5 The Magnetic Properties of Transition-metal Ions By R . C. SLADE Chemistry Department Queen Elizabeth College, Campden Hill Road London W8 7AH The early explanation of the magnetic properties of metal complexes in terms of crystal-field effects and orbital quenching represents a milestone in the develop-ment of the chemistry of transition-metal ions.’ Since this pioneering work, the measurement of magnetic moments has been much used and often misused, as a source of information as to the structure of and the bonding in such com-pounds. In this Report we shall examine recent work describing the magnetic behaviour of metal complexes and discuss our present understanding of the various factors which determine this behaviour. No attempt has been made to cover the entire literature comprehensively ; rather published reports are cited as examples or illustrations of the general themes developed in the first two sections ; also the proliferation of recent reviews in this subject makes such a task 1 Magnetic Behaviour of Theoretical Models Studies of the magnetic behaviour of transition-metal complexes having formally orbital triplet ground-terms have been particularly fruitful for the magneto-chemist because spin-orbit coupling and low-symmetry crystal-field components act to a first-order approximation on these ground terms.Consideration of the models proposed in recent years indicates that the magnetic properties of such systems could be successfully described by the simultaneous perturbation of spin-orbit coupling and an axial (tetragonal or trigonal) crystal-field ’ M.Gerloch and J. Lewis Rev. Chim. minerale 1969 6 19. B. N. Figgis and J. Lewis Progr. Inorg. Chem. 1964,6 37. E. Konig ‘Magnetic Properties of Transition Metal Compounds,’ Springer-Verlag, Berlin 1966. M. Kato H. B. Jonassen and J. C. Fanning Chem. Rev. 1964,64 99. ’ E. K. Barefield D. H. Busch and S . M. Nelson Quart. Rev. 1968,22 457. E. Konig Co-ordination Chem. Rev. 1968 3 471. ’ R. L. Martin ‘New Pathways in Inorganic Chemistry,’ ed. E. A. V. Ebsworth A. G. Maddock and A. G. Sharpe Cambridge University Press Cambridge 1968 ch. 9. P. W. Ball Co-ordination Chem. Rev. 1969,4 361. W. E. Hatfield and R. Whyman Transition Metal Chem. 1969 5 47. l o G. A. Webb Co-ordination Chem. Rev, 1969,4 107. “ E. Sinn Co-ordination Chem.Rev. 1970 5 313. l 2 B. Jezowska-Trzebiatowska and W. Wojciechowski Transition Metal Chem. 1970, 6 1 62 R. C. Slade component acting on the ground terms 2q 5q 3T1 and 4T1.13-16 These models (called the Figgis models with apologies to Professor Lewis) describe the magnetic behaviour by three parameters ; A-the spin-orbit coupling coefficient k-the orbital reduction factor and v-a parameter relating the ground-state splitting A and A (this form of parametrization was more convenient computationally than the alternative use of A directly). This three-parameter model (a fourth parameter A appears in the 3T1 and 4T1 models allowing for mixing of the corresponding TI terms from the P term) has been employed in an extensive series of studies by Figgis and Lewis and their co-workers of numerous transition-metal complexes involving different metal ions with a wide variety of ligands.Almost without exception the average magnetic moments measured on powdered samples over the temperature range 80-300 K could be fitted to the appropriate model by allowing reasonable variation in the three parameters. Thus for the first time the magnetic moments of complexes of metal ions of the first transition series could be described satisfactorily by a theoretical model. Despite the success of these apparently reasonable models, the authors were careful to stress the approximations both in the models them-selves and in the application of them to complexes of less than cubic symmetry. The values of the parameters obtained by fitting (either by manual interpolation using tables or by computer) the measured magnetic moments to the calculated ones represent those values required to reproduce the experimental results within that model.The question remains as to whether unique parameter values are obtained and whether the parameters have any meaning outside of the magnetic models. Thus if the models are in error then the parameters can disguise the error in such a way that although agreement between theory and experiment is maintained the parameter values have little or no significance. We shall discuss briefly the parameter values obtained their origins and their relationships (real and imagined) to general chemical concepts before considering theoretical extensions to the model. The axial ligand-field component was assumed to lift the orbital degeneracy of the triplet ground-states giving a singlet and a doublet the magnitude of the splitting being defined as A (positive when the singlet is lower).This representa-tion of the effect of the axial distortion is useful insofar as it does not require any particular type of metal-ligand interaction or any precise form of the geometric distortion although the sign and magnitude of A must be strongly related to this latter factor. In general the values of A obtained were found to become more ambiguous as the spin multiplicity of the ground term increased ; thus for axially distorted octahedral cobalt(I1) complexes with formal “Tlg cubic-field ground-terms equally acceptable fits were obtained for A values differing both in sign and magnitude so that correlations between these values and the sense of the B.N. Figgis Trans. Faraday SOC. 1961 57 198 204. l 4 B. N. Figgis J. Lewis F. E. Mabbs and G. A. Webb J . Chem. SOC. ( A ) 1967 442. l 5 B. N. Figgis J . Lewis F. E. Mabbs and G. A. Webb J . Chem. SOC. ( A ) 1966 141 1 . l 6 B. N. Figgis M. Gerloch J. Lewis F. E. Mabbs and G. A. Webb J . Chem. SOC. ( A ) , 1968 2086 The Magnetic Properties of Transition-metal Ions 63 distortion were not possible. On the other hand for the ,T2 ground terms of ‘tetrahedral’ copper(@ complexes unique values of A were obtained which correlated well with other data. Values of the spin-orbit coupling coefficients obtained from these models generally showed the anticipated reduction from the ones for free ions although again unique values were not often found; for Cs,CuCl, equally acceptable fits were found with A in the range -850 to - 550 cm- Reductions in the spin-orbit coupling coefficients have invariably been ascribed to covalency effects although following the early work of Owen,17 there has been considerable discussion about the precise cause of the reduction.The orbital reduction parameter or k factor has been and to some extent still is the most confused of the parameters in the Figgis model. This parameter was introduced by Stevens’ to account for the low e.s.r. g-value of the IrC162 - ion by introducing a mechanism for quenching orbital angular momentum additional to that provided by the rigorously cubic crystal-field. Stevens showed that in the formation of molecular orbitals the unpaired t, metal electrons involved in n-bonding could be considered as residing partly on the ligand atoms thus leading to a reduction in the orbital angular momentum of these electrons.This covalency was incorporated in an ad hoc fashion into the crystal-field formalism by replacing the orbital angular momentum operator L by kL ; accordingly values of k were assumed to lie in the range 0 < k < 1 the upper limit corresponding to no covalency. Although it appeared that the k values obtained from magnetic measurements could lead to some insight into the nature of the metal-ligand bond thus achieving one of the original aims of magnetochemistry it was soon found that any relation-ship between k and ideas about covalency was an obscure one.As pointed out by Figgis,I3 the observed k values did not correlate in any obvious manner with the n-bonding tendencies of the various ligands so that any hopes of interpreting k in terms of molecular orbital coefficients were not fulfilled. Despite these comments concerning the interpretation of the parameter values the Figgis approach has allowed virtually perfect agreement between theory and experiment both for the many compounds studied by Figgis and Lewis and for the five-co-ordinate compounds whose magnetic properties were examined by Wood.” The aims and achievements of this approach have been discussed. We have seen how ambiguities arose concerning the sign and magnitude of A, so that it was not possible to relate this parameter to molecular geometry.No better success was found for the parameters A and k which were considered essentially as chemical parameters reflecting (indirectly) the metal-ligand interaction. Thus for the [Fe(H20)6]’ + ions in FeSiF ,6H20 (with a rigorously trigonal axial distortion) and (NH,),Fe(SO,) ,6H,O (with an essentially l 7 J. Owen Proc. Roy. SOC. 1955 A227 183. l 9 K. W. H. Stevens Proc. Roy. SOC. 1953 A219 542. 