Experiment 7 Magnetic Susceptibility (2020-2021) PDF

Expt. 7: Preparation of Bis(tetraethylammonium) Tetrabromonicklate(II), [NEt4]2[NiBr4] Inorganic Chem Lab Experiment 7 - 1 Chemical Equations: NiBr2 + 2 N(C2H5)4Br → [N(C2H5)4]2[NiBr4] Materials Required: tetraethy...

Expt. 7: Preparation of Bis(tetraethylammonium) Tetrabromonicklate(II), [NEt4]2[NiBr4] Inorganic Chem Lab Experiment 7 - 1 Chemical Equations: NiBr2 + 2 N(C2H5)4Br → [N(C2H5)4]2[NiBr4] Materials Required: tetraethylammonium bromide, ethanol, nickel(II) bromide Procedure: Place 3.5 g tetraethylammonium bromide in 100 mL beaker on a stirrer hot plate in a fume hood. Add ethanol slowly until the solid has nearly dissolved (13–18 mL) and then gently warm the solution until the residue just dissolves. Add 1.75 g of finely ground nickel(II) bromide with rapid stirring. Heat the solution to boiling and simple filter the hot solution to remove any impurities or undissolved reagent. Evaporate the filtrate until dryness. Inorganic Chem Lab Experiment 7 - 2 Dr. Muayad Masoud Spring 2020-2021  Re-dissolve the solid in the minimum amount of boiling ethanol to which a few crystals of tetraethylammonium bromide has been added. Boil the solution until the bottom of the beaker is about 2/3 covered with crystals. Filter hot. Boil the filtrate further until more crystals appear and then again to obtain a third and final crop of crystals about 5 mL of solution should remain at this stage. Weigh the product and determine the yield. [NEt4]2[NiBr4] 1s2 2s2 2p6 3s2 3p6 4s2 3d8 28Ni Crystal field d-orbital splitting diagrams for common stereochemistries Inorganic Chem Lab Experiment 7 - 3 Electrons have a magnetic moment that can be aligned either with or in opposition to an applied magnetic field, ms, is +½ or –½. For an atom or ion with only paired electrons, the individual electron contributions to the overall spin magnetic quantum number, Ms, cancel one another, giving a zero net value of the overall spin quantum number; i.e., S = 0 (diamagnetic) If a diamagnetic material is placed between the poles of a strong magnet it will experience a repulsion for the applied field. The repulsion arises from circulation of the electrons caused by the applied field, resulting in an induced magnetic field in opposition. Suppose the sample is suspended between the poles of the magnet and is connected to the pan of an analytical balance. This is the experimental arrangement of a Gouy balance. As a result of the induced diamagnetic repulsion, the sample will appear to weigh less in the magnetic field, compared to its true weight outside the field. When removed from the applied field, the sample has no residual magnetic moment, and its apparent weight will be its true weight. Inorganic Chem Lab Experiment 7 - 4 Dr. Muayad Masoud Spring 2020-2021  If the sample contains unpaired electrons, the overall spin quantum number will be greater than zero; i.e., S > 0 (paramagnetic). If a paramagnetic species is placed between the poles of a strong magnet it will experience an attraction for the field, due to the alignment of the permanent paramagnetic moment with the applied field. If the sample is weighed with a Gouy balance, it will appear to be heavier in the magnetic field, compared to its true weight outside the field. With the exception of monatomic hydrogen, all atoms or ions with unpaired electrons also have paired electrons. In an applied field, these paired electrons and their associated induced diamagnetic moment slightly mitigate the paramagnetic attraction for the applied field. Nonetheless, the paramagnetic moment is always stronger than the opposing diamagnetic one, so the net effect is an attraction for the field. However, whenever we refer to a substance as paramagnetic, owing to an electronic configuration having unpaired electrons, we must realize that there is also a subtractive diamagnetic contribution to the overall magnetic moment of the sample. Inorganic Chem Lab Experiment 7 - 6 Dr. Muayad Masoud Spring 2020-2021  Transition metals, by definition, have at least one oxidation state with an incompletely filled d or f subshell and are consequently paramagnetic. The magnetic moment, µ, results from both the spin and orbital contributions of these unpaired electrons. The presence of coordinated ligands around the metal ion quenches the orbital contribution to greater or lesser degree, making the spin contribution most important. As an approximation, the expected magnetic