4 The Magnetic Properties of Transition-metal Ions By R. C. SLADE Chemistry Department Queen Elizabeth College, Campden Hill Road London W8 7AH 1 Introduction The previous Report' discussed at some length the application of parameterized theoretical models to the magnetic behaviour of transition-metal complexes. The extraction of numerical values for the various parameters and the subsequent interpretation and correlation of these values with structure and bonding was seen to be the primary concern of most magnetochemical investigations. This concern remains the dominant theme of work published during the year covered by this Report. As before the Report is divided into three sections the first section covers the broad area of magnetically dilute compounds in which the magnetic properties are essentially determined by structural aspects and ligand types ; the second section is concerned with spin-spin exchange interactions and their influence on magnetic behaviour; and a third section is devoted to 'cross-over' situations.Among several reviews published during the year two will attract the attention of many magnetochemists. A timely review of the techniques used for measuring single-crystal magnetic anisotropies together with a survey of anisotropy measure-ments has been written by Mitra2 and the late J. s. Griffith discussed3 the general theory of the magnetic susceptibilities of polynuclear complexes as applied to two and three interacting spin systems. 2 Theoretical Models and Magnetic Behaviour The theoretical magnetic behaviour of complexes with a cubic field 2T2 ground term has been discussed in two papers.An explicit expression has been derived4 for the susceptibility of octahedral d' ions allowing for the spin-orbit mixing of the excited ' E term wi?h the ground term and also for the reduction in orbital angular momentum. The model is similar to that used previously' to discuss the behaviour of d9 ions in distorted tetrahedral complexes. It has been shown6 ' ' S. Mitra Transition Metal Chem. 1972 7 183. R. C. Slade Ann. Reports (A) 1971 68 61. J. S. Griffith Structure and Bonding 1972 10 87. A. D. Westland Canad. J. Chem. 1972 50 1468. M. Gerloch J. Chem. SOC. ( A ) 1968 2023. 0. Kahn and S. F. A. Kettle Theor. Chim. Acta 1972 27 187. 10 102 R. C. Slade that the influence of vibronic coupling on the temperature dependence of the susceptibilities of cubic complexes with T2 ground terms is similar to that of covalency.The magnetic properties of the 2q term are considered within a four-parameter model incorporating the spin-orbit coupling coefficient A the covalency parameter k the vibronic coupling coefficient x and h q the frequency of the doubly degenerate modes of vibration. The coupling of these vibrations with the electronic parts of the wavefunctions is treated by the Born-Oppenheimer approximation. The results of this model indicate that increasing values of x lead to less-marked temperature dependence of the magnetic moments and that increasing covalency (decreasing k ) has the same effect. However the reduction in the orbital contribution to the magnetic moment owing to vibronic coupling is dependent on temperature whereas the reduction due to covalency is not so that the effects although similar are not the same.The principal and mean magnetic moments of V(urea),I have been measured' over the temperature range 300-80 K and the data interpreted in terms of the Figgis model8 for the 3T1g ground term of the d2 ion. Values of the three para-meters corresponding to 'best fit' of experimental and calculated moments were 2 = 45 cm-' A = 450cm-' and k = 0.5. The possible affect of including the excited spin triplet terms in the model was discussed and it was suggested that there would be no drastic changes in the parameter values on going to the more complex model presumably because of the high value of A,, in this complex.An X-ray analysis shows the trigonal distortion to be slightly temperature dependent since the small trigonal angular compression which exists at room temperature becomes a small angular extension at 90 K. The magnetic properties of some dithiocarbamate complexes of chromium(rir), manganese(irr) and iron(1ir) have been measured from 300 K to below 4.2 K9 The chromium complex has a moment that is substantially independent of temperature falling from a value of 3.80 pB at room temperature to 3.61 pB at ca. 4 K ; similarly the manganese complex has a temperature-independent moment consistent with that expected for an octahedral d4 ion. On the other hand an iron complex has magnetic properties that cannot be accounted for on the basis of a zero-field-splitting of the ,A, ground state.Previous Mossbauer spectral'* and far-infrared Zeeman-effect' studies have suggested a value of D, the z.f.s. parameter of -3.08 K although this value is not consistent with the magnetic moments at low temperatures. Furthermore positive values of D give no better agreement and neither does the reported crystal-structure determination suggest a pathway for magnetic exchange.' The substituted ligand (Et,dtc), however forms a tris complex with iron(1n) which is low-spin with moments of 2.10 and 1.85 pB at 83 and 4.2 K respectively and which are interpreted in terms of a low-symmetry splitting of the 2T2g ground term of 350 cm-'. ' B. N. Figgis and L. G. B. Wadley J.C.S. Dalton 1972 2182. B. N.Figgis J. Lewis F. E. Mabbs and G. A. Webb J. Chem. SOC. ( A ) 1966 1411. B. N. Figgis and G. E. Toogood J.C.S. Dalton 1972 2177. G. C. Brackett P. L. Richards and W. S. Caughey J. Chem. Phys. 1971,54,4383. l o R. Rickards C. E. Johnson and H. A. 0. Hill J. Chem. Phys. 1968,48 5231. l 2 P. C. Healy and A. H. White J.C.S. Dalton 1972 1163 The Magnetic Properties of Transition-metal Ions 103 Single-crystal anisotropies and powder susceptibilities have been reported for dichlorotetrakis( thiourea)-iron(1r) and -manganese(u) over the temperature range 300-80 K together with powder susceptibilities down to liquid-helium tempera-ture13 and single-crystal polarized spectra for the iron complex. The results were interpreted within the D crystal-field model parameterized by Dq Dt Ds, and A previously used for the corresponding nickel(@ and cobalt(r~)complexes.' 