2 o J. S. Wood J . Chem. SOC. ( A ) 1969 1582. J. Owen and J. H. M. Thornley Reports Progr. Phys. 1966 29 676 64 R. C. Slade tetragonal axial distortion) it was anticipated that the chemical parameters would have similar values so that the different magnetic moments 5.5 and 5-2 pB, respectively would be due to different values of A.However the k values were found to be ca. 1.0 for the fluorosilicate and ca. 0.7 for the Tutton salt. In terms of the then current interpretation of k as a measure oft, electron delocalization, this meant no delocalization in the former complex but some 30% in the latter! This difference in k values could obviously represent some undefined error in the theory or it could arise from some genuine difference between the two complexes which affected either directly or indirectly the magnitude of k . To clarify problems such as this the theoretical nature of the orbital reduction parameter has been re-investigated. Salzmann and Schmidtke,’ have examined the k values of an extensive series of complexes with pseudohalide ligands for metal ions having A E and T ground terms.For the complexes with either A or E ground terms k values ranging from 0.25 for [Cr(CN),I3- to 1.55 for FeBr,,- were obtained; values in excess of 1.0 were similarly found for all the tetrahedral iron(I1) complexes and for many of the octahedral chromium(rI1) ones. For the complexes with T ground terms, k was always less than 1.0. The orbital reduction was linked with the molecular orbital LCAO coefficients and normalization constants thus following Stevens, except that allowance was made for the effect of the a-bonding as well as the n-bonding interactions. On this basis the likely magnitude of the parameter k,, (including both a- and n-interactions) could only be established by the question-able procedure of neglecting the n-interaction completely. This being the case, it was found that k, could exceed 1.0 provided that the a-overlap integrals were larger than 0.5.Disregarding the problem as to whether or not such integrals could reasonably be expected there is an odd inconsistency in neglecting the n-bond interaction in a theoretical treatment to account for the k values observed in complexes that involve ligands with n-bonding capabilities. A lack of correla-tion between the observed k values and the Racah parameter B and spin-orbit coupling coefficients was discussed in terms of the radial part of the wavefunctions determining B and 3 and the angular part determining k . A more wide-ranging appraisal of the nature of the orbital reduction parameter was undertaken by Gerloch and Miller22 in an examination of the behaviour of k in isotropic octahedral and tetrahedral complexes.Inclusion of ligand-ligand overlaps in addition to the previously considered metal-ligand interactions,’”Y2 ‘ suggested quite narrow limits for the values of k in octahedral symmetry; for reasonable values of the LCAO molecular orbital mixing coefficients and the several overlap integrals the most likely values of k were shown to be 0.7 d k d 1.0. This lower limit arises because the ligand-ligand overlaps allow t, electrons on the ligand atoms to contribute to the total orbital angular momentum to an extent dependent upon the magnitude of the overlap integrals. In the tetrahedron, the lack of a centre of symmetry allows the metal 3d wavefunctions to mix with ’’ J. J. Salzmann and H. H. Schmidtke Inorg.Chim. Acta 1969 3 207 2 2 M. Gerloch and J. R. Miller Progr. Inorg. Chem. 1968 10 1 The Magnetic Properties of Transition-metal Ions 65 the 4p ones via the crystal field. This mixing is reinforced by the mixing between both the 3d and 4p orbitals and the ligand orbitals and since the orbital angular momentum of p-orbitals opposes that of d-orbitals this combination of crystal-and ligand-field effects was found to be capable of causing a considerable reduc-tion in the total orbital momentum. It was concluded that k values would be expected to be lower in tetrahedral complexes than in octahedral ones and, further that no simple correlation between k and chemical concepts such as ligand n-donor or -acceptor properties or reduction in A should be expected. Although these authors had concentrated upon metal ions in isotropic environ-ments they pointed out that any anisotropy in molecular geometry might well lead to anisotropy in k.A further complication not considered in the two previous studies is the Ham effect.23 The ccupling of the vibrational and electronic energy levels in the dynamic Jahn-Teller effect was shown to lead to a reduction in orbital angular momentum thus simulating the effects of covalent bonding. This effect has been largely ignored especially as proof of its existence in any particular system is not easy to find. Perhaps such proof could be furnished by the finding of a temperature dependence of the k values as the offending vibrations are trapped These studies have emphasized the theoretical nature of the orbital reduction parameter and although our present state of knowledge does not allow us to correlate k values with chemical concepts as was earlier hoped, at least we have a greater understanding of the various contributing effects; and more importantly we do not require too much of the parameter in relation to semi-empirical molecular orbital schemes.The above comments could perhaps be construed as lending some support for the Figgis models since the criticisms of nearly identical k values for widely different ligands are hereby removed. However criticisms of these models more specific and more serious than vague ambiguities in parameter values were found when systematic investigation of the magnetic anisotropies of single crystals of metal complexes was begun.The failure of magnetochemists to make use of single-crystal studies is sur-prising. There have existed for many years simple and reliable methods both for the measurement of single-crystal magnetic anisotropies and for the subse-quent interpretation of crystal data in terms of the molecular magnetic proper-Also it has long been recognized that the detailed interpretation of the e.s.r. and electronic spectra of metal complexes requires the use of single-crystal data. The experimental procedure employed for anisotropy measurements is an extremely sensitive one and capable of detecting very small anisotropies2’ but dificulties occur both in the determination of the crystal susceptibilities and 2 3 F. S. Ham Phys. Ret 1965 138 1727. 2 4 S. F. A.Kettle personal communication. 2 s 2 6 *’ K. S. Krishnan N. C. Chakravarty and S. Banerjee Phil. Truns. 