How does the thermodynamic stability, measured in terms of log k (where k is the thermodynamic stability constant) of a Nickel (II) ammine complex produced by reacting Ni(II) with excess of ligand depends on the basic strength (expressed in terms of pkb) of the ammine ligand used, determined using spectrophotometry?
The moment, I went through the HL part of Topic-3 (Periodic Table), I was fascinated to know about the co- ordination complexes as they find immense use whether it is cosmetics or pharmaceuticals or even industries. Though the ligands EDTA, ethylene diamine was introduced as a part of the course, I always wanted to delve more to understand the way these ammine ligands interact with the central metal ion and form the bonds. The immediate question that bothered me was – do ligands have dative bonds with the metal ion? or Is it just a strong electrostatic interaction between the transition metal ion and the ligands that donates the lone pair? This question became more interesting when I understood how the stability of the complex is an issue while synthesizing them. Several drugs used are metal ion complexes and they need to be designed in such a manner that they must decompose only at a certain part of the body and not anywhere else. Thus, it is imperative to understand the stability of the complex made and choose the ligands accordingly. What factors of the ligands determines the stability of the complex they form? Further research led me to know that among various factors the ability of the ligand to donate lone pairs or in other wordsthe basicity is an important factor. As the ammine ligands of various types with different basic strengths was easily procurable, I chose to use multiple ammine ligands of different basicity and elucidate how they could impact the stability of the metal ion complex they produce.
Ni is a transition metal from the first row of d block. At an oxidation state of +2, the Ni(II) ion shows an electronic configuration of [Ar]3d8 . It can react with ammine ligands (ligands that contains the ammine – NH2 group) to form metal-ammine complexes. The ammine ligands used in this investigation are of the type X- NH2 where X = H for ammonia (NH3), X = OH for hydroxyl amine (OH-NH2), X = H2N-CH2-CH2 for ethan- 1,2-diamine (H2N-CH2-CH2-NH2), X = C6H5 for amino benzene (C6H5-NH2) and X = NH2 for hydrazine (H2N- NH2). All of these ligand act as Lewis base due to the presence of lone pair on N atom. According to Valence bond theory, the ligands form a dative bond with the central metal ion by donating lone pairs to the empty d subshells of the central metal ion while according to the Crystal Field theory, the interaction between a metal ion and a ligand is considered as a strong electrostatic force of attraction between point charges of opposite nature and is thus simply an ionic interaction. Ni (II) can show both co-ordination number 4 in square planar complexes and 6 in octahedral complexes. This investigation deals with all octahedral complexes and thus the metal ion displays a co-ordination number of 6 showing six metal ligand bonds. In octahedral complexes, the Ni(II) shows an electronic state of t62e2g. All the metal-ammine complexes are low spin complexes and does not involve any pairing of ligand compromising the CFSE (Crystal Field Stabilization energy).
Ni2+ (aq) + 6 NH3 (aq) ------→ [Ni(NH3)6] 2+ (aq)
Hexaammine Nickel(II) ion
Ni2+ (aq) + 6 H2N-OH (aq) --------→ [Ni(NH2OH)6]2+ (aq)
Hexahydroxylaminonickel(II) ion
Ni2+ (aq) + 3 H2N-CH2-CH2-NH2 (aq) --------→ [Ni(H2N-CH2-CH2-NH2)3])2+ (aq)
Trisethan-1,2-diammine nickel(II)
Ni2+ (aq) + 6 C6H5NH2 (aq) -------→ [Ni(C6H5NH2)6] 2+ (aq)
Hexaaminobenzenenickel(II) ion
Ni2+ (aq) + 3 H2N-NH2 (aq) -------→[Ni(H2N-NH2)3] 2+
Hexahydrazinonickel(II) ion
All the complexes formed are octahedral and water soluble.
The thermodynamic stability constant is a measure of the ability of the complex to disassociate to separate out the ligands and the metal ion. Higher the thermodynamic stability, more is the un-reactivity of the complex towards any disassociation or ligand exchange reaction. The discussion below aims to deduce a mathematical formula to calculate the magnitude of thermodynamic stability constant using the value of molar concentration of Ni(II) ion at equilibrium.
The generalized equation is:
Ni2+ (aq) + n L --------→ [MLn]2+ (aq)
\(Stability \,constant \,(K) = \frac{[[ML_n]^{2+}]}{[Ni^{2+}][L]^n}\)
The table below narrates the molar concentration of the reactants and products in the reversible reaction of the complex formation at various stages:
[Ni2+] | [L] | [[MLn]2+] | |
---|---|---|---|
Initial | 0.01 | 0.10 | 0.00 |
Change | (0.01 − x) | (0.01 − x)n | (0.01 − x) |
Equilibrium | x | 0.10 − ((0.01 − x)n) | (0.01 − x) |
\(k = \frac{(0.01-x)}{x(0.10-0.01n+nx)^n}\)
Taking logarithm on both sides,
\(Using \,the \,formula log (\frac{a}{b}) = log (a) - log (b)\)
log(k) = log(0.01 − x) − log[x (0.10 − 0.01 n + nx)n]
using the formula log(ab) = log(a) + log (b) and the formula : log ab = b log a
log(k) = log(0.01 -x) - log (x) + n log(0.10 -0.01n + nx) (equation -1)
log(k) = stability constant
n = moles of ligand that reacts with 1 mole of the metal ion
x = molar concentration of Ni(II) ions at equilibrium
As the value of pkb decreases, the ammine becomes more basic and thus it has higher tendency to donate the lone pair from N atom and can form a stronger bond with the metal ion. Thus, it is predicted that with the decrease in pkb, the thermodynamic stability of the complex increases. There is a negative correlation between the pkb and the thermodynamic stability of the complex.