4 9 l5 The parameter values obtained for the iron complex were ambiguous although the sign and magnitude of the anisotropy fixed the relationship that Ds and Dt have the same sign and that Ds - 1.8Dt approximate values being given by lDtl < ca.85 cm- '. The data for the manganese complex were interpreted using the 6S 4G and 4P free-ion states perturbed by a D crystal field and spin-orbit coupling. It was shown that the sign of the z.f.s. parameter is determined by the sign of Dt and that the sign of the magnetic anisotropy reflects the sign of D. Thus pI > plr if D > 0 implying that Dq(equatoria1 ligand) > Dq(axia1 ligand), and vice versa. The very small anisotropies associated with the d5 configuration precludes any accurate determination of the parameter values but for Mn(tu),Cl, it was found that p > pIl and hence Dq(thiourea) > Dq(ch1oride).Long-standing discrepancies in the sign of the molecular magnetic anisotropy of ferrous ammonium sulphate hexahydrate have been resolved by a study16 of the principal crystal anisotropies over the range 300-90 K. The relationships between the molecular and crystal properties allows a determination of the angle between the symmetry axes of the two magnetically inequivalent molecules in the unit cell and the (010) crystal plane. It was found that K > KII gives a closer fit to the X-ray crystallographic angle than the alternative sign of the anisotropy and calculated values of the principal moments agree with the experimental ones.Values of the orbital reduction parameter k can apparently exceed 1.0 in low-spin iron(Ir1) complexes' ' by virtue of the mixing-in of excited t i e configurations into the tl ground configuration. Following Thornley18 and Griffith,lg the mixing is considered to arise by electrostatic interactions and results in an increase in the effective orbital angular momentum of the ground term. The electronic properties of trans-[FeX(NO)(das),J+ and trans-[FeX,(das),J+ where X = C1 or Br and das = o-phenylenebis(dimethy1arsine) have been measured.20 The complexes have a single unpaired electron with moments of 2.31-1.81 pB at room tempera-ture. For trans-[FeBr,(das),] + the room-temperature moment of 2.24 pUB falls to 1.8 p at 4 K whereas for the chloro-complex the moment is 2.31 pB at the higher temperature and 2.12 pB at 77 K.These results are consistent with a spin-paired tgg electronic configuration for the iron(m) complexes and so the experimental M. Gerloch J. Lewis and W. R. Smail J.C.S. Dalton 1972 1559. l 4 M. Gerloch J. Lewis and W. R. Smail J. Chem. SOC. ( A ) 1971 2434. I s M. Gerloch P. N. Quested and R. C. Slade J . Chem. SOC. ( A ) 1971 3741. l 6 A. K. Gregson and S. Mitra Chem. Phys. Letters 1972 13 313. l 7 S. A. Cotton Inorg. Nuclear Chem. Letters 1972 8 371. l 8 J. H. M. Thornley J. Phys. (C) 1968 1 1024. l 9 J. S. Griffith Mof. Phys. 1971 21 135. 'O R. D. Feltham W. Silverthorn H. Wickman and W. Wesolowski Inorg. Chem. 1972, 11 676 104 R. C. Slade data were fitted to the Figgis model for the 2T2 term.21 The low-symmetry splitting for the bromo- and chloro-complexes is 750 and lo00 cm- ' respectively, and I is reduced by 5-10 %.The nitrosyl complexes have moments which cannot be fitted to this model and it was suggested that these complexes have rhombic rather than axial symmetry. The phase transition that occurs in (Et4N)2CoC14 has been studied by low-temperature 20-1.5 K magnetic measurements.22 The room-temperature crystal structure has two magnetically equivalent molecules in the unit cell with the unique molecular axes of the distorted COCI,~- ions lying along the crystal-lographic c axis. The low-temperature studies indicate that below the transition temperature the unique axes lie in or near to the (001) plane and that the tetragonal symmetry of the crystal is maintained.The principal and mean magnetic moments of Cs,CoX and (Et,N),CoX, X = C1 or Br have been measured,23 3&80 K and the results interpreted within the 4F-4P free-ion basis set per-turbed by a D, crystal-field and spin-orbit coupling. The crystal-field energy levels and z.f.s. of the 4A2 ground term are found to be insensitive to the value of Cp the second-order crystal-field radial parameter particularly for 8 < O,, . The best fits of the experimental data to this theoretical model give 8 values of 52-54', compared with the X-ray crystallographic angle of 106.0" for the Cl-Co-C1 angle (equal to 28) in Cs,CoCl,. The data for the tetraethylammonium complexes were independently fitted above and below the transition temperature assuming that the molecules remain axially distorted and slight changes in the effective distortion angle were found.However although 'perfect fits' to values may be found it is not possible to obtain even approximate values for Cp in thesemolecules because of the marked insensitivity of the eigenvalues and eigenfunctions of the 4F-4P manifold to this parameter. Room-temperature moments have been calculatedt4 for several low-spin cobalt(I1) complexes as functions of the energy separations between the ground term usually 2 A lg and various excited terms. Values for these energy separations taken from spectral data together with the spin-orbit coupling coefficient and a constant value of k2 equal to 0.8 allows the experimental moments to be repro-duced.The formation of paramagnetic cobalt(rr1) complexes is reported in two recent papers. An extensive in~estigation~~ of the complexes formed by metal ions and tricyclic quadridentate Schiff-base ligands derived from o-aminobenzaldehyde and various diamines has produced paramagnetic cobalt(rI1) complexes of the type [Co(L)X] where L is a nitrogen-donor ligand and X is C1- Br- or I - . Moments of 2.8-2.7 ,uB were reported and the complexes tentatively assigned a five-co-ordinate square-pyramidal structure. The paramagnetism was rational-ized in terms of a strong tetragonal distortion giving energetically similar dZ2 and 2 1 B. N. Figgis Trans. Faraday Soc. 1961 57 190. 2 2 J. N . McClearney G . E. Shankle R. W. Schwartz and R. L.Carlin J . Chem. Phys., 1972,56 3755. 2 3 M. Gerloch J. Lewis and R. Rickards J.C.S. Dalton 1972 980. 2 4 Y. Nushida and S. Kida Bull. Chem. SOC. Japan 1972 45 461. 2 5 B. M. Higson and E. D. McKenzie J.C.S. Dalton 1972 269 The Magnetic Properties of Transition-metal Ions 105 d, orbitals and hence an electronic configuration (xz)z(yz)2(xy)'(zz)'. A planar cobalt(rr1) complex 3-n-propylbiuretatocobaltate(111) has been reported26 as being paramagnetic with a room-temperature moment of 3.51 pB. It was also found that addition of donor amines to this complex gives octahedral diamagnetic compounds. An interesting paper2' on the magnetic and thermal properties 12&530 K, of bis-N-(3-methoxysalicylidene)isopropylaminenickel(11) has shown that four separate crystalline forms may be isolated.Complexes of the general type bis-(N-R-X-salicylidene)nickel(II) have been extensively studied" and the structures and magnetic properties have been correlated with the nature of the groups R and X. In the present study the two Crystalline forms previously isolated,29 called I11 and IV were found together with two new forms I and I1 ; supercooled liquid and glassy states were also obtained by quenching the molten complex. Phase 111 is obtained as brown crystals by recrystallizing the crude product material from methanol. It is paramagnetic with the nickel ions in tetrahedral environments. The susceptibility of this form decreases smoothly as the tempera-ture is raised until at ca. 440 K a discontinuity