1933 A232 99; K. S. Krishnan and S. Ranerjee ibid. 1934 A234 265. K. S. Krishnan and K. Lonsdale Prac. Roy. Soc. 1936 A156 597. M. Gerloch J. Lewis and R. C. Slade J . Chern. Soc. ( A ) 1969 1422 66 R. C. Slade in the subsequent tensor transformation of these into the molecular suscepti-bilities for certain classes of crystal. These problems have been examined in detail for the case of monoclinic crystals by Gerloch and Quested in a study of the molecular magnetic ellipsoid in the ammonium cobalt Tutton salt.28 The first attempt to interpret magnetic anisotropies using the Figgis theory involved fitting the principal magnetic moments of the CUC~,~- ion in the complex Cs2CuC1 to the 2T2 model.The mean magnetic moments calculated by Figgis for the 2T2 term perturbed by an axial crystal-field distortion and spin-orbit coupling were fitted to the experimental data yielding parameter values u ca. - 6.6 A ca. - 800 cm- ' and k ca. 1.0. The value of A thus obtained + 5000 cm- ' was in excellent agreement regarding both its sign and magnitude, with the geometry2' and with the single-crystal polarized spectra,30 and hence strongly supported the validity of the theoretical model. Using this model and the parameter values obtained from the powder data the principal magnetic moments were calculated to be pll = 169 pB and pL = 2.08pB at 300K but experimentally3 p ll = 2-18 pB and pI = 1.79 pB (where and I refer to directions parallel and perpendicular to the axis of distortion).No combination of param-eter values could reproduce this sign of the anisotropy within the 2T2 model, so the agreement between the powder magnetic data the spectrum and the geometry for this ion must be fortuitous. It was found that the sign of the anisotropy could be accounted for if contribu-tions from the excited states (the 2 A and 2 B components of the 2 E cubic-field term) were included these states being mixed into the components of the ground T2 term by the spin-orbit coupling perturbation. Accordingly detailed calcu-lations of the principal and mean magnetic moments were performed using the 2D term as a basis perturbed by cubic and axial crystal-fields and spin-orbit coupling,32 and the values obtained were compared with those of the model restricted to the ground term.It was found that inclusion of the excited states markedly affected the anisotropies (reproducing the experimental sign) but not the average moments. This lack of sensitivity of the average powder moments means that such measurements could not distinguish between the 2T2 and 2D models whereas anisotropies could do so with ease. This revelation has strongly influenced subsequent studies in this area of magnetochemistry to the extent that the contributions of excited states have been incorporated into the theoretical models and these models tested using single-crystal anisotropy data. The 2D model was subsequently used to interpret the principal magnetic moments of a series of copper(r1) complexes with geometries ranging from nearly tetrahedral to square-~lanar.~' The signs of the anisotropies were found to require the inclusion of the 2E cubic-field excited state for all the complexes and the magnitudes of the anisotropies required anisotropy in k.The mean moments 2 8 M. Gerloch and P. N. Quested J . Chem. SOC. ( A ) 1971 2307. 2 9 B. Morosin and E. C. Lingafelter J . Phys. Chem. 1961,65 50. 3 0 J. Ferguson J . Chem. Phys. 1964 40 3406. 3 1 B. N. Figgis M. Gerloch J. Lewis and R. C. Slade J . Chem. SOC. ( A ) 1968 2028. 32 M. Gerloch J . Chem. Sac. ( A ) 1968 2023 The Magnetic Properties of Transition-metal Ions 67 were found to be geometry-dependent because of a corresponding dependence of k rather than by any direct correlation between the moment and u (or A).The inclusion of excited states in the magnetic models was extended to tetra-hedral nickel(i1) complexes previously fitted using the 3T model. The wave functions of the 3F and 3P free-ion terms formed the basis set perturbed by a tetrahedral crystal-field a tetragonal angular distortion and anisotropic spin-orbit coupling.33 This theoretical model is considerably more complicated than the 2D one because the excited states contribute via spin-orbit coupling and the low-symmetry field component. Also since the complete parametrization of the ground- and excited-state splittings and their mixings would make the subsequent magnetic description cumbersome recourse was made to the point-charge crystal-field theory as a means of relating the energies of the various levels to one another-and to geometry.The crystal-field energy levels were described by the Racah parameter B and by Dq Cp and 8 where Cp is a second-order crystal-field radial integral Dq the usual fourth-order radial integral and 8 an effective distortion angle ; the subsequent magnetic behaviour was further described by the parameters A and k. The general behaviour of the 3F-3P model was examined for many values of the various parameters and it was found that this model predicted magnetic moments up to 0 . 5 ~ ~ higher than those of the TI model emphasizing the contribution from the excited states. The theoretical model was applied to the magnetic behaviour of tetraethylammonium tetra-chloronickelate(r1) and bis-(N-isopropylsalicylaldiminato)nickel(Ir) measured over the temperature range 9&300 K and parameter values were determined in conjunction with data from electronic A discontinuity in the anisotropy of the chloro-complex was observed at ca.218 K and the magnetic data above and below this temperature were independently fitted. The relationship between geometry and magnetic behaviour was reflected by the values of 8 the effective angular distortion. In the high-temperature form of the chloro-complex the value of this parameter given by the best fit was 53-58 & 0.25" i.e. the angle subtended by the S axis was 106-75 i- 0.5" compared with the X-ray crystallo-graphic angle of 106.83 t- 0.3".35 For the isopropyl complex the small non-axial crystal-field component prevents a direct comparison of 8 found to be 51", with the crystallographic angle.The values of the chemical parameters il and k for the high-temperature form of the chloro-complex and for the isopropyl complex were found to be consistent with intuitive ideas of the relative degrees of covalency in the two complexes. An interesting comparison of the high- and low-temperature forms of the chloro-complex shows a small (ca. 3 %) reduction in k but a large (ca. 25 %) reduction in A ; these values were discussed and it was tentatively suggested that they could provide evidence for the occurrence of the Ham effect in the low-temperature form which from the 8 value obtained is almost a perfect tetrahedron. 3 3 M . Gerloch and R . C. Slade J . Chem. SOC. ( A ) 1969 1012. 34 M. Gerloch and R. C.Slade J . Chem. SOC. ( A ) 1969 1022. 3 5 C. D. Stucky J . B. Folkers and T. J. Kistenmacher Acta Cryst. 1967 23 1064 68 R. C. Slade The F-3 P magnetic calculations for tetrahedral nickel(I1) complexes have served as a model for more recent magnetochemical studies in which experi-mental magnetic moments often complemented by data from e.s.r. and electronic spectra have been interpreted using the crystal-field energy levels perturbed by spin-orbit coupling and the magnetic field. As a result of the use of such models the emphasis has shifted towards an understanding of the crystal-field radial integrals and the relationships between Cp and Dq as functions of geometry, co-ordination number and ligand type. However we must remain wary of trying to extract too much information from too little data.In this connection we note an investigation of the spectral and magnetic properties of complexes containing the TiF63- The 2D model appropriate to the d' electron configuration was used to obtain values for the k A and A ('T, splitting) para-meters and a possible correlation between these values and the extent of the distortion as a function of cation was examined. It was concluded that any such correlation would be tenuous in view of the lack of structural data. The inclusion of excited states was extended to octahedral iron(1r) complexes with 5Gg ground terms subject to trigonal or tetragonal low-symmetry crystal-field component^.