Transcript Conductance
Conductance A. Why are we doing this experiment? 1) One of the most basic pieces of information about your complex, is how many ions there are in the salt a) This helps us determine the oxidation state of the metal ion b) This helps us determine what the charged ligands and counter ions are c) This helps us determine the binding mode of the ligands d) A non-coordinating solvent (CH3CN) is needed, so that we don’t replace any of the coordinated ligands, which would give us the wrong answer e) Example + 0 N N O M M CH3 N O N CH3 O 0 ions 2) CH3CO2- N 2 ions N OH2 M N O O OH2 0 ions 2CH3CO2- N CH3 CH3 CH3 CH3 CH3 M N N N N CH3 O N O CH3 O N N N 2+ 0 CH3 CH3 CH3 3 ions What happens to our complex when we dissolve it (solution behavior)? a) Almost all reactions are done in solution b) Solid state (X-ray crystal) structures don’t tell us what happens in solution c) A range of coordinating solvents (MeOH = weakly coordinating; H2O = strongly coordinating) helps us determine the ease of ligand solvation d) Often, you need to replace some ligands to do the reaction you want B. How are we doing this experiment? 1) We are using an Oakton CON510 Bench Conductivity/TDS Meter a) Works much like a pH meter b) More ions in solution mean more conductance of electricity 2) The molar conductances are determined from readings at several dilutions and extrapolated to Lo 3) An Onsager plot for these complexes in each solvent is constructed to confirm the electrolyte type 4) The molar conductance value at infinite dilution is taken from the graph and compared to literature values, as a second method for finding electrolyte type C. Conductance Theory 1) L = specific conductance = 1/Resistance a) Voltage = (Current)(Resistance) [V = iR] so R = V/i or L = i/V b) Our conductance meter gives conductance directly, no correction needed c) Units of Resistance = ohm; Units of Conductance = ohm-1 = mho = Seimans 2) Molar Conductance = LM (1000)(L) ΛM Molarity 3) The Onsager Law L0e Le ( A wBL0e )c 1/ 2 e a) Le = equivalent conductance i. The charge on the metal ion influences the conductance ii. To remove this influence, we multiply the concentration by the metal ion charge to find the equivalent concentration (1000)(L) Λe Equivalent Concentrat ion b) L0e = equivalent conductance at infinite dilution i. Extrapolate from the five points you collect to ce1/2 = 0 ii. Plot Le vs ce1/2 c) Plot (L0e - Le) vs ce1/2 i. Slope = (A + wBL0e) ii. This is the number we will compare with data from the literature to determine the electrolyte type of our complex in that solvent iii. Feltham and Hayter J. Chem. Soc. 1964, 4587. 4. Molar Conductance Comparison a) We will plot LM vs (M)1/2 and extrapolate to M = 0 to find L0M b) Comparison with data from the literature will give us a second check of our electrolyte type comparison Geary, W. J. Coord. Chem. Rev. 1971, 7, 81. Water Water DMF DMF Methanol Methanol CH3CN Nitromethane Electrolyte Type L0M Onsager Slope L0M Onsager Slope L0M Onsager Slope L0M Onsager Slope 1:1 118131 92 65-90 150 80-115 256 120160 200 2:1 235273 185 130-170 ---- 160-220 514 220300 465 3:1 408435 285 200-240 ---- ---- ---- 340420 1020 4:1 ~560 526 ---- ---- ---- ---- ---- ---- D. How do we work up the data Excel Spreadsheet is available for download from my Shared Folder We will go over this now in Excel E. How do we interpret the results? 1) We compare our LM to the Geary data and try to assign an electrolyte type a) LM = 47.21 b) The 1:1 value from the table is 120-160. c) 2) F. Our electrolyte type must be less than 1:1; or zero ions We compare our Onsager Plot slope to the Feltham and Hayter date a) Our slope = 156 b) The 1:1 value from the table (for nitromethane) is 200 c) Again, our electrolyte type is less than 1:1 Conclusions 1) In acetonitrile, Fe(Bcyclam)Cl2 does not dissociate even one anion (fully) 2) Acetonitrile is a non-coordinating solvent (poor ligand), so it doesn’t replace Cl- G. Conclusions from other solvents/complexes Note: It is possible for a complex to behave as a 4:1 electrolyte in water, even if it has only a 3+ metal ion. Water can coordinate, and then be deprotonated to yield additional ions. [MLCl2]PF6 ---> ML(OH2)3+ + 2Cl- + PF6LM(OH2)3+ ---> LM(OH2)(OH)2+ + H+