Scheme 2. 2 Proposed direct nucleophilic attack and solvotic pathways of associative substitution reaction of square planar complexes
2.5 Factors Affecting the Rate of Substitution of Square Planar Complexes
2.5.1 The Effect on Rate of the Incoming Ligand
The second order rate constant, k2, of associative substitution mechanism is dependent on the nucleophilicity of the incoming ligand (nucleophile). Nucleophilicity is a measure of how readily the nucleophile attacks an electron-deficient metal centre such as Pd. Stronger nucleophiles
0.0 0.3 0.6 0.9 1.2
0.54 0.57 0.60 0.63 0.66
Absorbance
Time (s)
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exhibit higher rates of substitution at the metal centre than weak nucleophiles.40, 44 Several factors that influences the strength of nucleophilicity;7, 42, 55
i. Basicity: Basicity is the capacity of an electron-rich species to share its electrons with the proton75, 76 The basicity of the incoming nucleophile is characterised by its pKa which also correlates well with the nucleophilicity of the nucleophile towards the metal centre.77, 78 For reversible substitution reaction, the reverse reaction is largely dependent on the basicity of the nucleophile than for the forward reaction.79, 80
ii. Polarisability: Polarisability of the nucleophile is an important consideration for the rates rather than for equilibria of chemical reactions.9 Polarizability of a ligand can be explained by using Peasons’s “Hard Soft Acid Base” (HSAB)a† theory.42, 81-83 In ligand substitution reactions, high nucleophilicity values at the Pt(II) substrates are normally known for highly polarizable incoming ligands such as the sulfur containing nucleophiles and iodide.28, 29, 84, 85 Increasing the polarizability improves the electron donor ability of the incoming nucleophile.
iii. Oxidizability: Ligands that can easily lose their electrons, such as strong reducing agents are normally good nucleophiles. The ability to lose electrons are usually as a result of the values of their electrode reduction potentials (electrochemical data).55
iv. Solvation energy: Strongly solvated ligands are weaknucleophiles due to the requirement of high energy to free the nucleophile from the bonded solvent, prior to the ligand’s attempt to coordinate to the metal centre.
a† The theory states that hard acids, i.e. small metal ions have a high charge and possess a valence electron shell which is not easily distorted and hence prefer hard bases, e.g. Li+, Mg2+ and F-. Similarly, soft acids, i.e. large metal ions have a low charge with a valence electron shell which can easily be distorted or removed hence would prefer soft bases, e.g. Pt2+ and SCN-.
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v. Metal centre: The nature of the metal centre influences nucleophilicity of the incoming ligand since reaction show dependence on the nature of the metal center. In transition state, the lighter elements are less polarised than the heavier ones. This can be seen when isovalent metal ions are compared.86 Thus, the rate of substitution of d8 complexes, Ni(II), Pd(II), Pt(II) follows the trend Ni > Pd > Pt in the ratio of 5 x 106: 105: 1.87-89 Comprehensive studies were performed to rank the nucleophiles at a prototype Pt(II) complex, trans-[Pt(pyb)2Cl2] as the standard substrate in methanol at 30 ºC.7, 9 The nucleophilicity constant of the entering ligand, nºPt was measured for the rate of the chloro substitution from the standard Pt(II) according to the equation;
2.42 A set of nucleophilic reactivity constants,44, 90 nPt for an entering nucleophiles (Y), was initially defined as:
𝑙𝑜𝑔 (𝑘𝑌
𝑘𝑠) = 𝑛𝑃𝑡 2.43
where, 𝑘𝑌 = the measured rate constant for the entering nucleophile, Y 𝑘𝑠= rate constant for the solvent attack (methanol)
The above equation was found to be inadequate in taking care of the existing solvolytic pathway because 𝑘𝑌 and 𝑘𝑠 differ in their dimensions. Therefore, 𝑘𝑠 was substituted by the second-order rate since solvolytic pathway was associative in nature to give an expression;
𝑙𝑜𝑔 (𝑘𝑌
𝑘𝑠0) = 𝑛𝑃𝑡0 , 𝑏𝑢𝑡 𝑘𝑠0 = 𝑘𝑠[𝑀𝑒𝑂𝐻] 2.44
b Pyridine
trans-[Pt(py)2Cl2] + Yn trans-[Pt(py)2ClY](n+1) + Cl-
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Given that the concentration of pure methanol is 24.3 mol dm-3 at 30 °C, Equation 2.44 can further be simplified in Equation 2.45
𝑛𝑃𝑡0 = 𝑛𝑃𝑡+ 1.39 2.45
By definition, 𝑛𝑃𝑡0 for the methanol as the entering group is zero, being that it is the standard, while the highest value recorded was for triphenylphosiphine, Ph3P being 8.99. Nucleophilicity constants from several studies91-97 using the trans-[Pt(II)(py)2Cl2] was shown to follow the trend, Ph3P ˃ Et3As ̴ S2O32- ˃ CN- ̴ Ph3As, SO32- ˃ SCN- ̴ Me2Se ˃ I- ˃ N3- ˃ thiourea ̴ (Et2N)3P ̴ Et2S ˃ PhSH ̴ Br- ˃ NH2OH ̴ NH2NH2 ˃ Ph(CH2)2S2 ̴ imidazole ˃ Ph2S ̴ PhNH2 ̴ C5H5N ̴ NO2- ˃ Me2SO ̴ NH3 ̴ Cl- ̴ OH- ˃ H2O ˃ F- ˃ methanol.