^^ In particular it was hoped that the problem of the magnitudes of the mean moments of (NH,),Fe(SO,) ,6H20 and FeSiF6 ,6H20 might be resolved.The main part of the paper dealt with the 5D term perturbed by a trigonal crystal-field and spin-orbit coupling and the interpretation of the magnetic behaviour of the fluorosilicate and corresponding fluorogermanate within this framework. It was found that the magnetic moments were strongly affected by the mixing of the higher-lying E term with the E component of the ground term and that the degree of this mixing was dependent upon the magnitude of Cp the second-order crystal-field radial integral. When Cp > Dq the mean moments but more especially the anisotropies differ from those calculated using the restricted basis as might be anticipated ; more important however, is the variation of the moments with angular distortion at high Cp values.Thus, the anisotropy could change by 2.5 pB over the temperature range 8&300 K for a 1" trigonal angular compression of the octahedron. The low moments of the two trigonal molecules were thus assumed to derive from the sensitivity to small angular distortions of the ground-state splitting so that slight distortions could strongly quench the orbital angular momentum. In this way undue im-portance would not be ascribed to covalency to account for this effect. Similar but even more complex behaviour was found for cobalt(1r) complexes whose magnetic properties were described by the ,F-,P wavefunctions perturbed by trigonally3 * or t e t r a g ~ n a l l y ~ ~ distorted octahedral crystal-fields. For trigonal molecules the theoretical behaviour of the moments varied markedly with the magnitude of Cp and for the complexes hexakis(imidazole)cobalt(II) dinitrate and hexa-aquacobalt(r1) fluorosilicate and fluorogermanate the fitting procedures 3 h P.J . Nassiff T. W. Couch W. E. Hatfield and J . F. Villa Inorg. Chem. 1971 10 368. 3 7 M. Gerloch J. Lewis G. G. Phillips and P. N. Quested J . Chem. SOC. ( A ) 1970 1941. '' M. Gerloch and P. N . Quested J . Chem. SOC. ( A ) 1971 3729. 3 9 M. Gerloch P. N. Quested and R. C. Slade J . Chem. SOC. ( A ) 1971 3741 The Magnetic Properties of Transition-metal Ions 69 indicate that Cp is either < ca. 1000 cm- or > ca. 8000 cm- (compare values of Dq of ca. lo00 cm- l ) although unique parameter values were not obtained. The tetragonally distorted octahedral molecules were parametrized by the crystal-field integrals Dq Dt (fourth-order) and Ds (second-order) and the magnetic properties that were calculated within this theoretical model were used to interpret the behaviour of dichlorotetrakis(thiourea)cobalt(II) and dichloro-tetra-aquacobalt(1r) tetrahydrate.Again unique parameter values were not obtained. The effect of distortion in the tetragonal case is reflected by the high moment of a-dichlorobis(pyridine)cobalt(Ir) arising from the negative values of Ds and Dt which in turn could be associated with the stronger crystal-field lying along the molecular tetragonal axis rather than perpendicular to it.40 Where the low-symmetry crystal-field component and spin-orbit coupling act only to second-order on A ground terms the spin degeneracy of these terms is lifted giving a zero-field ~plitting.~' Although the effect of this splitting has long been recognized as playing an essential role in the e.s.r.spectra of transition-metal ions and also organic molecules in triplet states,42 it has been scarcely considered in descriptions of magnetic moments in relation to geometry. The occurrence of a zero-field splitting in high-spin iron(r1r) complexes leads to an observable magnetic a n i ~ o t r o p y ~ ~ which has been used to obtain values of the splitting in Fe(acetylacetone) and K,Fe(oxalate) ,3H20. A detailed investigation of the dependence of the zero-field splitting upon the crystal-field parameters Dq Ds, and Dt for the dichlorotetrakis(thiourea)nickel(Ir) complex of C4" symmetry, coupled with the subsequent magnetic behaviour has recently been reported.43 Parameter values obtained from the observed single-crystal electronic spectra (at room and liquid-helium temperatures) and magnetism (80-300 K) were consistent with one another within the crystal-field framework.Again the relationship between the crystal-field parameters was examined. These latest papers on the magnetic properties of cobalt(u) and nickel(r1) complexes illustrate fully the current direction of magnetochemical research discussed in this section. We have seen how crystal magnetic anisotropies revealed the need to include excited-state contributions and how over-para-metrization in these complex theoretical models was avoided by using the point-charge crystal-field model.In principle other empirical or semi-empirical bonding schemes could be used in this way but the crystal-field model was chosen because of its simplicity and proven utility. In many cases the magnetic models were found to be incapable of yielding unique parameter values and data from other techniques especially electronic spectra were simultaneously employed to limit the extent of these ambiguities and to determine the most probable values or ranges of values for the crystal-field parameters. In this connection we note a continuing series of investigations into the magnetic 40 R. B. Bentley M. Gerloch J. Lewis and P. N. Quested J . Chrrn. SOC. (A) 1971 3751. " 4 2 A. Carrington and A. D. McLachlan 'Introduction to Magnetic Resonance,' Harper 43 B . N .Figgis Trans. Faraday Soc. 1960 56 1553. and Row New York 1967. M. Gerloch J. Lewis and W. R. Smail J . Chem. SOC. (A) 1971 2434 70 R. C. Slade behaviour of lanthanide complexes using single-crystal data where again crystal-field parameters describing the f-electron splittings are d i s c ~ s s e d . ~ ~ . ~ ~ As a result of these studies the magnetic behaviour of transition-metal complexes is better understood now than only three years ago. We therefore have an appreciation of what magnetic data can and cannot achieve in discussions of structure and bonding ; what is clear is that the measurement of magnetic moments cannot be used with any confidence to diagnose unknown molecular geometries. 2 Spin-Spin Interaction The theories described in the previous section can be applied only to systems in which interactions between the paramagnetic ions are negligible.The magnetic properties of these 'magnetically dilute' compounds are then determined by the spin and orbital angular momenta of the individual ions. On the other hand, there exists an extensive series of metal complexes in which such interactions are present of suficient magnitude that they dominate the magnetic properties. This class of compounds can be further subdivided into systems in which the interaction occurs within well-defined polynuclear clusters in the crystal and those in which the interaction is a property of the whole crystal lattice. We shall not be discussing systems in this latter category because they are as yet mainly the concern of solid-state physicists rather than chemists.The magnetic properties of the polynuclear clusters may be i n t e r ~ r e t e d ~ ~ by an exchange term - 2CJijSi.Sj formally representing the spin-spin coupling between the magnetic ions the summation being over all interacting pairs. In this formalism the exchange is represented in a phenomenological way by the exchange integral J and although this provides a convenient mathematical procedure for calculating the magnetic suceptibilities the subsequent interpreta-tion of J in terms of chemical concepts is by no means straightforward. To the magnetochemist these systems are a fruitful field of study and the topic of magnetic exchange has received much a t t e n t i ~ n ~ * ~ - ~ * ' ' 1 2 but there is little clear understanding of the nature or pathway of the exchange interactions between the metal ions.Two important papers dealing with some of the general problems in the theoretical treatment of spin-coupled clusters have appeared in this past year. The difficulty of determining the energy levels and hence magnetic susceptibilities, in coupled systems subject to several distinct exchange interactions has been considered by Sage.47 The susceptibilities under these circumstances were deduced by an expansion method rather than by the usual Van Vleck-type perturbation method and provided that the spin-spin interaction is small compared with k'7 only the first few terms in the expansion are required. This so-called high-temperature expansion (because J i j < kT) was evaluated to third order in the coupling and susceptibilities calculated for both two and three 4 4 4 5 46 4 7 M.Gerloch and D. J. Mackey J . Chem. SOC. ( A ) 1970 3030 3040. M. Gerloch and D. J. Mackey J. Chem. Soc. ( A ) 1971 2605 2612 3372. K. Kambe J . Phys. Soc. Japan 1950,5 48. M. L. Sage Inorg. Chem. 1971 10 44 The Magnetic Properties of Transition-metal Ions 71 coupled spin-q atoms although the method appears more general. Although the values of the exchange integrals may not be uniquely determined if there are two distinct interactions