is observed as phase I is produced, which is also brown in colour and paramagnetic.On the other hand recrystal-lization from diethyl ether gives a green crystalline solid phase IV which is diamagnetic and with a square-planar structure. Heating this phase to ca. 350 K gives the paramagnetic form phase I. The crystalline phase I1 is obtained by holding the supercooled liquid at ca. 350 K for thirty minutes whereupon green crystals are formed. These are diamagnetic and like phase I crystals they may be obtained stable at room temperature by supercooling. The molten state itself has interesting magnetic properties in that a paramagnetic-to-diamagnetic transition occurs as the melt cools ; at the glass-transition temperature the transi-tion process is stopped and the glassy state corresponds to 26 % paramagnetism.Clearly in this very complex situation the factors determining the structural stabilities are very finely balanced. Magnetic susceptibility measurements have also been applied3' to a study of the paramagnetic-diamagnetic equilibrium in solution. The Quincke method for susceptibility measurements in solution was modified for use in magnetic titrations in which C-substituted ethylenediamines were added to nickel(@ solutions. It was found that increasing C-substitution results in an increase in the tendency to form diamagnetic bis-complexes rather than the paramagnetic tris-complexes formed with ethylenediamine and 1,2-propanediamine. This behaviour was attributed mainly to steric interactions between the more bulky substituents. Single-crystal polarized spectra electron spin resonance spectra and magnetic anisotropies have been measured3 for the eight-co-ordinate complex tetra-(6-aminohexanoic acid)copper(II) diperchlorate.A crystal-field model was used 2 6 J. J . Bour P. T. Beurskens and J. J. Steggarda J.C.S. Chem. Comm. 1972,221. 2 7 N. Arai M. Sorai and S. Seki Bull. Chem. SOC. Japan 1972,45 2398. '* R. H. Holm G. W. Everett jun. and A. Chakarvorty Progr. Znorg. Chem. 1966,7,83. 2 9 A. Takeuchi and S. Yama Bull. Chem. SOC. Japan 1969,42 3046. 30 G. R. Graybill J. W. Wrathall and J. L. Ihrig Znorg. Chem. 1972 11 722. 3 1 C. D. Garner P. Lambert F. E. Mabbs and J. K. Porter J.C.S. Dalton 1972 320 106 R. C. Slade to describe the electronic properties and the experimental data interpreted to establish values of Cp and Dq and the sequence of the one-electron orbital energies.The latter was found to be x y > x 2 - y2 > z2 > x z yz the same as previously r e ~ o r t e d ~ ~ . ~ ~ for CaCu(CH,CO,) ,6H,O and furthermore the crystal-field parameters are of similar magnitudes in the two complexes. It was concluded that the crystal-field model provides a good description of the behaviour of the energy levels in these copper(r1) complexes and that a reported angular overlap does not appear to account for the spectra of both complexes. The magnetic properties of some copper(I1) complexes of benzoxazole have been reported.35 The room-temperature moments are ca. 1.9 pB and the susceptibilities obey the Curie-Weiss law with small Weiss constants. The moment of dichloro-bis(benzoxazole)copper(II) was given as 1.97 pB in contrast to the previously reported value of 1.57 l(B.36 The scarcity of magnetic studies devoted to the 4d or 5d transition elements continues and only two studies are reported.The magnetic properties of some molybdenum(1v) complexes of the type Mo(chelate),X have been measured,37 where (chelate) = acetylacetone 8-hydroxyquinoline or N-substituted salicyl-aldimines and X = halide. The room-temperature moments are ca. 2.7-2.6 pB, similar to those found for other molybdenum(1v) and the tempera-ture-dependence studies on Mo(sa1-N-Et),Cl and Mo(sal-N-C6H4Me-p),C12 were discussed in terms of the 'TIg ground term perturbed by an axial crystal field and spin-orbit coupling. Comparable magnetic behaviour has been reported39 for two tungsten(1v) complexes WCl,(PEtPh,) and WC14(PBun2Ph) which have room-temperature moments of 2.06 and 2.02 pB respectively.Rather more information is available for the 4fand 5fions. The principal and mean moments of the twelve-co-ordinate D, complex Ce,Mg,(NO,) ,24H20 have been measured4' at 300-80 K. Twelve- and six-co-ordinate crystal-field models incorporating the orbital reduction parameter were applied in calcula-tions of the moments and the g-values and the parametric variations of ,u 11 pl gll , and g were examined. Best fits were used to obtain values of the crystal-field parameters p 2 p4 and p6 of ca. 450 100 and 50-100 cm- respectively for k values of 0.96-4.93. The reduction in k was suggested to be most probably due to covalent mixing of the 4forbitals with ligand a-orbitals although the possibility of metalf-p orbital mixing was not discounted.In a continuing series on the magnetic properties of the lanthanide elements the principal and mean moments 32 F. E. Mabbs and W. R. Smail J . Chem. SOC. ( A ) 1970 1716. 3 4 D. W. Smith J . Chem. SOC. ( A ) 1971 1209. 35 3 6 E. J. Duff and M. N. Hughes J . Chem. SOC. ( A ) 1968 2144. 37 A. van den Bergen K. S. Murray and B. 0. West Austral. J . Chem. 1972 25 705. 3 8 B. N. Figgis and J. I ewis Progr. Znorg. Chem. 1964 6 123. 39 A. V. Butcher J. Chatt G. J. Leigh and P. L. Richards J.C.S. Dalton 1972 1046. 40 C. D. Garner P. Lambert and F. E. Mabbs J.C.S. Dalton 1972 91. G . Gliemann and P. Morys Z . phys. Chem. (teipzig) 1970 243 28 1. G.J. Hamilton and E. Kokot Austral. J . Chem. 1972 25 2235 The Magnetic Properties of Transition-metal Ions 107 of several hexakis(antipyrine)metal(III) tri-iodide complexes were reported for the metals praseodymium:' europium,42 dyspro~ium:~ holmium,44 and thulium45 (antipyrine is 2,3-dimethy-1-phenyl-A3-pyrazolin-5-one). In all cases a crystal field of DJd symmetry parameterized by the integrals p2 p4 and p6 and the effective distortion angle 8 was applied to the relevant free-ion states corrected for the effects of intermediate coupling. The ground state together with the lowest-lying excited states are shown to provide a satisfactory restricted basis set for the calculation of magnetic properties and these models were thus used to interpret the experimental data.Values of the crystal-field parameters were obtained from fitting procedures although in some cases only approximate ranges of some parameters were found. The magnetic properties of several compounds of uranium(1u) have been r e p ~ r t e d . ~ ~ ~ ' The room-temperature moments of ca. 3.3-3.0 pB decrease to ca. 2.8-2.6 pB a t liquid-nitrogen temperatures. Adducts of uranium tetra-chloride and tetrabromide with various amines have been prepared and character-ized by spectral and magnetic measurement^.