Independent variable basicity of the ligand (measured in terms of pkb)
The purpose of the investigation is to use a variety of amine bases (that contains the group-NH2) which varies in basicity and check how the basicity impacts the stability of the complex they form with Ni(II). The ligands used are – ammonia (NH3), hydroxyl amine (HO-NH2), ethan-1,2-diamine (NH2-CH2-CH2-NH2), amino benzene (C6H5-NH2) and hydrazine (NH2-NH2). pKb is a measure of the basic strength. Lower the value of pKb, higher the pH of the aqueous solution of the base and stronger the basicity. For a more reliable and generalized conclusion, the bases have been chosen in such a way that they cover various electronic and structural effects like the electron withdrawing effect of the benzene ring in aminobenzene that reduces the basicity, negative electron withdrawing effect (-I) of the OH group in OH-NH2 that reduces the basicity. To quantify the basicity, the pKb values have been used.
The values of pKb has been taken from three different sources and a mean value has been used. The sources used are: Source-1-pubchem.com, Source-2-chem.libretexts.org and Source-3:chemguide.co.uk. All of these sources are academic databases and thus are reliable.
The thermodynamic stability of the complex measured as log k, where k is the stability constant will be computed using equation (1). The value of x (molar concentration of Ni(II) at equilibrium) will be determined using the values of absorbance of the solution at equilibrium using a spectrophotometer. The stability of a complex can also be expressed as kinetic stability but that is more of a qualitative indicator of how fast the complex can be made while thermodynamic stability indicates how difficult or easy it is to disassociate the complex. As, the investigation aims to deal with the stability of the complex when they are used in a particular chemical reaction, thermodynamic stability has been measured instead of kinetic stability.
Variable | Why is it controlled? | How is it controlled? |
---|---|---|
Type of the central metal ion | The stability of a coordination complex depends on the oxidation state, ionic radius as well as the outer shell electronic configuration of the central metal ion. | All the complexes made has Ni(II) as the central metal ion. |
Stoichiometry of the metal and ligand | The shape and the co-ordination number of a metal ion in a complex depends on the amount of ligand it reacts with. For example, Ni(II) can form square planar complexes with limited amount of ligand showing co- ordination number 4 as well as a octahedral complex while reacted with excess of ligands showing co-ordination number 6. | In all cases, the amount of ligand used is in excess. 0.01 moles of Ni(II) has been allowed to react with 0.10 moles of the ligand. The metal ion is kept as the limiting reactant in all cases to ensure that all complexes formed are octahedral. |
Shape of the complex ion | For a particular metal ion, the stability may depend on the shape and molecular geometry of the complex ion. | All the complexes formed are octahedral and the metal ion has a co-ordination number of 6. |
Time to reach equilibrium | As the determination of stability constant involves the deduction of the value of molar concentration of Ni(II) at equilibrium, it is essential to ensure that the reversible process of the complex formation has attained equilibrium before any data is recorded. | In all cases, Ni(II) was allowed to react with the ligand for 30.00 mins (monitored using a stop-watch) and allowed to reach equilibrium. |
Apparatus | Quantity | Least count | Absolute uncertainty |
---|---|---|---|
Digital mass balance | 1 | 0.01 g | ± 0.01 g |
Stop-watch | 1 | 0.01 s | ± 0.01 s |
Graduated pipette-1.00 cm3 | 1 | 0.05 cm3 | ± 0.05 cm3 |
Graduated pipette-10.00 cm3 | 1 | 0.05 cm3 | ± 0.05 cm3 |
Spectrophotometer – UV Visible | 1 | 0.001 AU | ± 0.001 AU |
Watch glass | 1 | --- | --- |
Spatula | 1 | --- | --- |
Glass beaker-100 cm3 | 5 | --- | --- |
Graduated measuring cylinder-100 cm3 | 1 | 1.0 cm3 | ± 0.5 cm3 |
Glass cuvette | 1 | --- | --- |
Soft tissues | 1 roll | --- | --- |
Burette-50 cm3 | 1 | 0.05 cm3 | ± 0.05 cm3 |
The ammine compounds used are potentially harmful and corrosive in nature. Exposure to these chemicals
may cause allergic reactions, respiratory disorders and even nausea.
An attempt has been made to minimize the use of consumable resources. For example, dilute solutions have
been used in the investigation to use least possible amount of chemicals.