possible limits can be found. The relevance of this treatment to clusters containing several coupled atoms additionally with inter-cluster interaction was suggested. In the description of the magnetic behaviour of spin-coupled systems using the exchange term given above the effects arising from orbital angular momentum are admitted by g-values differing from 2.00.The extension of this theory to include the orbital effects in a less arbitrary manner forms the subject of a paper by Lines.48 The theory was developed in some detail for cobalt(I1) cluster com-pounds and was applied to tetranuclear systems although experimental data are lacking. The basis set which consisted of the entire manifold of levels arising from the spin-orbit coupling effect on the 4T ground terms of the individual ions, was coupled via exchange forces in the usual way but with real spins Si and S j rather than the more commonly used fictitious ones. The k factor was employed to represent the reduction in orbital angular momentum in both the spin-orbit coupling and magnetic moment operators.Magnetic moments were calculated as functions of k J (the ferromagnetic intracluster exchange) and J’ (the anti-ferromagnetic intercluster exchange) and the dependence of magnetic behaviour on the separate effects represented by these parameters was investigated. It was concluded that at low temperatures (< 100 K) the magnetic behaviour was particularly sensitive to the magnitude of J‘ arising from the interaction between the tetramers behaving as single spin entities with the four tetramer spins aligned by the ferromagnetic coupling. Also a comparison of this theory with various approximations to it indicated the futility of attempting to describe the temperature variation of the magnetic moments using incomplete models.By far the greatest effort during recent years has been directed to studies of dinuclear antiferromagnetic complexes especially of the copper(r1) ion and this past year has been no exception. Continuing interest in the factors affecting the magnitude of the exchange and hence the magnetic properties in copper(I1) carboxylate dimers is evident. Following the pioneering work of Bleaney and Bowers,49 in which the antiferromagnetism in copper acetate monohydrate was described by a spin-spin interaction leading to a susceptibility that was de-pendent on the singlet-triplet equilibrium several workers have considered the various mechanisms by which such an interaction could occur. Figgis and Martin” suggested a direct Cu-Cu interaction with weak overlap of the d,2 - y 2 orbitals leading to 6-bond formation whereas Ballhausen with For~ter,~’ first suggested a a-bonded model with overlapping d, orbitals and later with Han~en,’~ considered a coupled-chromophore model without any direct metal-4 8 M.E. Lines J . Chem. Phys. 1971,55 2977. 4 9 B. Bleaney and K. D. Bowers Proc. Roy. Soc. 1952 A214,451. 5 0 B. N. Figgis and R. L. Martin J . Chem. Soc. 1956 3837. 5 1 L. S. Forster and C. J. Ballhausen Acta. Chem. Scand. 1962 16 1385. ” A. E. Hansen and C. J. Ballhausen Trans. Faraday Soc. 1965 61 631 72 R. C. SIade metal bonding ; various approximate molecular orbital treatment^^^,^^ have also favoured the &bond model. An alternative to the Bleaney-Bowers model was recently suggested by Jotham and Kettle5 and applied by them to copper acetate monohydrate and some of its homologues.s6i57 The Jotham-Kettle model extends the earlier one by the inclusion of a weak metal-metal &-bond leading to a six- (rather than a four-) times two-electron basis set for the two interacting metal orbitals the two addi-tional functions corresponding to both electrons pairing in one orbital or the other.As a result of this extension an additional term was required in the Hamiltonian to allow for the mixing of the metal orbitals the effect of this term on the eigenvalues being represented by various splitting or covalency para-meters y,. The observed magnetic susceptibilities could be fitted precisely to the model and unique values of J and y (y arising from S-bond formation) were derived.An interesting point to emerge from this model is that the spin-exchange interaction was found to be ferromagnetic in nature thus indicating a direct metal-metal rather than an indirect superexchange mechanism. However, as a result of this metal-metal interaction the arrangement of the energy levels is such that a spin singlet remains as the ground term whatever the sign of J , so that the overall pattern is antiferromagnetic. Both the Bleaney-Bowers and Jotham-Kettle models have been useds8 to analyse the magnetic susceptibilities and e.s.r. spectra of amine adducts of arylcarboxylic acid complexes of copper(II) the spin singlet-triplet separations of which were found to be ca. 300 cm- '. Within experimental error satisfactory fits were found using the Bleaney-Bowers model and consequently no evidence was found to suggest that the covalency parameter y was important in these complexes.However the authors admit that a small but significant fraction of magnetically dilute impurity found to be present in these and similar complexes,59 might well affect the applicability of the Jotham-Kettle model. In discussing the factors affecting the magnitude of the exchange integral no correlation was found between steric factors acid pK values and J for pyridine adducts whereas some correlation was found for aniline adducts. Of these latter complexes, ortho-substituted benzoic acids show strong antiferromagnetism whereas the meta- and para- forms often do not indicate any exchange interaction. Some of the dangers in assigning dinuclear structures to several copper(I1) carboxylate complexes on the basis of magnetic behaviour alone have been stressed.60 This group of compounds generally have magnetic moments only slightly reduced from the monomeric values although the temperature de-5 3 E.A. Boudreaux Inorg. Chem. 1964,3 506. 5 4 M. L. Tonnet S. Yamada and I . G. Ross Trans. Faraday Soc. 1964 60 840. 5 5 R. W. Jotham and S . F. A. Kettle J . Chem. Soc. ( A ) 1969,2816. 5 6 R. W. Jotham and S . F. A. Kettle J . Chem. Soc. ( A ) 1969 2821. S T R. W. Jotham and S. F. A. Kettle Inorg. Chem. 1970,9 1390. 5 8 F. G . Herring B. Landa R. C. Thompson and C. F. Schwerdtfeger J . Chem. Soc. ( A ) , 1971 528. 5 9 J. Lewis F. E. Mabbs L. K. Royston and W. R. Smail J . Chem.Soc. ( A ) 1969,291. 6 o R. C. Komson A. T. McPhail F. E. Mabbs and J. K. Porter J . Chem. Soc. ( A ) 1971, 3448 The Magnetic Properties of Transition-metal Ions 73 pendence can be fitted to the Bleaney-Bowers dinuclear models with singlet-triplet separations (2J) < 160 cm- ' compared with the values of 25G350 cm- ' found in dinuclear clusters with moments of ca. 1 . 4 ~ ~ at room temperature. An example of this group of compounds which was considered to be dinuclear, dipropionatocopper(r1)-(p-toluidine) was subsequently found to be a one-dimensional polymer involving different types of bridging carboxylate groups.6 ' Based on this structure determination it would appear that the magnetic proper-ties should be described by the more relevant one-dimensional Ising model.The determination of the structure of diacetatocopper(1r)-bis(pto1uidine) trihydrate6' also suggested the use of the Ising model and accordingly the magnetic behaviour was fitted to this model. Complete agreement between theory and experiment was not obtained over the entire temperature range 8&300 K but at temperatures below 200 K the data were fitted with a value of J = - 23 cm- The mechanism of the exchange was presumed to be a super-exchange one uia bridging water molecules as the large Cu-Cu separation (4.73 A) probably precludes a direct metal-metal interaction. Although it might be thought that such low J values were more consistent with essentially poly-nuclear structures rather than with dimers the situation becomes more compli-cated when the magnetic behaviours of the aniline and p-toluidine adducts of copper dipropionate and dibutyrate are analysed.These compounds were assumed to have structures similar to Cu(propionate),(p-toluidine) but their magnetic behaviours are best described by the dinuclear Bleaney-Bowers model with J ca. - 100 cm- rather than by the Ising model. Even more relevant is the small antiferromagnetic singlet-triplet separation of 18 cm- found for the proven dinuclear complex Na,Cu[( k)-tartrate],5H,0.63 The coupling mechanism in these carboxylate complexes has been considered as arising essentially from a direct metal-metal interaction or a superexchange coupling involving the bridging ligands. Unfortunately the situation is not always clarified by the experimental data.The direct interaction has been claimed to be insignificant on the basis of a correlation between measured C u - C u separa-tions and exchange integrals ; thus for (Me4N),[Cu(HC0,),(NCS)1 the Cu-Cu distance64 is 2.716 A and J = - 485 cm- ' while the strictly analogous acetate complex has the Cu-CU distance equal to 2.643 A and J = - 305 cm-'. These data were taken as evidence for the superexchange mechanism. On the other hand. in [Cu(dien)(HC0,)]HC02 (dien = diethylenetriamine) and anhydrous Cu(HCO,) (royal-blue form) involving bridging formate groups there is no evidence of spin-spin coupling despite the short Cu-0 bond length^.