moment for an ion with a certain number of unpaired electrons can be estimated from the spin-only magnetic moment, µs, which disregards orbital contributions: s = g S ( S + 1) (1) g is the gyromagnetic ratio (g = 2.00023) S = n(½), where n is the number of unpaired e–’s in the configuration. Substituting g = 2 and S = n(½) into equation (1), we can calculate the spin-only moment in terms of the number of unpaired e–’s from the expression s = n ( n + 2 ) (2) Inorganic Chem Lab Experiment 7 - 7 E.g. for d1, as Ti3+, µs =[1(1+2)]½ = 3 = 1.73. The units of the magnetic moment are Bohr magnetons (BM). Actual magnetic moments tend to be somewhat larger than the spin- only values obtained from either equation (1) or (2), owing to incomplete quenching of the orbital contribution. Nonetheless, the experimentally obtained value of the effective magnetic moment, µeff, taken as approximately the spin-only value, often serves as a practical means of determining the number of unpaired electrons on the transition metal in a complex. This, in turn, gives information about the spin state of the metal and can suggest its oxidation state or mode of bonding. The magnetic field inside a substance that is placed in an external magnetic field will depend not only on the magnitude of the applied field but also upon the ability of that substance to produce its own field, which will add to (paramagnetism) or subtract from (diamagnetism) the applied field. This "apparent" magnetic field, B, felt by the sample may then be represented as B = H0 + ΔH = H0 + 4πI Inorganic Chem Lab Experiment 7 - 8 Dr. Muayad Masoud Spring 2020-2021  B = H0 + ΔH = H0 + 4πI where H0 is the applied magnetic field and I is the induced magnetic moment of the sample or the magnetization induced from its electrons with diamagnetic and paramagnetic effects combined. Divide through by H0: B/H0 = 1 + 4π(κ) or B/H0 – 1 = 4π(κ) where κ , = I/H0, is a measure of the susceptibility of the substance to interact with an applied field and is called the volume magnetic susceptibility (what we measure!) The magnetic susceptibility is defined as 'The ratio of the intensity of magnetism induced in a substance to the magnetizing force or intensity of field to which it is subject. For diamagnetic repulsion (B < H0) so κ < 0 (−ve values) For paramagnetic attraction (B > H0) so κ > 0 (+ve values) This volume susceptibility is usually converted to susceptibility per gram of substance or mass susceptibility, χg, by dividing by its density, d. It is called the specific susceptibility is defined by χg= κ/d Inorganic Chem Lab Experiment 7 - 9 Experimental Determination of Magnetic Moments Magnetic moments are not measured directly. Instead, they are calculated from the measured magnetic susceptibility, χ. Over the years there have been a number of techniques used to determine magnetic susceptibilities of transition metal complexes. These include the Gouy method, the Faraday method, and the NMR method. Of these, only the Faraday and NMR techniques are suitable for microscale samples of 50 mg or less. In 1974, D. F. Evans of Imperial College, London, developed a new type of magnetic susceptibility balance suitable for semimicroscale samples, which is commercially available from Johnson Matthey. The Evans balance employs the Gouy method in a device that is compact, lightweight, and self-contained. It does not require a separate magnet or power supply, and is therefore portable. The instrument has a digital readout that provides quick and accurate readings, with sensitivity matching traditional apparatus. Inorganic Chem Lab Experiment 7 - 10 Dr. Muayad Masoud Spring 2020-2021  It can be used with solids, liquids, and solutions. As such, the Evans balance is ideal for our purposes. In the Gouy method, the balance measures the apparent change in the weight of the sample created by the sum of the diamagnetic repulsion and paramagnetic attraction for the applied field. The Evans balance uses the same principle, but instead of measuring the force that the magnet exerts on the sample, it measures the equal and opposite force the sample exerts on suspended permanent magnet. The Evans balance determines this force by measuring the change in current required to keep a set of suspended permanent magnets in balance when their fields interact with the sample. The magnets are on one end of a balance beam, and when interacting with the sample change the position of the beam. This change is registered