Magnetic susceptibility measurements of transition metal containing compounds

Transcription

1 Magnetic susceptibility easureents of transition etal containing copounds Introduction: Measureents of agnetic properties have been used to characterize a wide range of systes fro oxygen, etallic alloys, solid state aterials, and coordination coplexes containing etals. Most organic and ain group eleent copounds have all the electrons paired and these are diaagnetic olecules with very sall agnetic oents. All of the transition etals have at least one oxidation state with an incoplete d subshell. Magnetic easureents, particularly for the first row transition eleents, give inforation about the nuber of unpaired electrons. The nuber of unpaired electrons provides inforation about the oxidation state and electron configuration. The deterination of the agnetic properties of the second and third row transition eleents is ore coplex. The agnetic oent is calculated fro the agnetic susceptibility, since the agnetic oent is not easured directly. There are several ways to express the degree to which a aterial acquires a agnetic oent in a field. The agnetic susceptibility per unit volue is defined by: where I is the intensity of the agnetization induced in the saple by the external agnetic field, H. The extent of the agnetic induction (I) depends on the saple. The induction ay be visualized as an alignent of dipoles and/or by the foration of charge polarization in the saple. H is the strength of the external agnetic field in units of oersteds (Oe). The κ is unitless. Generally, it is ore convenient to use ass units, therefore the ass or gra susceptibility is defined as: I H g d where d is the density of the solid. The olar susceptibility is the ass susceptibility ultiplied by the forula weight. g (F.W.in g ol The ters κ, χg, and χ are all easures of the agnetic oent of a substance in a agnetic field. -1 )

2 Relating susceptibility to unpaired electrons: The relationship between the applied agnetic field and the oents resulting in the diaagnetic and paraagnetic susceptibilities, cobined with the contribution for other effects including Van Vleck paraagnetis, can be described in ters of the effective agnetic oent eff, where k = Boltzann s constant, T = absolute teperature, = Bohr Magneton, N is Avogardo s nuber, and A is the susceptibility per gra of the paraagnetic ion. ' 3k AT ' eff 2.84 AT 2 B.M. N The units are in B.M. (Bohr Magnetons), which is a unit of agnetic oent and equal to eh/4πc = erg/gauss. The a is the atoic susceptibility corrected for the diaagnetic coponents of the ligands and associated ions. The diaagnetic corrections for cations, anions, and individual atos and are given in units of 10-5 /g ato. Table 1 (left) Diaagnetic corrections and (right) Pascal s Constants in 10-5 /g ato or 10-6 erg*g -2 ol -1. Cations Anions Molecules Li F H 2O 13 Na Cl NH 3 16 K Br en 47 Rb I py 49 Cs CH 3CO PPh Tl C 6H 5CO NH CN - 13 Hg CNO - 23 Mg 2+ 5 CNS - 34 Zn ClO 4 34 Pb CO 3 28 Ca C 2O Sr HCO 2 17 Ba NO 3 19 Fe O 2-6 Cu OH - 11 Co S Ni SO 4 38 Cu S 2O Ag acac - 55 Cd Other 13 First Row Transition Metals Pascal s Constants H 2.93 C 6.00 N (ring) 4.61 N (open chain) 5.57 N (ono aide) 1.54 N (diaide iide) 2.11 O (ether alcohol) 4.61 O (ketone aldehyde) O (carboxyl) 3.36 F 6.3 Cl 20.1 Br 30.6 I 44.6 S 15.0 Sr 23 Te 37.3 P 26.3 As(V) 43.0 As(III) 20.9 Sb(III) 74.0 Li 4.2 Na 9.2 K 18.5 Si 20 Pb 46 Sb(IV) 30 Mg 10.0 Ca 15.9 Al 13 Zn 13.5 Hg(II) 33