^^ Room-temperature moments of 2.8-2.6 pB were reported together with variable-temperature data for (Et4N)4[U(NCS)8] and K,[U(NCS),]. The non-linear variation of the suscepti-bility with temperature for the tetraethylammonium complex was interpreted as evidence of a strong second-order Zeeman effect the moment being close to the spin-only value and not varying much with temperature.In contrast the potas-sium salt has a lower moment at room temperature and the variation with temperature is greater. These differences were attributed to changes in the second-order paramagnetism arising from small changes in the structure of the anion. The relationships between magnetic properties and structure are nicely illustrated by the prediction of possible site symmetries from magnetic data for some uranium(1v) and neptunium-(111) and -(Iv) complexes with the cyclopenta-diene ion.49 The magnetic susceptibilities 100-2.5 K were correlated with the properties of the crystal-field energy levels in fields of various symmetries. For example the complex U(Cp) has a moment of 2.76 pB up to 25 K whereas above 60 K the susceptibility becomes independent of temperature.This behaviour is consistent with a magnetic ground state lying some 3 0 c m - ' below an excited singlet state and hence with a crystal field of symmetry less than tetrahedral but with a three-fold axis i.e. C, or C . 4 1 M. Gerloch and D. J. Mackey J.C.S. Dalton 1972 410. 4 2 M. Gerloch and D. J. Mackey J.C.S. Dalton 1972 42. 4 3 M. Gerloch and D. J. Mackey J.C.S. Dalton 1972,415. 44 M. Gerloch and D. J. Mackey J.C.S. Dalton 1972 1555. 4 5 M. Gerloch and D. J. Mackey J.C.S. Dalton 1972 37. 46 R. Barnard J. I . Bullock and L. F. Larkworthy J.C.S. Dalton 1972 964. 4 7 R. Barnard J. I. Bullock B. J. Gellatly and L. F. Larkworthy J.C.S.Dalton 1972, 4 8 4 9 1932. P. Gans and J. Marriage J.C.S. Dalton 1972 1738. D. G. Karraker and J. A. Stone Inorg. Chem. 1972 11 1742 108 R. C. Slade 3 Spin-Spm Interactions A great deal of interest continues to be shown in polynuclear metal complexes exhibiting spin-spin exchange interactions. The theoretical models used to in-vestigate these systems lead to values of J the exchange integral and many workers hate attempted empirical correlations between the sign and magnitude of J and molecular structure nature of bridging ligands metal-metal separations, etc. These studies although rationalizing the magnetic behaviour of individual complexes do not suggest general methods of tackling exchange problems in a wide variety of complexes. For general problems the review by Griffith3 is of interest as well as a paper concerned with the interaction of four metal ions with orbitally non-degenerate ground states.” The magnetic properties of complexes containing the Ti2Clg3- anion have been interpreted” using the dipolar coupling model.In addition to the exchange interaction the result of distortion and spin-orbit coupling was considered for the individual 2qf ground terms. The separate effects were parameterized by: J, the exchange interaction between orbitals Q and b of the two atoms; 6 the crystal-field axial distortion of the ground terms; and [ the one-electron spin-orbit coupling coefficient. Numerical values of these parameters were varied and the eigenfunctions and eigenvalues of the 2T2-2& interaction plus Zeeman effect were calculated.The experimental momentss2 of (Et2NH,)Ti2C1 were found to be reproduced by values of [ = !50cm-’ 6 = 500cm-’ and J = - 300 cm- ’. The exchange integral in this case corresponds to the interaction when each electron occupies a tzr orbital of the trigonally quantized d-orbital set. Oxovanadium(1v) carboxylate complexes have susceptibilities which are low in comparison with many other oxovanadium(1v) compounds and which rise to maxima at 250-200 KS3 The Ising anisotropic exchange model was found to give a good description of this behaviour with J values of CQ. - 170 cm-’ and g = 2.1-1.9. These data together with some spectroscopic results were inter-preted in terms of a V=O-*.V=O interaction with additional interaction via bridging carboxylate groups.The oxovanadium(1v) ion has been showns4 to form polynuclear complexes with N-(2-hydroxyphenyl)-2-hydroxynaphthalidene-imines. Several of these complexes are antiferromagnetic with moments less than 1.73 pB and the interaction was suggested to arise by direct a-type overlap of the metal d, orbitals assuming that the configuration about each vanadium atom is a distorted square pyramid. Pure samples of p-0x0-bis[pentammine chromium(rrr)] halides have been prepared by a low-temperature procedure and the magnetic properties ~tudied.’~ 5 0 J. S. Griffith Mol. Phys. 1972 4 8 3 3 . C. G. Barraclough and A. K. Gregson J.C.S. Faraday 11 1972 68 177. 5 2 P. C. Couch G. W. A. Fowles and R. A. Walton J . Chem. SOC. ( A ) 1969,972. 5 3 A. T. Casey B.S. Morris E. Sinn and J. R. Thackeray Austra!. J . Chem. 1972 25, 1195. 5 4 G. 0. Carlisle and D. A. Crutchfield Znorg. Nuclear Chem. Letters 1972 8 443. 5 5 E. Pederson Acra Chem. Scand. 1972 26 3 3 3 The Magnetic Properties of Transition-metal Ions 109 The failure of previous investigator^^^-^* to obtain consistent results for studies of the magnetic properties of basic rhodo-salts of chromium(Ir1) has been attri-buted to preparative difficulties and the probable contamination with basic erythro-salts. For the pure complexes the magnetic behaviour 300-50 K is reproduced by the dipolar coupling model for two interacting high-spin chrom-ium(m) ions to give J values of ca. -450 a-' whereas pure erythro-complexes have J values of -23 cm-' when fitted to the same model.The possibility of explaining the magnetism in terms of the coupling of low-spin chrornium(II1) ions (strong tetragonal distortion acting on the octahedral t, orbitalss9) was investigated but although the room-temperature moments could be reproduced the temperature dependence could not. An ,,X-ray investigation has shown hydroxo-bridges in the complex di-p-hydroxo-tetraglycinatodichromium(II1) and the magnetic susceptibility was measured from 100 to 4.2 K 6 0 The data were described by the Van Vleck expression for exchange coupled high-spin chromium-(111) ions and best fits gave 2J = - 8.4 cm-' with g = 1.95. The complex di-p-oxo-tetrakis-(2,2'-bipyridine)dimanganese(111,~~ perchlorate trihydrate contains manganese in different oxidation states61 bridged by two oxygen atoms.The moment of 1 . 7 9 ~ ~ per manganese at room temperature decreases to 1.33 pB at 80 K and the susceptibility varies with temperahre in a way consistent with a superexchange coupling between the d3 and high-spin d4 ions. Magnetic exchange interactions. between pairs of similar and dissimilar metals have been studied6 in the dinuclear complexes Cu(TSB)MCl where M = manganese(II) iron(m) or COP~~X~II) TSB is a Lridentatezchiff base and n = 2 or 3. The Cu-Mn and Cu-Fe complexes have temperature-dependent moments and J values of - 24 and - 55 cm-' respectively and were obtained by fitting the data to the dipolar coupling model. The Cu-Cu complexes also exhibit pairwise antiferromagnetic interactions and the susceptibilities are described by the Bleaney-Bowers equation with J values of ca.