by a pair of photodiodes set on opposite sides of the balance beam's equilibrium position. The diodes send signals to an amplifier that in turn supplies current to a coil that will exactly cancel the interaction force. Inorganic Chem Lab Experiment 7 - 11 Inorganic Chem Lab A digital voltmeter, connected across a precision resistor in series with the coil, measures the current directly. This current is displayed on the digital readout. The sample's magnetic susceptibility per gram is called the mass magnetic susceptibility, χg. For the Evans balance, the general expression for the mass magnetic susceptibility is L  = C ( R − R ) +   A  (3) m  g 0 L = sample length in centimeters m = sample mass in grams C = balance calibration constant (different for each balance) (Calculated in this experiment using a calibration standard). R = reading from the digital display when the sample is in the balance R0 = reading from the digital display when the empty sample tube is in place in the balance χv' = volume susceptibility of air (0.029 10–6 erg∙G–2cm–3) Experiment 7 - 12 A = cross-sectional area of the sample The volume susceptibility of air is usually ignored with solid samples, so equation (3) can be rewritten as CL ( R − R 0 ) (4) g = m 109 Dr. Muayad Masoud Spring 2020-2021  Equation (4) gives the mass magnetic susceptibility in the cgs-units of erg∙G–2∙cm–3 (where G is Gauss). The calibration standards usually employed in magnetic susceptibility measurements are Hg[Co(SCN)4] (χg =1.644 10–5) or [Ni(en)3]S2O3 (χg = 1.104 10–5 erg∙G‒2cm–3) A preferred method to evaluate C in equation (4) is to perform the experiment with one of these calibration standards employing the appropriate value of χg. Inorganic Chem Lab Experiment 7 - 13 Calculation of Magnetic Moment from Magnetic Susceptibility The molar magnetic susceptibility, χM, is obtained from the mass magnetic susceptibility by multiplying by the molecular weight of the sample in units of g/mol; i.e., M = M g (5) The units of χM are erg∙G–2. This experimentally obtained value of χM includes both paramagnetic and diamagnetic contributions, which we may identify as χA and χα, respectively. All sources of paired electrons (e.g., ligands, counter ions, core electrons on the paramagnetic species) contribute to the diamagnetic portion of the overall susceptibility. In 1910, Pascal observed that these contributions were approximately additive and consistent from sample to sample. Consequently, the diamagnetic contribution to the observed molar susceptibility can be estimated as the sum of constants (called Pascal's constants) for each diamagnetic species in the sample. Inorganic Chem Lab Experiment 7 - 14 Dr. Muayad Masoud Spring 2020-2021  We are interested in the paramagnetic molar susceptibility, which can be obtained by removing the diamagnetic contributions from χM. Thus we may write  A =  M −   (6) Values of χA (sometimes called the corrected magnetic susceptibility, M corr ) are inherently positive, while those of χα are inherently negative. Thus, for a paramagnetic substance, it must be that χA > χM. 3kT  A (7) eff = N 2 The value of the effective magnetic moment, µeff, can be determined from χA by the Curie Law equation where k is the Boltzmann constant, T is the absolute temperature (K), N is Avogadro's number, and β is the Bohr magneton. If the appropriate constants are substituted, equation (7) becomes eff = 2.828  AT (8) Inorganic Chem Lab Experiment 7 - 15 Operation of the balance: The first measurement will be made with the calibration standard [Ni(en)3]S2O3 in order to obtain a value of C, the instrument constant in equation (4). The procedure is then repeated with [N(C2H5)4]2[NiBr4], and the experimentally determined value of C for the balance system is used to calculate the magnetic susceptibility of the sample. Other compounds used or provided by your instructor will be treated identically to your compound. 1. Turn the RANGE knob to the x1 scale and allow a 30 minute warm- up period before use. If the balance is to be used frequently, it should preferably be left on continuously. 2. Adjust the ZERO knob until the display reads 000. Zero should readjusted if the range is changed. Inorganic Chem Lab Experiment 7 - 16 Dr. Muayad Masoud Spring 2020-2021  NOTE: The zero knob on the balance has a range of 10 turns. It is best to operate the balance in the middle of this range. This is accomplished by turning the knob 5 turns form one end and then, ignoring the bubble level, adjusting the back legs of the balance until the digital display reads about zero. Inorganic Chem Lab Experiment 7 - 17 Once this is done at the beginning of the laboratory period, all further adjustments can be made with the zero knob on the front of the instrument. 