^^^^^ 6 1 6 2 C. G. Barraclough and C. F. Ng Trans. Furuday Soc. 1964,60,836. 6 3 R. L. Belford R. J. Missavage I. C. Paul N . D.Chasteen W. E. Hatfield and J. F. Villa Chem. Comm. 1971 508. " D. M. L. Goodgame N . J. Hill D. F. Marsham A. C. Skapski M. L. Smart and P. G. H. Troughton Chem. Comm. 1969,629. '' M. J . Bew R . J . Dudley R. J . Fereday B. J. Hathaway and R. C. Slade J . Chem. Soc. ( A ) 1971 1437. 6 6 R . L. Martin and H. Waterman J . Chem. Soc. 1959 1359. D. R. W. Yawney and R. J . Doedens J . Amer. Chem. Soc. 1970,92 6350 74 R. C. Slade Also for the several linear antiferromagnetic copper(r1) compounds whose magnetic behaviour has been analysed using the Ising model the values of J have been found to increase with increasing Cu-Cu separation suggesting a superexchange mechanism. Clearly the assignment of structure on the basis of magnetic behaviour is fraught with complications ; similar magnetic behaviour does not necessarily imply similarity in structure or vice versa The magnetic susceptibility of (4-nitroquinoline N-oxide)copper(rI) chloride has been measured in the temperature range 4.2-37.5 K.67 In view of the ob-served antiferromagnetism in similar substituted heterocyclic N-oxide complexes of copper(I1) halides the distinctly monomeric behaviour of this complex in the higher temperature range 77-299 K was considered unusual.68 This has been variously attributed to the electron-withdrawing nitro-substituent indirectly affecting the bridging oxygen atoms,69 chlorine-atom bridges rather than the oxygen ones,68 or dimeric ferromagnetic interaction^.^' The magnetic data in the lower temperature range could be fitted to the theoretical models for both linear and dimeric interactions although the latter model was more appropriate.A positive J value of 135 cm- ' was obtained but because of the lack of structural data no attempt was made lo interpret this value in terms of the nature of the magnetic interaction. The observation that the antiferromagnetic exchange in Cu(adenine),Cl ,3H,O is of the same order of magnitude as in copper acetate monohydrate has been interpreted as evidence for a superexchange mechanism in view of the relatively long Cu-Cu separation (3.066 The X-ray crystal-structure determination of the adenine complex showed the nucleotide base as bridging two copper atoms, and it was suggested that this arrangement might be of biological significance in that it provides a means of holding pairs of metal ions closely together.The surprising magnitude of the antiferromagnetic interaction in [Cu(pyrazine)-(NO,),] in view of the Cu-Cu separation of 6-7128 has been interpreted as arising from a superexchange mechanism transmitted through the bidentate heterocyclic amhe.' The magnetic data measured over the temperature range 2-9-65 K were fitted to the one-dimensional Ising model giving a value of J = - 6 cm- ' although it was observed that some features of the e.s.r. spectrum were more consistent with a dimeric model. The complex tetrakis-(NN-diethyldithiocarbamato)dicopper(rr) consists of dimers involving sulphur-atom bridges and with a Cu-Cu separation of 3.54 A.73 The magnetic data for this complex have been reported for the temperature range 4.2-56 K and interpreted using a modified Langevin equation derived for 6 7 J .A. Barnes W. C. Barnes and W. E. Hatfield Inorg. Chim. Acta 1971 5 276. 6 8 R. Whyman D. B. Copley and W. E. Hatfield J . Amer. Chem. SOC. 1967 89 3135. 6 9 Y. Muto and H. B. Jonassen Bull. Chem. Soc. Japan 1966,39 5 8 . 'O E. Sinn Inorg. Nuclear Chem. Letters 1969 5 193. ' l P. de Meester D. M. L. Goodgame K. A. Price and A. C. Skapski Nature 1971, 229 191. '* J. F. Villa and W. E. Hatfield J . Amer. Chem. SOC. 1971 93 4081. 7 3 M. Bonamico G. Dessy A. Mugnoli A. Vaciago and L. Zambonelli Acta Crysl., 1965 19 886 The Magnetic Properties of Transition-metal Ions 75 two coupled spins.74 The value of 2J was found to be +25 cm-' indicating a spin triplet ground-state and ferromagnetic exchange rather than the more normal antiferromagnetism.Although such a result has been observed in oxygen-bridged copper(1r) complexe~,~ this is the first reported case involving sulphur bridges. The mechanism of this exchange was considered in some detail and both dipolar-coupling and superexchange mechanisms were investigated although the former was found to play only a minor role. The superexchange mechanism was assumed to involve a-orbital overlaps between the out-of-plane orbitals of the square-pyramidal copper 'monomer' units and the bridging apical sulphur atom rather than the more usually considered in-plane interactions. Within this description the triplet ground-state arises from a combination of partial electron transfer between overlapping copper dx2-yZ and dZ2 orbitals and sulphur p x and p, orbitals and a coupling between these orthogonal p-orbitals.The ad-vantage of the lower temperature range used in this study compared with the temperatures used in previous studies of this complex76 is that a much greater temperature dependence is observed thus allowing the value of 2J to be fitted with a greater degree of certainty. The magnetic exchange in the Schiff-base complex N-salicylidene-1.-valinato-copper(r1) has been investigated over the temperature range 77-380 K and the data were fitted to the theoretical model for coupled dimers giving a 2J value7' of - 115 cm- '. It was claimed that these data were consistent with a tetranuclear cluster as found by molecular-weight determinations in which the exchange was transmitted through a n-bonding pathway involving out-of-plane orbital interactions.However in view of the lack of structural data and the complexities of structure determination from such magnetic data these conclusions should be treated with reserve. Magnetic exchange in chromium(II1) and molybdenum(II1) halide complexes of general formula A3M2X9 (A = univalent cation M = Cr'" or Mo"' and X = C1 or Br) has been studied and the relative contributions from different mechanisms have been assessed.78 Following Earnshaw and Lewis,79 the data were fitted to the susceptibility expression for dinuclear d3 ions and values of the exchange integrals were obtained. Comparison of these and previously reported values indicated that the exchange was some fifty times greater in the molybdenum complexes and that replacing chlorine by bromine reduced the exchange for both metals.These data were correlated with the structural data available for several of the compounds and for the molybdenum series it was suggested that a direct metal-metal interaction occurred in the chloro-complexes although the superexchange mechanism became more important in the analogous bromo-'4 J . F. Villa and W. E. Hatfield Inorg. Chem. 1971 10 2038. '' W. E. Hatfield J. A. Barnes D. Y . Jetter R. Whyman and E. R. Jones jun. J . Amer. " A. K. Gregson and S . Mitra J . Chem. Phys. 1968 49 3696. " G. 0. Carlisle K. K. Ganguli and L. J. Therist Inorg. Nuclear Chem. Letters 1971, Chem. Soc. 1970 92 4982. 