Interestingly, the effect of the incoming group on Pd(acacc)2 demonstrated the same trend with the few incoming groups studied98 as shown in Table 2.1.
Table 2. 1 The entering group effects on the rate of substitution of Pd(acac)2. X k2 (x 102 s-1)
H2O 3.2
OH- 3.2
Cl- 8.9
Br- 32
I- Fast
SCN- Very fast
The increasing rates of substitution following the order; H2O ≈ OH- < C1- < Br- < I- < SCN- for Pd(acac)2 was an indication that the polarizability is a crucial factor in determining nucleophilic reactivity which corroborated with that of the Pt(II) complexes.99
c Acetylacetonate
60 2.5.2 Solvent Effects
The substitution reactions are carried out in the solvent medium which can swiftly substitute the leaving ligand making the rate to be independent of the incoming ligand due to the solvotic pathway (as shown in Equation 2.17 and scheme 2.2) as have been reported in Pd(II) and Pt(II) complexes.73, 77, 78, 100 The coordinating ability of the solvent proportionally determines the overall rate of the reaction.9, 10, 44 A solvent that have electron rich-donor atoms as well as lone pairs of electrons exhibits acceleration in their coordinating tendency.
An example which have been used to quantify the effect of solvent was demonstrated in the reactions involving the substitution of chloride from trans-[Pt(py)2Cl2] by the radio-labelled chloride (36Cl-) depicted in Equation 2.46 and the data shown in Table 2.2.
(2.46) Table 2. 2 Effect of solvent on the rate of chloride exchange from the trans-[Pt(py)2Cl2]101
Coordinating Solvents
k-2/(10-5 s-1) Non-/weakly Coor- dinating solvents
k2/M-1 s-1
DMSO 380 CCl4 104
H2O 3.5 C6H6 102
EtOH 1.4 i-BuOH 10-1
PrOH 0.4 Me2CO 10-2
DMF 10-3
From Table 2.2, a highly coordinating solvent medium leads to the reaction proceeding predominantly via the direct solvent attack which is independent of the concentration of the entering nucleophile. The k2, (second order rate constant) for direct nucleophilic substitution is therefore much less than k-2 of the solvolytic pathway. The rate of substitution by the solvent follows the trend, ROH < H2O ≈ CH3NO2 < DMSO in agreement with the nucleophilicity of the solvents. This indicates that the Pt–S bond making process in the reaction of DMSO takes pre-
trans-[Pt(py)2Cl2] + 36Cl- trans-[Pt(py)2Cl(36Cl)] + Cl-
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eminence over the bond breaking process in the transition state. For example, if the solvent solvates the Cl-, then DMSO should be less efficient solvent than H2O. DMSO coordinates to Pt(II) via its high polarizable S atom and hence has high nucleophilicity at Pt(II) centre.102 As a result, rate of exchange is higher for DMSO than for H2O. On the other hand, weakly coordinating solvents such as CCl4, C6H6 and sterically hindered alcohols are dominated by direct nucleophilic attack towards the complex since their k2 is significantly larger than k-2. In addition, higher rates are observed in non-polar solvents such as CCl4, which prevents solvation by the Cl- unlike in the polar and highly solvating solvents like DMF or H2O.