3. Place a clean, dry, empty sample tube of known weight (analytical balance) into the tube guide (the tube guide is the brass hole in the top of the instrument) and take the reading R0 (will probably be negative). NOTE: Instrument signal can drift over short periods of time. It should be rezeroed before each measurement. On the x 1 scale the digital display should fluctuate by no more than ±1. However, when you record R or R0 take a "visual average" of this fluctuation and use this as your reading. 4. Carefully, pack the (dry) sample carefully into the glass tubes supplied to give a sample height in the range 2.50 – 3.50 cm after baking (at least 1.5 as a lesser amount will not give a stable reading of R). Add the sample to the tube in small amounts, gently tapping the base of the tube on the wooden bench (not the table the balance is on) a number of times between additions to shake the solid down and ensure even packing. Inorganic Chem Lab Experiment 7 - 18 Dr. Muayad Masoud Spring 2020-2021  5. When it is well packed, weigh the tube with sample so mass of sample in grams can be determined. Record the sample mass m (in grams), the sample length L (in cm, 2 decimal places), making sure the top of the sample is horizontal, and using a thermometer placed or suspended near the instrument, determine the temperature to 0.1 C (convert into K). 6. Rezero the instrument, place the packed sample into the tube guide and take the reading R. 7. If the reading goes off scale, turn the RANGE knob to the x10 scale, re- zero and multiply the reading by 10. 8. Using equation (4), calculate the instrument constant C (reference standard) or the mass susceptibility (sample) from your recorded values of L, R0, R, m, and (for the sample) C. You will need the value of T for the calculation of µ from the Curie Law, equation (8). 9. Remove the sample from the tube by inverting it and gently tapping it on a piece of weighing paper on a hard surface. Do not tap too hard, since the glass lip can be easily broken. After the tube is empty, rinse with an appropriate solvent from a disposable pipette with a fine tip (Pasteur pipette) and return the glass tube. Inorganic Chem Lab Experiment 7 - 19 Inorganic Chem Lab Experiment 7 - 20 Dr. Muayad Masoud Spring 2020-2021  Calculations: Calculate χM , (molar magnetic susceptibility) using: C L (R − R0 ) M M = 3 10−6 in cgs units 10 m Where C = calibration constant – measured as …………. L = length of the sample in cm R and R0 are the balance readings for the full and empty tubes, espectively M is the Formula weight of the sample m is the mass of the sample in grams Use Pascal's constants to evaluate the diamagnetic correction,  , (see below). Calculate the corrected molar susceptibility of the sample: Calculate μeff, the magnetic moment (in Bohr Magneton). eff = 2.83  AT BM where T : temperature in K Inorganic Chem Lab Experiment 7 - 21 Diamagnetic Corrections: DIAMAGNETIC CORRECTIONS (x 10–6 mol–1) Cation d Anion d Ligand d Li+ –1 F− –9 Urea –34 Na+ –7 Cl− –23 thiourea –42 K+ –15 Br− –35 ammonia –18 Tl+ –36 I− –51 water –13 NH4 + –13 NO3− –19 1,2–diaminoethane –46 NEt4 + –112 NO2− –10 pyridine –49 Hg2+ –40 ClO3− –30 pentane-2,4-dione –52 Zn2+ –15 ClO4− –32 benzene –55 Pb2+ –32 BrO3− –39 PPh3 –200 Ca2+ –10 IO3− –51 cyclopentadienyl –65 Fe2+ –13 IO4− –52 bipyridine –105 Cu2+ –13 CN− –13 phenanthroline –128 Co2+ –13 SCN− –31 salen –182 Ni2+ –13 SO42– –40 Mn2+ –13 CO32− –30 Mg2+ –5 OH− –12 CH3CO2− –30 C2O42− –25 All data taken from "Introduction to Magnetochemistry" A. Earnshaw, Academic Press 1968 Inorganic Chem Lab Experiment 7 - 22 Dr. Muayad Masoud Spring 2020-2021  Example: Calculation of μeff for Fe(NH4)2(SO4)2∙6H2O M = 392.14 Diamagnetic correction: T = 287 K Fe2+ –13 x 10–6 L = 2.85 cm 2 x NH4+ –26 x 10–6 R0 = –36 2 x SO42– –80 x 10–6 R = 1961 6 x H2O –78 x 10–6 m = 0.196 g   = –197 x 10–6 C = 1.044 C L (R − R0 ) M M = 10−6 in cgs units 103 m 1.044  2.85  1961 − ( −36 )   392.14 = 10−6 103  0.196 = 11888 10−6 Inorganic Chem Lab Experiment 7 - 23 χA = χM –  = [11888–(–197)] 10–6 = 12085 10–6 eff = 2.83  AT BM = 2.83 12085 10−6  287 = 5.27 BM Inorganic Chem Lab Experiment 7 - 24 Dr. Muayad Masoud Spring 2020-2021