-200 to - 100 cm-'. The magnetic properties of p-0x0-bis[protoporphyrin IX dimethyl ester iron(~~r)] 293-1.5 & were i n t e r ~ r e t e d ~ ~ in terms of a dimeric model with strong antiferromagnetic exchange between the high-spin iron(n1) ions. Using the dipolar coupling model a J value of 380 K was obtained and a comparison with the J values obtained for related compounds suggested an Fe.-O.Fe linear 5 6 H. Kobayashi T. Haseda E. Kanda and M. J. Mori J. Phys. SOC. Japas 1960 15, 1 646. A. Earnshaw and J. Lewis J. Chem. SOC. 1961 396. International Conference on Co-ordination Chemistry' Uppsala 1962 p. 50. 1962 p. 102. P. M. Plaksin R. C. Stoufer M. Mathew and G. J. Palenik J. A m v . Chem. SOC. 19?2, 94 2121.* B. Jezowska-Trzebiatowska and W. Wojciechowski in 'Proceedings of the VIIth 5 9 C. J. Ballhausen 'Introduction to Ligand Field Theory' McGraw-Hill New York, 6 o D. J. Hodgson J. T. Veal and W. E. Hatfield J. Coordination Chem. 1972 2 1 . 6 1 6 2 S. Kokot C. M. Harris and E. Sinn Austral. J. Chem. 1972 25 45. 6 3 T. H. Moss H. R. Eillienthal C. Moleski G. A. Smythe '2. C. McDaniel and W. S. Caughey J.C.S. Chem. Comm. 1972,263 110 R. C. Slade bridging unit in this complex. Halogeno(quinolin-8-olato)iron(111) complexes have moments below the spin-only value at room temperature and which decrease markedly with temperat~re.~~ The susceptibilities were fitted to the dipolar coupling model for two interacting iron(rr1) ions to give negative J values. The interpretation of these data was substantiated by Mossbauer studies and it was suggested that the metal ions were bridged by halogen or oxygen atoms.Magnetic and spectroscopic properties of the 0x0-bridged iron(II1) dimers in complexes of the type (enH,) [(FeHedta),0],6H20 have been rep~rted.~' The susceptibilities were fitted to the isotropic Heisenberg exchange model and, although it was not possible to distinguish between coupled 3 spins or coupled 3 spins with J values of ca. -95 cm-' it was clearly shown that the spectral data were consistent only with the former alternative. The spin-spin coupling model for this system was suggested to be more appropriate for describing the magnetic properties than the alternative three-centre delocalized molecular-orbital model of Dunitz and Orge1.66 Tetrahedral dinuclear cobalt(I1) complexes Co,LX where L = phthalazine or pyrazine ligands were reported67 and characterized by spectral and magnetic measurements.The magnetic behaviours were described by a model based on the antiferromagnetic coupling of the two high-spin cobalt@) ions6* and allowing also for the modification of the d-orbitals by covalency by use of a delocalization coefficient kZ. The experimental data were fitted to this model to give J values of ca. - 1.0 to -5.0 cm-' and k2 values of ca. 1.05 to 0.5. The values of these parameters were correlated with the electronegativities and nephelauxetic effects of the ligands X where X = Cl Br I or NCS. Preliminary results of magnetic susceptibility measurements have been reported6' for a series of di- and tri-nuclear complexes of cobalt(Ir) nickel@) and copper(@ with the ligand 1,7-diphenylheptane-1,3,5,7-tetraone.The cobalt complex was found to be trimeric with antiferromagnetic exchange the nickel complex was also a trimer but with strong ferromagnetic exchange and the copper complex exhibited inter- and intra-molecular exchange interactions. An investigation7' of some M(pyrazine),X complexes were M = cobalt(I1) or nickel@) and X = C1 Br, or I using vibrational spectroscopy and susceptibility measurements showed that the complexes have a sheet structure with bridging pyrazine groups between the metal ions and with trans terminal halogen ligands; previo~sly,~' they had been considered to have bridging halide atoms. The magnetic data of the nickel complexes together with those of Ni(pyridine),Cl and Ni(pyridine),Br were interpreted using the dipolar coupling model for a linear system and the pyridine 64 D.Cunningham M. J. Frazer A. H. Qureshi F. B. Taylor and B. W. Dale J.C.S. 6 5 H. J. Schugar G. R. Rossman C. G. Barraclough and H. B. Gray J. Amer. Chem. 6 6 J. D. Dunitz and L. E. Orgel J . Chem. SOC. 1953 2594. 13* A. Earnshaw and J. Lewis J. Chem. Soc. 1961 396. 6 9 B. Andrelczyk and R. L. Lintvedt J . Amer. Chem. SOC. 1972 94 8634. ' O M. Goldstein F. B. Taylor and W. D. Unsworth J.C.S. Dalton 1972 418. A. B. P. Lever J. Lewis and R. S. Nyholm J . Chem. Soc. 1964 4761. Dalton 1972 1090. SOC. 1972 94 2683. A. B. P. Lever L. K. Thompson and W. M. Reiff Inorg. Chem. 1972 11 104 The Magnetic Properties of Transition-metal Ions 111 complexes gave J values of + 6.6 and + 4.7 cm- respectively in agreement with the expected7 ferromagnetic interactions in systems with halogen-bridge bond angles of close to 90".However the pyrazine complexes were found to be virtually magnetically dilute and the very small J values were independent of the halogen atom. These data were claimed to support a linear Ni-pyrazine-Ni system and an antiferromagnetic exchange was predicted. The failure to observe any such exchange was attributed to some ferromagnetism arising from pyrazine n-orbital overlap with metal eg orbitals or by some unspecified metal-metal interaction. The magnetic susceptibilities of some dimeric nickel(@ ethylenediamine complexes [Ni,(en),X,]Y, where X,Y = C1,Br or X = NCS Y = I have been measured over the temperature range 3-1.5 K and also as a function of magnetic field strength.73 The moments are in the range for octahedrally co-ordinated nickel(@ ions at room temperature but on lowering the temperature the moments rise to ca.3.4 pB in the range 15-25 K and further cooling causes a rapid decrease in their values. This behaviour was interpreted in terms of a ferromagnetic intracluster interaction with exchange integrals of ca. 5-10 cm-together with a much weaker antiferromagnetic interaction between the dimers and/or a zero-field splitting of the ground terms of the individual nickel@) ions. The complex [Ni,(en),(NCS),]I represents the first example of a ferromagnetic-ally coupled nickel(I1) cluster involving polynuclear bridges ; also the magnitudes of the exchange integrals in these complexes are very similar despite large differ-ences in the Ni-Ni separations emphasizing that distance is no barrier to exchange provided that an efficient pathway is available.On the basis of magnetic and spectral data it was suggested74 that solid-state complexes of nickel(I1) with Schiff bases derived from 5-chloro-2-hydroxybenzophenone and ethylene-diamine contain both octahedral and square-planar nickel(1r) in the ratio 1 2, and a trimeric unit was used to account for the magnetic properties. Polynuclear Schiff base complexes have been formed75 by reaction of copper(@ or nickel@) nitrate with monomeric copper(I1) or nickel(I1) Schiff-base complexes. The copper series is dinuclear and shows moderately strong antiferromagnetic interactions with J values of ca.