7 527. I . E. Grey and P. W. Smith Austral.J . Chem. 1971 24 73. l9 A. Earnshaw and J . Lewis J . Chem. SOC. 1961 396 76 R. C. Slude complexes. A plot of J uersus Mo-Mo separation for the chloro-complexes showed a sharp increase in exchange at small metal -metal separations indicative ofa direct mechanism. These same trends were also observed for the chromium(rI1) complexes and the marked decrease in exchange for this series was rationalized by the greater Cr-Cr separations in the M2Xg2- dimers together with a de-crease in the radial extension of the 3d orbitals compared with that of the 4d orbitals of molybdenum. The magnetic properties of systems such as [Fe(salen)],O and [(phen),-Fe(OH)]Cl arising from strong spin exchange uiu an assumed linear Fe-0-Fe system continue to be studied. The X-ray crystallographic structure and magnetic anisotropy of [Fe(salen)],O,CH,Cl were determined so as to further investigate the factors affecting the magnetic behaviour of these systems." The observed anisotropies were attributed to anisotropy in the g-value rather than in the exchange interaction so that a previous interpretation' of the powder data was confirmed.It was concluded that within the dipolar coupling formalism, the spin-spin interaction should be regarded as being between two spin-free iron(@ atoms in the dinuclear cluster. The spin state of the iron(rr1) atoms and the extent and nature of polymerization in a series of hydroxyl-bridged iron(II1) sulphate complexes containing amines having high molecular weights have been investigated by magnetic studies.' The susceptibilities measured over the temperature range 8&300 K were fitted to various theoretical models and it was found that a trinuclear cluster of interacting $ spins gave the most satisfactory fits.Also the bridging groups were suggested as being hydroxy-groups rather than oxy-groups on the basis of additional chemical and physical evidence. Part V of a continuing series of studies devoted to magnetic exchange in transition-metal complexes is concerned with the structure of Co,(OMe),-(acac),(MeOH) and its nickel(I1) analogue and an interpretation of the magnetic properties of the nickel tetramer.'3 The room-temperature moment of 3.31 pB is in the range observed for octahedral nickel(I1) ions with 3A, ground terms, so that any spin exchange must be weak.Reduction of the temperature to 1.63 K causes a steady increase in the moment increasing to a value of 580pB at the lowest temperature. These data were interpreted as arising from a ferromagnetic coupling of the unpaired eg electrons of the four nickel atoms giving a value for the exchange integral of 7 cm- '. The continuing increase in the moment below 20 K was attributed to a weak ferromagnetic interaction between the tetramer clusters leading to Curie-Weiss behaviour with 8 = +04" in this lower tem-perature region. This behaviour together with that of structurally related clusters was compared with that of the linear trimer Ni3(acac)6,'4 and it was noted that the exchange was larger in the trimer (in which an antiferromagnetic 'O P. Coggon A. T. McPhail F.E. Mabbs and V. N. McLachIan J . Chem. Soc. ( A ) , 1971 1014. J. Lewis F. E. Mabbs and A. Richards J . Chem. SOC. ( A ) 1967 1014. 8 2 R. W. Cattrall K. S. Murray and K. I. Peverill Znorg. Chem. 1971 10 1301. 8 3 J. A. Bertrand A. P. Ginsberg R. I. Kaplan C. E. Kirkwood R. L. Martin and R. C. 8 4 A. P. Ginsberg R. L. Martin and R. C. Sherwood Inorg. Chem. 1968 7 932. Sherwood Inorg. Chem. 1971 10 240 The Magnetic Properties of Transition-metal Ions 77 inter-cluster interaction was found) than in the tetramer cluster compounds as previously discussed by Andrew and Blake.' Finally a superexchange mechan-ism via the bridging methoxy-groups was suggested as being the principal contribution to the exchange in the tetramer. A tetrameric copper(r1) cluster compound [Me,N],[Cu,OCl 0] has been shown to exhibit antiferromagnetic exchange,86 the susceptibility over the temperature range 4.2-295 K being described by the Van Vleck equation for a tetrahedral arrangement of coupled atoms and giving a value of J = 16 cm- '.The magnitude of J was correlated with the number of Cu-C1-Cu bridges by a comparison with the number of bridges and the magnitude of J ( - 8.6 cm- I ) in the dimeric [CU,C~,]~- ion present in the complex [Co(en),],[Cu2C1,]C1,, 2H o . ~ ~ Antiferromagnetic exchange in nickel(I1) complexes with bridging nitrito-groups has been discussed in relation to possible n-bond pathways consistent with the structures of these polymers.88 For bridges of the type Ni-ON(0)-Ni, agreement was obtained between experimental data and theory by fitting the former to the linear-chain model for interacting S = 1 ions and a mechanism was suggested in terms of overlap between the half-filled d x 2 - - y Z orbitals of the metal and either the filled n or empty n* molecular orbital of the nitrito-group.An essentially similar n pathway for the exchange had been suggested previously, although the earlier data had not been fitted to any theoretical model.89 A second type of bridge Ni-O(N0)-Ni found in addition to the previous type in the trimeric cluster [Ni(3-Mepy)2(N0,),],c6H6,90 was considered to be capable of both ferro- and antiferro-magnetic interactions depending upon the relative contributions from c- and n-type overlaps. The net interaction in the trimer however remains antiferromagnetic but with a J value smaller than those of complexes containing the first type of bridge.Again the danger of interpreting magnetic data with insufficient structural information is shown in a report on the properties of NN'-propylenebis(sa1i-cylaldiminato)oxovanadium(~v).~~ It was found that the vanadyl oxygen bridge does not apparently provide a pathway for exchange although a mechanism involving this bridge had been previously suggested for vanadyl acetate and other oxovanadium(1v) c o m p l e x e ~ . ~ ~ - ~ ~ Vanadium(i1) double chlorides e.g. RbVC1, and (Me,N)VCl, have magnetic properties consistent with antiferromagnetic interactions presumably through bridging chloride atoms although the hydrated 8 5 J. E. Andrew and A. B. Blake J . Chew. Soc. ( A ) 1969 1456." J. A. Barnes G. W. Inman jun. and W. E. Hatfield Inorg. Chem. 1971 10 1725. *' J. A. Barnes W. E. Hatfield and D. J. Hodgson Chew. Phys. Letters 1970 7 378. D. M. L. Goodgame M. A. Hitchman and D. F. Marsham J . Chew. Soc. ( A ) 1971, 259. 8 9 B. J . Hathaway and R. C . Slade J . Chem. Soc. ( A ) 1967 952. 90 D. M. L. Goodgame M. A. Hitchman D. F. Marsham P. Phavanantha and D. 9 1 9 2 A. T. Casey and J. R. Thackeray Austral. J . Chew. 1969 22 2549. 9 3 D. R. Dakternicks C. M. Harris P. J. Milham B. S. Morris and E. Sinn Inorg. " A. P. Ginsberg E. Koubek and H. J . Williams Inorg. Chew. 1966,5 1656. Rogers Chew. Comm. 1969 1383. D. M. L. Goodgame and S . V. Waggett Inorg. Chim. Acta 1971 5 155. Nuclear Chern. Letters 1969 5 97 78 R. C. Slade complexes NH4[VC13(H20),] and Cs,[VC14(H20),] have normal monomer proper tie^.^^ The magnetic properties of terdentate Schiff-base complexes of manganese(I1) have been interpretedg6 in terms of an antiferromagnetic exchange arising from a polynuclear structure.The results were fitted to both S = 2 dimeric and infinite-chain models and J values of ca. - 2 cm- were obtained in both cases; thus no conclusions could be reached regarding the most likely structures in view of this ambiguity. 3 Miscellaneous Structural Applications In this section we shall comment briefly upon the literature reports on systems which generally fall outside of the themes developed in Sections 1 and 2. The anomalous magnetic behaviour observed in. five-co-ordinate complexes of iron(II) cobalt(rI) and nickel@) halides and thiocyanates with the terdentate ligands 2,6-di-(2-diphenylphosphinoethyl)pyridine (pnp) and the corresponding methyl ligand (pmp) has been interpreted in terms of a high-spin-low-spin e q ~ i l i b r i u m .