All waste chemicals were diluted and thrown into a safety bin for disposal.
Note: Nickel(II) chloride is a green solid. When dissolved in water, the chloride ions are replaced by water and thus hexaaquanickel (II) complex ion is formed.
NiCl2 (s) + 6H20 (l) ------→ [Ni(H2O)6]2+ (aq) + 2Cl- (aq)
Moles of NiCl2 added = Moles of [Ni(H2O]6]2+
Mass of NiCl2 added = 0.13 g
\(Moles \,of \,NiCl2 \,added = \frac{mass}{molar\ mass}= \frac{0.13}{129}≅ 0.001\) \(\)
Moles of [Ni(H2O)6] 2+ = 0.01
\(Molar \,concentration \,of \,[Ni(H2O)6] 2+ = \frac{mass}{Volume}= 0.01 mol dm-3\)
Wavelength ± 0.01 nm | Absorbance ± 0.001 AU | Wavelength ± 0.01 nm | Absorbance ± 0.001 AU | Wavelength ± 0.01 nm | Absorbance ± 0.001 AU |
---|---|---|---|---|---|
400 | 505 | 605 | |||
405 | 510 | 610 | |||
410 | 515 | 615 | |||
415 | 520 | 620 | |||
420 | 525 | 625 | |||
425 | 530 | 630 | |||
430 | 535 | 635 | |||
435 | 540 | 640 | |||
440 | 545 | 645 | |||
445 | 550 | 650 | |||
450 | 555 | 655 | |||
455 | 560 | 660 | |||
460 | 565 | 665 | |||
465 | 570 | 670 | |||
470 | 575 | 675 | |||
475 | 580 | 680 | |||
480 | 585 | 685 | |||
485 | 590 | 690 | |||
490 | 595 | 695 | |||
495 | 600 | 700 | |||
500 |
Absorbance of [Ni(H2O)6]2+ in visible range (400-700 nm).
This will be a smooth line scatter plot and the maxima of the curve will be marked. A perpendicular will be drawn from the maxima to the x axes to determine the wavelength at which the absorbance of [Ni(H20)6]2+ complex is maximum. The complex is green in color. Thus, according to the color wheel, this complex should absorb red color and the wavelength of maximum absorbance is supposed to lie within the range of 640 nm to 700 nm. The value reported in literature is 670 nm.
This will be a scatter plot. A linear trend line passing through the origin will be drawn. The equation of the trend line will give a mathematical relationship between absorbance and molar concentration. For example, of the equation is y = mx ; y is absorbance and x is the molar concentration.
\(\text{Thus, molar concentration (y) = }\frac{absorbance\ (x)}{gradient\ of\ Graph - 2\ (m)}\ mol\ dm^{-3} \) (equation - 2)
All the ligands used are in the physical state of liquid at room temperature. The number of moles of the ligand used is 1.00 moles in all cases and it is taken in excess of the metal ion so that the metal ion is the limiting reactant and is completely consumed. The table below shows the volume of ligand to be used.
Formula -
\(\text{Volume of ligand to be used = }\frac{mass}{density}= \frac{moles\ ×\ molar \ mass\ (in\ g)}{density\ at\ room \ temperature\ (in \ g\ cm^{-3} )}\ cm^{3}\) \(\)
Ligend used | Number of moles to be taken | Density In g cm-3 * | Molar mass | Volume to be taken in ± 0.05 cm3 |
---|---|---|---|---|
NH3 (aqueous solution of NH3 – NH40H was used) | 0.10 | 0.697 | 32.04 | 5.10 |
NH2NH2 | 0.10 | 1.028 | 32.04 | 3.10 |
OH-NH2. HCl | 0.10 | 1.679 | 69.49 | 4.20 |
C6H5-NH2 | 0.10 | 1.0210 | 93.13 | 9.10 |
NH2-(CH2)2-NH2 | 0.10 | 0.9011 | 60.10 | 6.70 |
*All values of density are taken from the pubchem.ncbi.nlm.nih.gov database and as this is a government website, the data can be relied upon.
The same process was repeated for other ligands. Refer to Table - 2 for the volume of the ligand to be used in Step-5.
Ligand | pKb | Mean absorbance (± 0.001 AU) | Molar concentration of Ni(II) at equilibrium inmoldm-3 | Moles of ligand that reacts with 1 mole of the metal ion (n) | Stability constant (log k) |
---|---|---|---|---|---|
NH3 | 4.75 | ||||
NH2NH2 | 5.90 | ||||
OH-NH2. HCl | 7.97 | ||||
C6H5-NH2 | 9.13 | ||||
NH2-(CH2)2-NH2 | 4.11 |
Formula to be used: Refer to equation-2 to calculate molar concentration from absorbance and equation-1 to calculate log k. The values of n are there in the equations written in the background information.
Graph-3: Thermodynamic stability (log k) of Ni(II)-ammine complexes at room temperature against the basic strength (pkb) of the ammine compound used as a ligand.