- 200 cm- obtained from the Bleaney-Bowers equation. The trinuclear nickel complexes however are essentially magnetically dilute although there is some slight indication of a weak ferromagneticinteraction. Polynuclear complexes of the copper(n) ion continue to be a rich source for magnetochemical studies and more papers have been devoted to this topic than to any other. Because of this relatively large number of publications some work is given only a cursory mention although readers are warned that longevity and significance are related only in the mind of the author. The sensitivity of exchange interactions to small changes in the geometry of 7 2 J. Kanarnori J.Phys. and Chem. Solids 1959 10 87. 7 3 A. P. Ginsberg R. L. Martin R. W. Brookes and R. C. Sherwood fnorg. Chem. 1972, 7 4 G . M. Mockler G. W. Chaffey E. Sinn and H. Wong Inorg. Chem. 1972 11 1308. '' J . 0. Miners. E. Sinn. R. B. Coles and C. M. Harris J.C.S. Dalton 1972 1149. 11 2884 112 R. C. Slade the bridging units has been shown76 for several antifcrromagnetic dinuclear copper complexes although an extensive e~amination~~ of a large number of copper carboxylate complexes using the singlet-triplet-singlet model suggests that variations in J (singlet-triplet separation) and A (singlet-singlet separation) are rationalized by the nephelauxetic effects of the terminal ligands rather than by tetragonality changes. The singlet-triplet-singlet model has also been applied78 to a series of dinuclear copper complexes with bridging aromatic N-oxide groups.For an extensive series of 1 1 complexes values of J of ca. - loo0 cm-' and A values of infinity were interpreted on the basis of a contribu-tion to J from a mainly superexchange mechanism rather than from an interaction between the d,2-y2 orbitals the b-bond together with little or no a-overlap of the metal d, orbitals. Pathways for the superexchange were discussed in terms of Cu-0 a-bonding with copper d, orbitals overlapping with oxygen a-orbitals in the assumed idealized C, distorted-square dimeric unit. A simple MO model has been applied7' to the exchange interactions in the dinuclear complex with the Schiff base derived from pyrrole-2-carbaldehyde and 3-aminopropanol and other oxygen-bridged dimers.Assuming D, symmetry for tlie four-membered-ring system Cu Cu the MOs transform as a, bzg, 2b,, and 2b3,. The first two are non-bonding and the latter two are mixed in the bonding and antibonding MOs. The energy separation between the antibonding b, and b3g orbitah was related to the exchange integral J and it was shown how the energies of these orbitals and hence the value of J was influenced by the 0-Cu-0 angle via overlap considerations and also by the bonding in the rest of the molecule. The first observation of a spin-triplet ground state for a metal amino-acid complex was reported for tetrakis-(L-tyrosinato)dicopper(Ir).*' The magnetic properties 93-2.5 K were fitted to a modified Langevin equation for interacting dimers and the triplet-singlet separation was found to be 19.1 cm-'.A mechan-ism involving out-of-plane a-orbital overlap was suggested to be compatible with the ferrornagnetism. Small variations in the Cu-0-Cu bond angles were suggested" as being responsible for the differences in the magnetic properties of di-ph ydroxo-bis-2 -(2-ethylaminoethyl)pyrid-nedicopper(11) perchlorate and simi-lar complexes. Two studies of the susceptibilities of [Cu(amine)(OH)],X have been reported for amine = 2,2'-bipyridylE2 and amine = 2,2'-bipyridyl or 1,lO-phenanthr~line.~~ The hydroxy-bridged dimers are weakly ferromagnetic for 0 / \ \ / 0 7 6 E. Sinn and W. T. Robinson J.C.S. Chem. Comm. 1972 359. 7 7 R. W. Jotham S. F. A. Kettle and J. A. Marks J.C.S. Dalton 1972 428. 78 R.W. Jotham S. F. A. Kettle and J. A. Marks J.C.S. Dalton 1972 1133. 7 9 J. A. Bertrand and C. E. Kirkwood Inorg. Chim. Acta 1972 6 248. J. F. Villa and W. E. Hatfield Inorg. Chem. 1972 1 1 1330. D. Y. Jeter D. L. Lewis J. C. Hempel D. J. Hodgson and W. E. Hatfield Inorg. Chcm., 1972 11 1958. J. A. Barnes D. J. Hodgson and W. E. Hatfield Inorg. Chem. 1972 11 144. 83 A. T. Casey Austral. J . Chem. 1972 25 231 1 The Magnetic Properties of Transition-metal Ions 113 X = $SO4 or I whereas for other anions zero or small negative J values were obtained. The exchange interactions in dimeric copper(I1) ad-dichloropropionate and its dioxan adduct have been studied ;84 room-temperature moments of 1.50 and 1.44 pB decrease with decreasing temperature and the susceptibilities obey the Bleaney-Bowers equation with 2J values of ca.- 250 cm-'. Bridging rn-phenyl-ene groups have been to give a very weak exchange interaction 2J = ca. - 1 .O cm - ' in di-~-NN'-m-phenylenetetrakis(salicylideneiminato)dicopper(II), where the copper ions are separated by 8.5A. Nitrogen atoms provide the bridging links in bis-(6-hydroxypurinato)copper(11) and bis-(6-aminopurinato)-copper(I1) complexes.86 These complexes resemble copper(@ acetate mono-hydrate in general stuctural properties and the susceptibilities may be fitted to the Bleaney-Bowers equation to give J values of ca. - 208 to - 156 cm- these values being only slightly lower than that for the acetate despite the appreciably longer copper-copper distance in the purine complexes. The interaction was considered to occur through a superexchange mechanism via the pn orbitals of the heterocyclic bridging ligands.The susceptibilities of four trinuclear copper(1r) complexes have been reported8' at 3 M . 2 K. The moments were compared with those calculated from a model based on a triangular array of copper ions with the central ion interacting with its neighbours on each side but with no interaction between these latter ions. The model is that previously ~ s e d ~ ~ * ~ ~ to interprete the liquid-nitrogen-temperature data for these complexes. This earlier work was confirmed and similar J were reported. A study of the susceptibilities 3 G 1 . 5 K of a group of tetrameric complexes cu4ox&4 where X = C1 L = (C,H,),PO and X = Br L = (C,H,),PO or C,H,N has been rep~rted.~' All complexes have moments of ca.1 . 9 2 ~ ~ at room temperature which rise to 2.3-2.1 pB at 40-65 K and then rapidly de-crease at lower temperatures. In these complexes the copper ions are arranged in a tetrahedron about the central oxygen atom and each copper ion has a trigonal-bipyramidal structure. It is argued that the 'normal' order of the d-orbital energy levels in trigonal-bipyramidal complexes found in say CuC1 -, may be altered significantly by the phosphine oxide or pyridine ligands so that an orbital doublet term becomes the ground state. The magnetic behaviour of four such ions coupled in a tetrameric unit was examined as functions of two exchange interactions the interaction between each pair of copper ions bridged by the central oxygen atom (parameterized by J 2 ) and the interaction through the bridging halogen atoms (parameterized by J1).The measured data were 8 4 8 5 D. Y . Jeter and W. E. Hatfield Znorg. Chim. Acfa 1972 6 440. 