Theoretical Models in Explaining the Behavior of Transition Metal Coordination Complexes - Assignment Example

INORGANIC CHEMISTRY Chemistry Assignment 11 February Part A Structure, Stereoisomerism, and Nomenclature Sketches of the transition metal complexes and their stereoisomers a) [Co(NH3)6]3+ NH3 NH3 3+ 3HN Co NH3 3HN NH3 Pentamminechlorocobalt (III). There are no other possible isomers Transition metals: the d group elements. b) [FeCl4]- Cl Cl - Fe Cl Cl c) [Co(NH3)4Cl2]+ Cl NH3 + Cl NH3 + Cl Co NH3 3HN Co NH3 H3N NH3 3HN Cl Cis-tetramminedichlorocobalt (III) Trans-tetramminedichlorocobalt (III) d) [Cr(en)3]3+ where en=H2NCH2CH2NH2 3+ H2N H2N NH2 Cr H2N NH2 H2N Stability of Complexes 2. A) The values of LgKn for the Cu/NH3 system decrease as you move across the table. This is because the stability constants decrease as NH3 ions are attached to the Cu2+ ion. B) The values of Igβ for the Cu2+/en system are larger than those of the Cu2+/NH3 because there is the formation of chelates. Chelates are stable compared to complex ions formed from simple monodentate ligands (Substitution in complex ions - stability constants n.d.). This usually leads to the chelate effect because reactions involving the formation of 1,2-diaminoethane increases the entropy of the system by escalating the disorder of the system. Changes that increase the entropy of a system increase the likelihood of a reaction taking place. Compounds such as ammonia and water bind to a “metal ion through a single atom or ion” and are known as monodentate ligands (Burton 2000). This accounts for the differences in the overall stability constants between the Cu2+/NH3 and Cu2+/en systems. Theoretical Models in Explaining the Behavior of Transition Metal Coordination Complexes 3. A) The electronic structure of Cr and Cr(III) using s, p, and d notations Cr 1S22S22p63s23p63d54s1 Cr (III) is the same as Cr3+ 1S22S22p63s23p63d3 The number of 3d electrons present in Cr (III) is 3 (Ryan 2000). b) The arrangement of the d electrons for Cr (III) before and after the electronic transition using the crystal field splitting diagram The crystal field splitting is the division in energy between two arrays of d orbitals due to uneven repulsion of the d electrons of the metal by the negative charges, which are set octahedrally around the innermost metal ion (Reger, Goode & Ball 2009). Therefore, the energy of the d orbitals augments as the negative charges approach the metal ion (Crystal field theory: energy level splitting n.d.). Cr (III) before excitation z2 x2-y2 Δ0 Energy xy xz yz Cr (III) after excitation Energy z2 x2-y2 Δ0 xy xz yz c) Estimating the value of the crystal field splitting in KJ/mol using the provided λmax values: i. [CrCl6]3- 740nm Photon energy = E = Δo= h c/λ where h is Planck’s constant= 6.62608 x 10-34 J sec and c= 3 x 108 E=[(6.62608 x 10-34 J)× (3 x 108)]÷(740×10-9) =2.686×10-19 J/ion. This energy is converted into KJ/mol by multiplying by Avogadro’s constant (6.023×1023) and dividing by 1000. = (2.686×10-19) × (6.023×1023)/1000 =161.78 KJ/mol. ii. [Cr(NH3)6]3+ 460 nm E= [(6.62608 x 10-34 J) × (3 x 108)]÷(460×10-9) =4.321×10-19J/ion = (4.321×10-19) × (6.023×1023)/1000 =260.28KJ/mol iii. [Cr(H2O)6]3+ 575 nm E= [(6.62608 x 10-34 J) × (3 x 108)]÷(575×10-9) =3.457×10-19J/ion = (3.457×10-19) × (6.023×1023)/1000 =208.22KJ/mol d) The ligands arranged in order of increasing ligand field strength are [CrCl6]3-, [Cr(H2O)6]3+, and [Cr(NH3)6]3+. 4. The magnetic moment of [Mn(H2O)6]2+ is 5.9 BM while the magnetic moment of [Mn(CN)6]4- is 1.7 BM because [Mn(H2O)6]2+ contains more unpaired electrons compared to [Mn(CN)6]4-. According to Khandelwal, the magnetic moment of a substance increases with the increase in the number of unpaired electrons (n.d.). 