~ ~ * ~ * The complexes of pnp were suggested to have distorted trigonal-bipyramidal structures and those of pmp to have distorted square-pyramidal ones. The occurrence of the anomalous behaviour in both structural types was explained by a thermally controlled equilibrium between the high-and low-spin forms and the various factors affecting this equilibrium were discussed. Corresponding equilibria have been observed in some iron(I1) amidine complexes99 and also in a number of NN-dialkyldithiocarbamato-complexes of iron(m)."' The former group has been studied by magnetic measurements over the temperature range 9 3 4 0 3 K and by Mossbauer spectra at 300 and 4.2 K, whereas magnetic data only were reported for the latter group of compounds.Low-symmetry crystal-field components have been seen to affect the magnetic properties of transition-metal ions quite markedly. Further to this earlier dis-cussion a study of cobalt(Ir1) complexes of the type [CoN,X] where X = halide and N4 represents a quadridentate nitrogen-donor ligand has shown"' that the unusual paramagnetic behaviour is entirely dominated by the low-symmetry field component in these molecules which are assumed to be square pyramidal. The temperature dependence of the mean moments of tetraethylammonium tetrachloro- and tetrabromo-nickelate(I1) over the temperature range 4.2-80 K has been reportedlo2 and the moments were found to decrease rapidly from ca.3.5 to ca. l.OpB as the temperature was decreased. Since the relevant theoretical model has not as yet been extended to these temperatures no detailed interpreta-tion was attempted although it was suggested that the larger moment of the " 96 9 7 W. S. J. Kelly G. H. Ford and S . M . Nelson J . Chem. Soc. ( A ) 1971 388. 9 8 W. V. Dahlhoff and S. M. Nelson J . Chem. SOC. ( A ) 1971 2184. 9 9 M. J. Boylan S. M. Nelson and F. A. Deeney J . Chem. SOC. ( A ) 1971 976. M. Gerloch B. M. Higson and E. D. McKenzie Chem. Comm. 1971 1149. 1971 7 721. L. F. Larkworthy K. C. Patel and D. J . Phillips J . Chern. Soc. ( A ) 1971 1347. K. D.Butler K. S. Murray and B. 0. West Austral. J . Chem. 1971 24 2249. l o o E. Kokot and G. A. Ryder Austral. J . Chem. 1971 24 649. l o ' G. W. Inman,jun. W. E. Hatfield and E. R. Jones jun. Inorg. Nuclear Chem. Letters The Magnetic Properties of Transition-metal Ions 79 bromo-complex is compatible with a greater low-symmetry field component than that present in the chloro-complex. Despite our earlier comments regarding the restriction of the Figgis calcula-tions magnetic data are often still interpreted using the relevant ground-term perturbed by an axial field and spin-orbit coupling. Thus the temperature dependence (8@-300 K) of the susceptibilities of complexes of lutidines has been foundlo3 to be consistent with axially distorted octahedral and tetrahedral structures for cobalt(I1) and nickel(I1) ions respectively.A fairly wide range of acceptable parameter values was obtained by fitting the data to the ,T1 and 3T1 magnetic models ; the ground-state splitting were generally found to be positive and the values of A found to be close to those expected for weak-field ligands, consistent with the reported spectral data. The moments of a series of octahedral high-spin iron@) complexes Fe(isoquinoline),X Fe(pyridine),X and Fe(phen-anthroline),X (X = C1 Br I or N3) were used to evaluate the parameters k, A and 2 in the Figgis 'T2 model.lo4 The sign of A indicated an orbital singlet ground-term in all the complexes except Fe(isoquinoline),I and a correlation between k and A(cornplex)/;l(free ion) was found. The angular trigonal distortion of 0-35" found in the hexamminechromium(m) ion in complexes of the type [Cr(NH,),] [Cucl,] and [Cr(NH&] [CdCI,], together with an intramolecular spin-spin interaction was foundlo5 to be sufi-cient to produce a zero-field splitting of the ,Azg ground term.The departure of the experimental susceptibilities (measured at temperatures down to 4.2 K) of the cadmium complex from those calculated allowing for the zero-field splitting was interpreted as arising from an antiferromagnetic exchange interaction as seen from the positive Weiss constant 0 = 1.4K. Information concerning the nature of the 2E2,(al,)2(e2,)3 ground electronic state of the ferricinium ion has been sought from a study of the temperature-dependent susceptibilities of several ferricinium and analogous iron(m) compounds.' O 6 The magnetic behaviour of this ground state calculated as a function of orbital reduction and low-symmetry field was not consistent with the observed data and either a temperature-de-pendent distortion or thermal population of a higher-lying ,A lg(ulg)1(e2g)4 state was suggested to account for the discrepancy. The possibility of a tempera-ture-dependent distortion effect arising from the interactions between various anions and the CuN plane in bis-(NN-diethylethylenediamine)copper(rr) complexes has been suggested' O7 to account for the thermochromic behaviour of these complexes. Various studies of the magnetic properties of lanthanide and actinide ions have been reported. Amongst these is a study of the susceptibility of tervalent ytter-bium in the octahedral complex CS2NaYbC16 measured over the temperature I o 3 D.J . Machin.and J. F . Sullivan J . Chem. SOC. ( A ) 1971 658. ' 0 4 G. J. Long and W. A. Baker jun. J . Chem. SOC. ( A ) 1971 2956. W. E. Estes D. Y. Jeter J . C. Hempel and W. E . Hatfield Inorg. Chem. 1971 10, 207 4. l o 6 D. N. Hendrickson Y. S. Sohn and H. B . Gray Znorg. Chem. 1971 10 1559. l o ' A. B. P. Lever E. Mantovani and J. C. Donini Znorg. Chem. 1971 10 2424 80 R. C. Slade range 2.5-100 K. lo' The octahedral crystal-field energy-levels r6 and r8 arising from the J = %ion were found to be separated by 60 cm- ',with the former as the ground level ; from this result the crystal-field fourth- and sixth-order radial integrals were evaluated and their magnitudes discussed in comparison with similar data from other systems.A comparable study of the susceptibility of the quadrivalent plutonium ion in octahedral environments has been reported lo9 by the same author and the crystal-field components of the 51 ground-state have been deduced. To end this section on a different note we include a reported study of the dia-magnetic anisotropies of metal-free nickel(n) and zinc(I1) phthalocyanines. l l O A theoretical model based on Pauling's assumption' l1 that the pn electrons are responsible for the large anisotropy in aromatic systems was found to be in poor agreement with the experimental data for these systems. Better agreement was obtained using a semi-empirical treatment based upon the estimation of Pascal's constants for the various atoms in both parallel and perpendicular directions.4 Concluding Remarks The development of the parametrized theoretical models to a state of considerable complexity may well be viewed with either indifference or concern by chemists who wish to correlate magnetic and perhaps spectral data with geometry when the latter is unknown; this is especially true since it is apparent that molecular geometry is a prerequisite for magnetochemical investigations and not an end-product. However such complexities are seen to be necessary for the detailed interpretation of single-crystal magnetic anisotropies and for the extraction of reliable parameter values which may be used in correlations of structure bonding, and magnetism. Gross misunderstandings can occur and indeed have occurred, in the interpretation of the parameters obtained by fitting powder susceptibility data to incomplete theoretical models although even at the present time ambigui-ties still arise. The availability of a commercial instrument for studies at liquid-helium temperature based on a vibrating magnetometer design,' '' has opened a new dimension in magnetochemistry which has been exploited in the past year mainly in studies of spin-spin exchange interactions. The awaited extension of such low-temperature studies to other systems may allow some of the present ambiguities to be resolved ; it may also of course lead to a re-thinking of current trends and ideas but whatever the result it will surely provide valuable informa-tion for the continuing investigation of magnetism and chemical bonding. D. G. Karraker J . Chem. Phys. 1971,55 1084. l o 9 D. G. Karraker Inorg. Chem. 1971 10 1564. ' l o C. G. Barraclough R. L. Martin and S. Mitra J . Chem. Phys. 1971 55 1426. I " L. Pauling J . Chem. Phys. 1936,4 673. ''' S. Foner Rev. Sci. Instr. 1969 30 548