8 6 T. Asakawa M. Inoue K. Hara and M. Kubo Bull. Chem. Soc. Japan 1972,453,1054. " B. N. Figgis and D. J. Martin J.C.S. Dalton 1972 21 74. " S. J. Gruber C. M. Harris and E. Sinn J . Znorg. Nuclear Chem. 1968 30 1805. 8 9 S. J. Gruber C. M. Harris and E. Sinn J . Chem. Phys. 1968 49 2183. 90 M. E. Lines A. P. Ginsberg R. L. Martin and R. C. Sherwood J . Chem. Phys. 1972, M. Melnik Acta Chem. Scand. 1972 26 697. 57 I 114 R. C. Slade interpreted within this model to give values of J of ca. 28-45 K indicating a ferromagnetic exchange from bridging halides with near 90" bridge-bond angles and J values of ca.- 40 to - 67 K arising from an antiferromagnetic interaction via the .n-orbitals of the bridging oxygen. The crystal and molecular structure and susceptibility 3&90 K of tetrakis-[(aquo)(N-2-pyridylsalicylaldiminato)copper(11)] tetranitrate have been reported?' Although the molecule forms a tetrameric cluster with copper ions in an almost square arrangement the magnetic data are consistent with the Bleaney-Bowers equation for coupled dimers so that the cluster may be viewed as two independent dinuclear units. Some copper complexes with terdentate Schiff bases derived from substituted salicylaldehydes and S-methyl dithiocarbazate have been prepared.' The 5-nitrosal complex exhibits ferromagnetism with a moment of 2.03 pB at 293 K rising to 2.43 pg at 83 K.This behaviour was considered to be consistent with a cluster of tetrahedrally arrayed copper ions and a J value of +38cm-'. The antiferromagnetic exchange shown by Cu(NH,),CO has been inter-~ r e t e d ' ~ in terms of a direct interaction between the copper ions of neighbouring chains rather than by a superexchange through bridging carbonate groups and a modified Van Vleck equation94 gave a better description of the magnetic behaviour than the anisotropic Ising model for infinite chains the former giving a J value of -4.5cm-'. The failure to observe any appreciable spin-spin interaction in a-bis-(8-hydroxyquinolinato)copper(11) having a chain structure and the corresponding p form with a dimeric structure has been interpreted9' in terms of out-of-plane copper-oxygen distances of greater than 2.8A pre-cluding the transmission of the exchange.The susceptibilities of 2-(2-aminoethyl)-pyridine complexes of copper(I1) chloride or bromide have been measured, 296-2.8 K.96 Both CuLCl and CuLBr exhibit antiferromagnetism whereas the CuL,Br complex is paramagnetic. This behaviour is accounted for by the structures of the 1 1 and 2 1 complexes in that the latter has a five-co-ordinate structure of the [CuL,Br]+ ion with Cu-Cu separations of 11.858 along the chains and 7.41 A between the chains whereas the 1 1 complexes have dimeric structures with Cu-Cu separations of 3.9 A. For these complexes the Van Vleck equation for coupled dimers gives 25 values of - 5.7 and - 3.6 cm-' for the chloro- and bromo-complexes respectively.The complex polybis-[p-(2-picoline N-oxide)-chlorocopper(~~)-di-p-chloroldi-aquocopper(r1) dihydrate Cu,Cl,(C6H,NO) ,2H,O contains alternating linear chains of diamagnetic Cu,C1,(C6H,N0) units and paramagnetic CuCl ,2H,O groups linked by long copper-oxygen Measurements of the suscepti-9 1 9 2 9 3 D. Y. Jeter D . J. Hodgson and W. E. Hatfield Inorg. Chem. 1972 11 185. 94 9 5 G. W. Inman jun. W. E. Hatfield and R. F. Drake Inorg. Chem. 1972 11 2425. 96 D. Y. Jeter W. E. Hatfield and D. J. Hodgson J . Phys. Chem. 1972 76 2707. 97 H. Miyoshi H. Ohya-Nishiguchi and Y. Deguchi Bull. Chem. SOC. Japan 1972 45, J. Drummond and J . S. Wood J.C.S. Dalton 1972 365. M. Akbar Ali S. E. Livingstone and D. J. Phillips J.C.S.Chem. Comm. 1972,909. M. Inoue M. Kishita and M. Kubo Inorg. Chem. 1967 6 900. 682 The Magnetic Properties of' Transition-metal Ions 115 bility of this complex 300-1.6 K have shown that in the diamagnetic units the copper(1I)ions are so strongly coupled that the singlet state is almost totally populated whereas the CuCl ,2H,O units are magnetically dilute and the susceptibility obeys the Curie-Weiss law. This interpretation was supported by electron spin resonance spectral measurements showing a typical anistropic spectrum for the paramagnetic ion and a weak half-field resonance from the dimeric units. The antiferr~magnetism~~ observed by susceptibility measurements, 292-4.2 K for hexamminecobalt(uI) tribromodichlorocuprate(I1) has been suggested as arising from intermolecular exchange via the axial ligands of the trigonal-bipyramidal units.Out-of-plane copper-ligand interactions have been studied99 in dibromobis-(2-methylpyridine)copper(11) and its chloro-analogue. X-Ray structural data show that the molecules consist of weakly bound dimers with the individual copper ions bonded by trans bromines and nitrogen atoms in a basal plane with the bridging bromine atoms in the axial position of the square-pyramidal structure. The susceptibilities of both complexes were fitted to the Van Vleck equation for coupled dimers to give singlet-triplet separations of 5 and 7.4 cm- ' for the bromo- and chloro-complexes respectively. The Ising model for infinite linear chains was found"' to provide an acceptable inter-pretation of the antiferromagnetism of dichloro-(2,3-dimethylquinoxalinato)-copper(I1) and dichloro-(2-methylquinoxalinato)copper(11) with J values of - 45 and - 19 cm-'.Previous that the 2-methyl complex has chloride bridges compared with the quinoxaline bridges in the 2,3-methyl complex were supported by the relative magnitudes of these exchange integrals. Far-infrared spectral and magnetic susceptibility studies were found to be consistent with strongly interacting polymeric chains in complexes of pyrazine with copper(I1) halide~.''~ Both the Ising model for linear chains and the Bleaney-Bowers equation for coupled dimers gave acceptable fits to the measured data, although the infrared spectra were consistent with a polymeric structure. Mag-netic properties have been reported for dichloro(pyridazine)copper(II) and related compounds at 3 0 M .2 K ' O4 and for Cu(pyridine),Cl and Cu(pyridine),-Br at 297-8 K,'05 and the observed antiferromagnetism was discussed using the Ising and Heisenberg models. Finally some copper complexes of N-hydroxy-alkylsalicylideneimines were found to form paramagnetic or antiferromagnetic types106 of general formula Cu(sa1-N-ROH) or alternatively mainly ferro-magnetic complexes of the type Cu(sa1-N-RO). 