3 Measureent of agnetic susceptibility: The Guoy ethod, Faraday ethod, and deterination of agnetic susceptibility by nuclear agnetic resonance (NMR) are all experiental techniques to deterine the agnetic susceptibility of transition etal containing coordination copounds. The Evans balance, developed by Professor D.F. Evans of Iperial College London, is a copact and self contained experiental apparatus. The digital readout provides rapid and accurate readings and the sensitivity rivals that of traditional ethods. The instruent also has the advantage that saples as sall as 50 g ay be easured. The Evans balance has the sae basic configuration as found in the Guoy ethod. The saple is suspended between two poles of a agnet. The balance easures the apparent change in the ass of the saple. The saple is repelled or attracted to the agnetic field for diaagnetic and paraagnetic substances, respectively. The Evans balance doesn t easure ass directly or the force the agnet exerts on the saple. It easures the force the stationary saple exerts on the suspended peranent agnets, which is accoplished by easuring the change in current required to keep a set of peranent agnets in balance after the agnetic fields interact with the saple. The two pairs of agnets are located on one end of a balance bea. When the saple is placed between the poles of one pair of agnets, the bea is deflected and changes position. The change is detected by a coil or photodiode on opposite side of the equilibriu position of the balance bea. A current is passed through a coil to exactly cancel the interaction force. The current needed is proportional to the force exerted by the saple and is used to easure the agnetic susceptibility. The current is easured with a digital volteter connected across a precision resistor, in series with the coil, and displayed on the digital display. The general ass agnetic susceptibility, χg, by the use of an Evans balance is: L g C( R Ro) L = saple height in centieters = saple ass in gras C = balance calibration constant (printed on back of balance) R = reading fro the digital display when the saple and tube is in place in the instruent R0 = reading fro the digital display when the epty saple tube is in the instruent v = volue susceptibility of air ( erg G -2 c -3 ) A = cross-sectional area of the saple Calibration of the Evans balance is typically done with Hg[Co(SCN)4] or [Ni(en)3]S2O3, which have values of erg G -2 c -3 and erg G -2 c -3, respectively. The volue susceptibility contribution fro air is relatively sall and can usually be neglected for the easureent of the agnetic susceptibility of solid saples. The ass agnetic susceptibility can be rewritten as: CL( R Ro) /[110 9 ( )] g and has units of erg G -2 g -1. Use this equation to calculate the χg. X ' v A

4 The olar susceptibility, χ, is calculated by: and has c.g.s. units of erg G -2 ol -1. g (F.W.in g ol Diaagnetic corrections to the olar susceptibility are ade to account for the inner core electrons, ligands, atos, and ions in the copound or aterial, which ake the apparent olar susceptibility saller than it really is fro the contribution fro the unpaired electrons. The diaagnetic contributions are added to the value of χ to obtain the χ A, the olar susceptibility of the paraagnetic ato. Select diaagnetic corrections are given in table 1 and ore extensive tables are found in the literature.[1, 2] The diaagnetic correction for ligands or other groups not found in the table can be obtained by suing the Pascal s constants for each ato or type of ato in the group as found in the table 1 or priary literature. su of all diaagneti c correction s A The easured χ A can be related to the effective agnetic oent in Bohr Magnetons by: eff 3 kt N where k is the Boltzann constant, N is Avogadro s nuber, and is the Bohr agneton. Thus, the equation siplifies to: 1/ ( T A 2 eff A ) The easured eff can be copared to the calculated value, s, fro the spin-only forula, where the orbital angular oentu is assued to be quenched by the ligand field. s g S( S 1) where S is the total spin of the paraagnetic center with n unpaired electrons. n s (B.M.) The easured value of eff varies slightly fro one copound or aterial to another, as shown in tables 2 and 3 for transition etals in an octahedral and tetrahedral geoetry, respectively. The nuber of unpaired electrons for other coordination geoetries can be predicted by considering the splitting of the d-orbitals, and crystal field stabilization energy.[3] -1 1 / 2 )