5. The limitations of the Crystal Field Theory as applied to transition metal complexes are that it ignores the central metal atom and the ligands. The Crystal field Theory views ligands as point charges and does not account for chemical bonds. The Molecular Orbital Theory is a more useful theory because it explains atoms in terms of orbitals and the arrangement of electrons in the orbitals (electron configuration). This helps in accounting for chemical bonds as a result of transfer or sharing of electrons. It takes into account the ionic and covalent involvements in the formation of complexes. Using Standard Electrode Potentials to Predict the Redox Properties of 3d Transition Metal Compounds 6. How to construct Latimer diagrams from standard electrode potential data The Latimer diagram indicates the standard reduction potential for changes between each of the oxidation states of an element in order, starting with the highest oxidation state on the left and the lowest oxidation state on the right (Chandra 2006). The first step involves identifying the element with the highest oxidation state and writing it down. An arrow is drawn from this element to that with the second highest oxidation state. The value of the electrode potential that connects these elements is then written on top of the arrow. The same steps are repeated until the element with the lowest oxidation state is written on the far right side of the diagram. For the given standard electrode potential values, the Latimer diagram is as follows: +1.00 +0.344 -0.255 -1.13 VO2+ VO2+ V3+ V2+ V 7. a) the balanced half equation for the VO2+/V3+ half equation is: VO2+(aq) +2H+(aq) +2e- V3+(aq) +H2O(l) E=+0.334V b) Vanadium reacts with aqueous acid because aqueous acid contains hydrogen ions, which reduce vanadium hence resulting in a chemical reaction. c) The E0 for the half reaction VO2+→V2+ is found by using the half equations for the individual steps. The half equation for the conversion of VO2+ to V3+ is VO2+(aq) +2H+(aq) 2e- V3+(aq) + H2O(l) E=+0.334V (Reaction 1) The half equation for the conversion of V3+ to V2+ is given by V3+(aq) +e- V2+(aq) E=-0.255V (Reaction 2) The free energy for reaction 1 where the number of electrons transferred is 2(n=2) is given by ΔG1 = -nFE = -0.344V The free energy for reaction 2 is ΔG2 = -nFE = -0.255V. The free energy for the conversion of VO2+→V2+ is ΔG = ΔG1 + ΔG2 =0.599V’ but the conversion of VO2+→V2+ is a three electron transfer reaction where n=3. Therefore, E = ΔG/-nF E =0.599F/3F E =0.1996V 8. a) The strongest oxidizing agent is MnO-4. This is because it has the largest positive value of standard electrode potential (Applications of standard electrode potentials n.d.). b) Determining whether VO2+(E=1.0) can oxidize I- to I2-(E=0.535). According to the standard electrode potentials for the two species, VO2+ has a high positive value as compared to I-/I-2. Therefore, it is expected that VO2+ will oxidize I- to I2-(Vanadium n.d.). c) The E0 value for the reaction between Cr2O72- and Fe2+ in acid solution. Cr2O72- is a strong oxidizing agent and oxidizes Fe2+ to Fe3+, whereas Cr2O72- becomes reduced to Cr3+. The half equations are Cr2O72- + 14H+ +6e- 2Cr3+ + 7 H2O (E= -1.33V) Fe3+ + e- Fe2+ (E=-0.771V) The first reaction has a high positive E0 value; therefore the equilibrium of the reaction will move to the right. In the second reaction, the equilibrium of the reaction tends to shift to the left because the reaction has a less positive E0 value. Therefore, the E0 value of the second reaction will change to positive when the reactants on the right hand side shift to the left hand side as follows: Fe2+ Fe3+ + 6e- (E=+0.771V) Cr2O72- + 14H+ +6e- 2Cr3+ + 7 H2O (E= -1.33V) Adding the two equations and their E values gives the following overall equation: Cr2O72- + 14H+ +6e- + Fe2+ 2Cr3+ + 7 H2O + Fe3+ + 6e- (E=-0.559V) Therefore, the E0 value of the reaction is 0.559V. d) Hydrochloric acid cannot be used in titrations using MnO4-because it is extremely volatile and takes part in the chemical reaction. References Applications of standard electrode potentials, n.d., viewed 11 February 2013, . Burton, G Chemical ideas, 2nd edn, 2000, Heinemann Education Publishers, Oxford. Chandra, S, 2006, Comprehensive inorganic chemistry, New Age International, New Delhi. Crystal field theory: energy level splitting, n.d., viewed 11 February 2013, . Khandelwal, B. L n.d., Inorganic chemistry: chemistry of transition elements, viewed 11 February 2013, . Reger, D. L., Goode, R. S., & Ball, D. W., 2009, Chemistry: principles and practice, 3rd edn, Cengage Learning, Belmont, CA. Ryan, L., 2000, Advanced chemistry for you, Nelson Thornes, United Kingdom. Substitution in complex ions - stability constants, n.d., viewed 11 February 2013, . Transition metals: the d group elements, n.d., viewed 11 February 2013, http://faculty.smu.edu/jmaguire/Gen%20Chem/20.%20transition%20metals.pdf Vanadium, n.d., viewed on 11 February 2013, . Read More