G. Ryder, A ustr. J. Chem. 24, 649 (1971). 41. J. S. Griffith, J. inorg, nucl. Chem. 2, 1 (1956). 42. A . A . G . Tomlinson, J. chem. Soc. (A) 1409 (1971).

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as octahedral, i.e. Co(LNO)2(NOa)2 as a monomer involving two bidentate nitrato groups and Co(LNO)2(NCSh as a NCS-bridged polymer [ 10]. Nevertheless, more recent work established the existence of pentaco-ordinated Co(II) complexes with pyridine N-oxides (e.g. [Co(2-picNO)5]2+)[7, 43]. Comparison of the solid state electronic spectra of the two Co(II) complexes under study (Table 4) to that of [Co(2-picNO)5](C104)2[7] is clearly indicative of pentaco-ordinated structures for the former compounds; in fact, the three complexes exhibit (d-d) transitions at 700-850 nm; bands in this region are not observed in the spectra of octahedral Co(II) complexes with pyridine N-oxides [1,7]. The magnetic moments of these Co(II) complexes are also suggestive of pentaco-ordinated configurations [7, 44]. It should be noted that Herlocker has studied Co(LNO)2(NCS)2 and concluded that its properties point to a pentaco-ordinate configuration [45]. Co(LNO)~(NOa)2 is partially solvated in nitromethane, yielding a mixture of penta- and hexacoordinated species, as shown by the intensification of the band at ca. 1100 nm (Table 4). The Co(II) thiocyanate complex is obviously de-dimerized in the same solvent and a tetrahedral species (most probably the Co(LNO)2(NCS)~ monomer) is formed, as suggested by its electronic spectrum (Table 4). In methanol, the above complexes are solvated, yielding octahedral species as shown by the intensities of the bands at 500-520 nm (E = 8-22)[10]. The Ni(II) complexes exhibit solid-state electronic spectra and magnetic moments (Table 4) which may be interpreted in terms of pentaco-ordinated configurations[19, 44]. In nitromethane, these compounds show significantly different electronic spectra than those of the solid-state (Table 4); the solution spectra of the Ni(II) compounds are typical of hexaco-ordinated pyridine N-oxide complexes of this metal ion[l, 2,4, 7, 30] and indicative of solvation of the solid complexes by nitromethane. It is of interest to note that hexamethylphosphoramide (HMPA) behaves in a similar way as LNO towards Ni(NCS)2 [46]: In fact, octahedral complexes of the type NiI_~(NCS)2(OH2)2 are initially obtained with both these ligands. Dehydration of these products leads to the formation of yellow-orange complexes of the type NiI<(NCS)2; the dehydrated LNO complex is quite stable in the atmosphere, while the H M P A analogue is hygroscopic and converts rapidly to the dihydrate in the air [46]. It is, therefore, quite probable that Ni(HMPA)2(NCS)2 is dimeric and pentaco-ordinated, as is the case with its LNO analogue. The solid-state electronic spectrum of Mn(LNO)~(NO3)2 is also suggestive of a pentaco-ordinated structure [44]. Cu(LNO)~(NOa)2 is obviously tetraco-ordinated, involving an essentially planar CuO4 moiety, as shown by the occurrence of the (d-d) transition at 604 nm [7, 9] (Table 4). A tetragonally distorted, octahedral structure for Cu(LNO)2(NCS)2 is evidenced by the fact that the (d-d) band is split in this complex and lies some 5000 cm -1 lower in energy than that of the tetraco-ordinated Cu(NO3)2 complex [7]. The magnetic moment of the Cu(NCS)2 complex is slightly below the 43. B. A. Coyle and J. A. Ibers, lnorg. Chem. 9,767 (1970). 44. M. Ciampolini and N. Nardi, lnorg. Chem. 5, 41, 1150 (1966); M. Ciampolini and G. P. Speroni, lnorg. Chem. 5, 45 (1966); F. Lions, I. G. Dance andJ. Lewis, J. chem. Soc. (A) 565 (1967). 45. D.W. Hedocker, made this point during the discussion of the present work at the 162nd National Meeting o f the Am. Chem. Soc. 46. E. LeCoz, J. Guerchais and D. M. L. Goodgame, Bull. Soc. chim. Ft. 3855 (1969).

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normal values for the Cu(II) ion (Table 4); demagnetization via magnetic exchange is a common phenomenon in bi- or polynuclear, ligand-bridged Cu(II) complexes (cf., for example, Ref. 15). The fact, that Cu(NCS)~ yields a polynuclear hexacoordinated LNO complex, whereas the Co(II) and Ni(II) analogues are pentacoordinated dimers, is attributed to the steric effects introduced by the 2,6-methyl substituents of LNO. In fact, a variety of polynuclear hexaco-ordinated Co(II), Ni(II) and Cu(II) thiocyanate complexes with amine ligands, of the type [ML2(NCS)2]x, have been reported; Co(I1) and Ni(II) complexes of this type are essentially octahedral, but the Cu(II) analogues exhibit significant tetragonal distortion, owing to the Jahn-Teller effect [34, 47]. In M(LNO)2(NCS)z (M = Co, Ni), the steric features of LNO favour the stabilization of a dimeric pentacoordinated structure, by hindering the formation of essentially octahedral polymers. In Cu(LNO)2(NCS)2 the LNO ligands are most probably t r a n s to each other and co-ordinated along the z axis; this is suggested by the fact that Vc,,o(LNO) occurs at a considerably lower frequency (Table 3) than the corresponding band in [Cu(PNO)6](CIO4)2, which appears at 368cm-1120]. The Cu-LNO bands are, therefore, relatively long, owing to the Jahn-Teller distortion, with the steric interference of the LNO groups insufficiently severe to prohibit the attainment of a polynuclear hexaco-ordinated structure for this complex. The conclusions of the present study, insofar as steric effects during the formation of complexes between LNO (when in excess) and metal nitrates or thiocyanates are concerned, are as follows: In the cases of divalent metal salts these effects are evidenced by the fact that M(LNO)2X~ rather than M(LNO)4Xz (X = NOa, NCS) complexes tend to be stabilized; only Cd(NO3)~ forms a complex of the latter type, while Hg(SCN)2 yields a 1 : 1 complex with LNO. However, in the case of trivalent metal nitrate complexes there is apparently no steric interference during co-ordination of the ligand, and [M(LNO)6] 3+ cationic complexes are formed. M 3+ ions appear to be generally less susceptible than M 2+ ions to the steric effects of bulky or sterically hindered monodentate oxo-ligands [9, 48]. 47. M. A. Porai-Koshits and G. N. Tischenko, Kristallografiya 4, 239 (1959); H. C. A. King, E. K6r6s and S. M. Nelson, J. chem. Soc. 5449 (1963); 4832 (1964); S. M. Nelson and T. M. Shepherd, lnorg. Chem. 4, 813 (1965). 48. J. T. Donoghue and R. S. Drago, Inorg. Chem. 2, 1158 (1963); R. S. Drago, J. T. Donoghue and D. W. Herlocker, lnorg. Chem. 4, 836 (1965).