5 Table 2 Measured agnetic oents, d-configuration, and nuber of unpaired electrons for transition etal ions with an octahedral geoetry. Metal Ion d configuration Nuber of Unpaired Electrons Magnetic oent 1 (B.M.) Ti 4+, V 5+ d Ti 3+, V 4+ d V 3+ d Cr 3+, Mn 4+ d Cr 2+, Mn 3+ d 4 4 (high spin) 2 (low spin) Fe 3+, Mn 2+ d 5 5 (high spin) 1 (low spin) Fe 2+, Co 3+ d 6 4 (high spin) 0 (low spin) Co 2+ d 7 3 (high spin) 1 (low spin) Ni 2+ d Cu 2+ d Cu + d Table 3 Measured agnetic oents, d-configuration, and nuber of unpaired electrons for transition etal ions with a tetrahedral geoetry. Metal Ion d configuration Nuber of Unpaired Electrons Magnetic oent (B.M.) Ti 4+, V 5+ d Cr 5+ d Cr 4+ d Fe 5+ d d Mn 2+ d 5 5 (high spin) Fe 2+ d 6 4 (high spin) Co 2+ d 7 3 (high spin) Ni 2+ d Cu 2+ d Cu + d Data taken fro Magnetic Susceptibility Balance Instruction Manual, Johnson Matthey, Issue 3, 10 th March 2004, ECN 306

6 Instructions for operation of the Evans Balance by Johnson Matthey: 1. Place the balance on a flat surface and level using the bubble. Plug in/turn on. Note: the balance is sensitive to breakage and should not be oved. 2. Turn the RANGE dial to 1 and allow the balance to war up for inutes. 3. Adjust the ZERO dial until the display reads 000. The zero should be readjusted if the range is changed. Note: The zero dial has a range of about 5 turns and works best in in the iddle of the range. If needed, adjust the dial 5 turns fro one end and adjust the back legs of the balance until the display reads near zero. All further adjustents should be done fro the front of the Evans balance. 4. Weigh an epty tube and place into the tube guide. Record the reading R0. The instruent can drift over tie, therefore it should be rezeroed before each easureent. The digital display should fluctuate by no ore than ±1 on the 1 setting. Note a range of readings if there is significant fluctuation. 5. Fill the saple tube with the saple of interest. Gently tap the botto of the tube on a hard surface to increase the packing density. The height of the saple in the tube should be at least 1.5 c, ideally between 2.50 and 3.50 c. Measure and record the ass of the tube and saple using an analytical balance. Measure the height of the level saple contained in the tube. 6. Rezero the epty balance and place the tube containing the saple into the tube guide. Record the reading R. a. If the reading of off-scale, adjust the RANGE dial to 10, rezero, and take an additional reading and ultiply the reading by Gently tap the tube containing the saple on a hard surface again to further pack the saple, and then take another reading of R. Repeat until several readings are within ±1. 8. Deterine the teperature (T) to 0.1 C with a theroeter claped near the balance. 9. The saple ay be reoved fro the tube by gently tapping the tube upside down on a piece of weighing paper. Avoid breaking the glass lip on the tube. 10. After reoving ost of the cotton fro a cotton swab, scrub the inside of the tube to reove any reaining loose powder. 11. Rinse the epty tube with ethanol and then acetone with a fine-tip disposable pipette, drying between each step. Ensure the tube is dry before using again. 12. Tabulate the data R, R0, L, M, and T with proper units and significant figures. 13. Calculate the eff and copare to predicted values fro s calculated by assuing a spin-only contribution. References 1 Figgis, B. N.; Lewis, J. In Lewis, J., Wilkins, R. G., Eds.; Modern Coordination Cheistry; Interscience Publishers Inc.: New York, 1960; pp Angelici, R. J. In Synthesis and technique in inorganic cheistry. W.B. Saunders: 1977; 3 Housecroft, C. E.; Sharpe, A. G. In Inorganic Cheistry; Pearson Education Liited: 2008; Vol. 3