9 8 D. Y. Jeter and W. E. Hatfield J . Coordination Chem. 1972 2 39. q 9 P. Singh D. Y . Jeter W. E. Hatfield and D. J. Hodgson Inorg. Chem. 1972,11 1656. D. E. Billing A. E. Underhill D. M. Adams and D. M. Morris J. Chem. SOC. ( A ) 1966, 902. l o * M. J. M. Campbell R. Grzeskowiak and F. B.Taylor J . Chem. SOC. ( A ) 1970 19. G. W. Inman jun. and W. E. Hatfield Inorg. Chem. 1972 11 3085. ' 0 4 S. Emori M. Inoue and M. Kubo Bull. Chem. SOC. Japan 1972,45 2259. D. Y . Jeter and W. E. Hatfield J . Inorg. Nuclear Chem. 1972 34 3055. T. Tokii Y . Muto M. Kato K. Imai and H. B. Jonassen J. Inorg. Nuclear Chem., 1972,34 3377. l o o G. W. Inman jun. J. A. Barnes and W. E. Hatfield Inorg. Chem. 1972 11 764 116 R. C. Slade 4 High-spin-Low-spin Equilibrium As a result of the close proximity of alternative ground-state energy levels, differing in spin multiplicity large variations in magnetic behaviour may be observed for small changes in structure or ligand type. Such variations in behaviour usually arise from the thermal equilibria between the ground states and they are often described as resulting from 'crossover' situations.In strongly distorted octahedral iron@) complexes it is possible that a 3 A , ground state is pr~duced.'~' Such a situation has been shown to exist in the complex [Fe(phen),(ox)],5H20 where phen = 1,lO-phenanthroline and ox = oxalate by magnetic susceptibility measurements down to 1.2 K and Mossbauer spectra at 4.2 K.'08 The complex has a room-temperature moment of 4.00 p B and its susceptibility obeys the Curie-Weiss law down to 77 K ; the moment is effectively constant down to 15 K when it then decreases rapidly to a value of 2.68 pB at 1.2 K. This behaviour is interpreted on the basis of a 3 A ground term with a zero-field splitting of 4.6 cm-' with the M = 0 level lowest. Magnetic properties associated with the TZg-'Alg crossover in iron@) have been reported for several complexes.The syntheses and magnetic properties of a series of complexes Fe(N-N-N),X where N-N-N = acr'a"-tri-imine-2,6-(dibenzothiazol-2-yl)pyridine have been described."' For X = C1 Br I or NCS the room-temperature moments are ca. 5.3 pB and they are independent of temperature as illustrated by the iodo-complex. These complexes thus contain high-spin iron(I1). However the perchlorate complex has a room-temperature moment of 4.40 pB falling to 2.23 pB at 83 K and so this complex exhibits 'crossover' behaviour. It is shown that the ligand-field strength of (N-N-N) is less than that of 2,2',2"-terpyridyl since [Fe(terpy),]Br,,H,O is diamagnetic at room temperature. Similar magnetic behaviour has been reported' lo for FeL,X,,nH,O complexes where L is a ring-substituted analogue of 1,lO-phen-anthroline or 2,2'-bipyridyl and X = NCS or NCSe.The magnetic properties of these complexes arise from three effects viz. a phase change leading to a change in susceptibilities over a narrow temperature range a thermal equilibrium between high- and low-spin states and a low-symmetry splitting of the octahedral energy levels leading to moments of intermediate value4 and independent of temperature. A simple thermal equilibrium between the nearly equi-energetic ST, and lAl states accounts for the magnetic behaviour of some iron@) com-plexes of potentially terdentate chelating ligands,' ' ' and the ligand-field strengths of these chelates were found from the spectra of the analogous nickel(1r) complexes to be in the region normally associated with the 'crossover' point.The apparently moderate changes in ligand characteristics required to give spin-paired iron(I1) complexes close to the 'crossover' region are shown1l2 by the preparation of l o ' E. Konig and R. Schnakig to be published. lo' E. Konig and B. Kanellakopulos Chem. Phys. Letters 1972 12 485. Io9 S. E. Livingstone and J. D. Nolan J.C.S. Dalton 1972 218. ' l o A. J. Cunningham J. E. Fergusson H. K. J. Powell E. Sinn and H. Wong J.C.S. ' I 1 H. A. Goodwin and D. W. Mather Austral. J . Chem. 1972 25 715. I l 2 C. M. Harris S. Kokot H. R. H. Patil E. Sinn and H. Wong Austral. J . Chem. 1972, Dalton 1972 2155. 25 1631 The Magnetic Properties of Transition-metal Ions 117 low-spin Fe(bipy) + and high-spin Fe(pq) + complexes where pq is 2-(2'-pyridy1)quinoline.In iron(1rr) complexes the spin crossover occurs with the 6Alg and the ,T2, states and the factors influencing this crossover in iron(rI1) dithiocarbamates have been examined.' The room-temperature moments measured in solution to avoid complicating solid-state interactions were correlated with the pK, values of the secondary amines in a series of complexes of the type Fe(S,CNR,) . The formation of the high-spin state is favoured by a high pK value for HNR, since under these circumstances a carbon-nitrogen double-bond is favoured as in (2). On the other hand when form (2) is sterically disfavoured by a suitable choice of R then form (1) is preferred.These sterically hindered amines give rise to low-spin complexes so that (1) involves a stronger ligand field than (2). How-ever it appears that the steric effect is only secondary since the primary function S /-y /R Fe :C-N \ -7 \ S R (1) S / \ /R Fe C=N \ / \ (2) S R of the substituents R in determining magnetic behaviour is to act as an electron-releasing group. This interpretation contrasts with an earlier one.'I4 The anticipated contraction in the iron-sulphur distances in going from high- to low-spin complexes has been demonstrated' ' by X-ray crystal-structural determin-ations of Fe(CS,N(CH,),) moment = 5.90 pB and Fe(CS,NMePh) mo-ment = 3 . 0 ~ ~ . The thermal equilibrium between the 6A,g and 2T2g states in four tris(monothi0-P-diketonato)iron(m) complexes has been studied' by variable-temperature Mossbauer spectra and susceptibility measurements.In some cases both high- and low-spin forms coexist in varying proportions and give intermediate values for the moments and distinct Mossbauer spectra. A linear relationship between the Mossbauer isomer shifts and the magnetic moments of iron(II1) dithiocarbamates has been claimed to show the increasing importance of back-donation of the d electrons into empty ligand n-orbitals as the low-spin state is approached.' Cobalt(r1) complexes of the type Co(sa1en)B and related molecules where B is an aromatic amine have been prepared'" and their magnetic properties meas-ured in methylene chloride solution at 293-193 K and in the solid state. The room-temperature moments 3.5-1.9 pB and their temperature dependence were interpreted in terms of a thermal equilibrium between the 4A" and 'A' states of the assumed five-co-ordinate square-pyramidal molecules. R. R. Eley R. R. Myers and N. V. Duffy Znorg. Chem. 1972 11 1128. A. H. Ewald R. L. Martin E. Sinn and A. H. White Znorg. Chem. 1969 8 1837. 'I6 M. Cox J. Darken B. W. Fitzsimmons A. W. Smith L. F. Larkworthy and K. A. Rogers J.C.S. Dalton 1972 1192. R. R. Eley N. V. Duffy and D . L. Uhrich J. Inorg. Nuclear Chem. 1972 34 3681. L. G. Marzilli and P. A. Marzilli Znorg. Chem. 1972 11,457. '' P. C. Healy and A. H. White J.C.S. Dalton 1972 1163