A solution and solid state study of niobium complexes University of ...

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5.4 Conclusion Chapter 5 The values of K1 obtained from two separate experiments are the same within experimental error. This is a further indication that the reaction scheme and resulting rate law provides a good understanding of the formation reactions studied here. The value of K2 is three orders of magnitude smaller than K1 (e.g. at 25.0 °C, K1 = 1975 (201) M -1 vs K2 = 2.0 (5) M -1 ). This is to be expected since the rate-determining ring- closure is slow. Similar examples, where this has been observed, are known for bidentate complexes. 7 The relative rate of the ring-closure is best illustrated by comparing k1 to k2. The rate data cannot be compared simply by looking at the results in Table 5.2, since the k2 rate constant is only measured in s -1 . The rate constant, k1 (at 25.0 ° C), is converted to a first-order rate constant by multiplying k1 (237 (4) M -1 s -1 ) with [acacH] (0.05 M) and adding k-1 (0.12 (1) s -1 ). When this value of 11.97 (2) s -1 is compared with k2 (3.7 (3) x 10 -5 ), a difference of 6 orders of magnitude is obtained. The difference between k2 and k-2 is between 1.5 and 2 times at all the temperatures studied. This is also a very good indication of the small value obtained for the equilibrium constant, K2. Comparison of the kinetic data obtained for this study with that of the corresponding tantalum study 8 yielded the following: Similar rate laws were obtained for both studies, except that no k-2 step was observed for the tantalum study. The values obtained for the rate constants, k1 and k2, of the tantalum study (316 (9) M -1 s -1 ; 4.43 (7) x 10 -5 s -1 ) was slightly higher than that obtained for the niobium study (237 (4) M -1 s -1 , 3.7 (3) x 10 -5 s -1 ) at 25.0 °C. The equilibrium constant, K1, for both studies are, within experimental error, similar at ± 2000 M -1 . The high positive value obtained for ∆S ‡ (k1), points strongly towards a dissociative type mechanism for the first step, as expected for octahedral substitution reactions. 7 H. J. van der Westhuizen, R. Meijboom, M. Schutte, A. Roodt, Inorg. Chem., 49, 20, 9599, 2010. 8 R. Koen, H. G. Visser, A. Roodt, M.Sc. Thesis, University of the Free State, 2012. 93
Chapter 5 However, more information (such as high-pressure kinetic data) is required to make a more reliable conclusion of the type of mechanism. The large negative value obtained for ∆S ‡ (k2) might be misleading. It is expected that a η 1 -acac ligand will free rotate quite substantially and that the resulting ring-closure, to form η 2 - coordination, could override the observed enthropy for the dissociation of the chlorido ligand, thus resulting in a more ordered transition state for the second step. 94
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A solution and solid state study of
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Table of contents Abbreviations and
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3.4 Infrared Spectroscopy (IR) ....
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Abbreviations and Symbols Abbreviat
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Abstract 93 Nb NMR was successfully
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Opsomming 93 Nb KMR is met sukses g
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Chapter 1 metals. 3 Due to Wollasto
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Synopsis... 2. Literature Review of
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Chapter 2 Niobium resembles tantalu
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2.1.2 Uses Chapter 2 Niobium has a
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2.2 Separation of Nb and Ta 2.2.1 M
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Chapter 2 Buachuang et al. 16 repor
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Chapter 2 Niobium oxide surfaces ex
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2.4.6 Water absorption Chapter 2 Th
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Chapter 2 been reported in literatu
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Chapter 2 containing niobium as the
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Chapter 2 conclusions, with regard
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Chapter 2 Figure 2.5: Structure of
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2.6.3.3 [NbCl3O(ttbd) - ] Chapter 2
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Chapter 2 (a) (b) Figure 2.10: Stru
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2.7 Alkoxides Chapter 2 Specific kn
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Chapter 2 Reactions of dialkylamid
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Chapter 2 other NbCl5-x(OMe)x produ
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Chapter 2 In 1991 Lee et al. 83 pub
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EtO EtO EtO EtO Cl Nb Cl Cl Nb Cl R
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Chapter 2 The hemicarbonate formed
Page 53 and 54: Synopsis... 3. Synthesis and Charac
Page 55 and 56: Chapter 3 with γ = magnetogyric ra
Page 57 and 58: Chapter 3 electromagnetic spectrum
Page 59 and 60: 3.5.1 Bragg’s law Chapter 3 Bragg
Page 61 and 62: 3.5.3 ‘Phase problem’ Chapter 3
Page 63 and 64: Chapter 3 A = ∑ ε cl (3.14) In
Page 65 and 66: Chapter 3 t1/2 = 54 = . 3.7 Sy
Page 67 and 68: 3.7.2.4 Synthesis of [NbCl4(acac)]:
Page 69 and 70: 4. Crystallographic Synopsis... Cha
Page 71 and 72: Chapter 4 packages 2 respectively.
Page 73 and 74: Chapter 4 4.3 Crystal Structure of
Page 75 and 76: Chapter 4 longer bonds (C1-C2, C2-C
Page 77 and 78: Chapter 4 Figure 4.5: Packing of [N
Page 79 and 80: Chapter 4 Figure 4.7: Molecular str
Page 81 and 82: Chapter 4 Table 4.5: Selected bond
Page 83 and 84: Chapter 4 Figure 4.11: Molecular st
Page 85 and 86: Chapter 4 Figure 4.12: The phacac p
Page 87 and 88: Chapter 4 Table 4.9: Hydrogen bonds
Page 89 and 90: Chapter 4 According to our knowledg
Page 91 and 92: 5.2 Experimental procedures 5.2.1 K
Page 93 and 94: Chapter 5 corresponds to one specie
Page 95 and 96: 5.3 Results and Discussion 5.3.1 Pr
Page 97 and 98: Chapter 5 Figure 5.5: Typical UV/Vi
Page 99 and 100: 5.3.3 Derivation of the rate law Ch
Page 101 and 102: Chapter 5 The Stopped-flow data (fa
Page 103: Chapter 5 Figure 5.9: Eyring plot,
Page 107 and 108: Chapter 6 second reaction entailed
Page 109 and 110: Appendix A Table 1.3: Bond angles (
Page 111 and 112: Appendix A A 2 Supplementary Data f
Page 113 and 114: Appendix A C(16)-H(16B) 0.96 C(16)-
Page 115 and 116: Appendix A O(7)-C(10)-C(11) 123.6 (
Page 117 and 118: Appendix A H(18E)-C(18B)-H(18F) 109
Page 119 and 120: Appendix A H(18F) 6243 6605 575 75
Page 121 and 122: Appendix A Table 3.3: Bond angles (
Page 123 and 124: Appendix A Table 3.4: Anisotropic d
Page 125: Appendix B Appendix B B 1 Supplemen
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A solution and solid state study of niobium complexes University of ...

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