Chemistry HL
Chemistry HL
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Effect of basicity of ammine ligands on thermodynamic stability of Ni (II)-ammine complexes

Table of content

Research question

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?

Rationale

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.

Background information

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  • Ni(II) – ammine complexes

    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).

    Reaction of Ni(II) with the ammine ligands to form the complex ions

    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.

    Deduction of mathematical formula to determine thermodynamic stability constant

    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:

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  • [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)
    Figure 1

    \(k = \frac{(0.01-x)}{x(0.10-0.01n+nx)^n}\)

     

    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

    Hypotheses

    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.

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  • 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.com3 , Source-2-chem.libretexts.org4 and Source-3:chemguide.co.uk5 . All of these sources are academic databases and thus are reliable.

    Figure 2 - Table On pkb values of the ligands used

    Dependent variable thermodynamic stability – log (k)

    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.

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  • Controlled variable

    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.
    Figure 3 - Table On Controlled Variable

    Apparatus required

    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

    Figure 4 - Table On Apparatus Required

    List of materials required

    Figure 5 - Table On List Of Materials Required

    Safety precautions

    The ammine compounds used are potentially harmful and corrosive in nature. Exposure to these chemicals
    may cause allergic reactions, respiratory disorders and even nausea.

    • A laboratory coat was worn always.
    • To restrict inhaling the fumes and smokes, a safety masks was used and gloves as well.
    • All solutions were prepared carefully under strict supervision of a laboratory technician to avoid
      spillage.

    Ethical considerations

    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.

    Environmental considerations

    All waste chemicals were diluted and thrown into a safety bin for disposal.

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  • Experimental procedure

    Finding the peak absorbance wavelength for hexaaquanickel(II) complex ion.

    • A 100 cm3 glass beaker was taken.
    • A watch glass was taken and placed on a top pan digital mass balance.
    • The balance was tared to a reading of 0.00 ± 0.01 g.
    • Nickel(II) chloride, green solid was transferred to the watch glass using a spatula until it reads 0.13 ± 0.01 g.
    • The weighed solid was transferred to a 100 cm3 glass beaker.
    • Distilled water was added till the mark.
    • A glass rod was used to dissolve the solid.
    • Two cuvettes were taken and one of them was filled with distilled water while the other one was filled with the 0.10 moldm-3 Ni(II) solution.
    • The distilled water was used as a sample and the Ni(II) solution was used as blank.
    • The absorbance of the Ni(II) sample solution was recorded in the range of 400 nm to 700 nm at an
      interval of 5 nm.

    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
    Figure 6 - Table On Data Collection For Absorbance At Wavelength In The Visible Region (400 nm To 700 nm) For Ni(II)

    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 nm6.

    Determination of calibration curve for Ni (II) solution

    • A 100 cm3 glass beaker was taken.
    • On a watch glass, 0.13 ± 0.01 g (0.001 moles) of NiCl2 was weighed using a top pan digital mass balance.
    • The weighed solid was transferred to the same beaker.
    • Distilled water was added till the mark of 100 cm3
    • A glass rod was used to stir the solution.
    • A cuvette was filled with the solution (to be used as sample) and another one was filled with (to be used as blank).
    • The absorbance of the sample was recorded at (insert the wavelength entered from Figure - 6).
    • Steps 6 and 7 were repeated for two more times.
    • All of the above steps were repeated using 0.26 ± 0.01 g (0.002 moles), 0.39 ± 0.01 g (0.003 moles), 0.52 ± 0.01 g (0.004 moles) and 0.65 ± 0.01 g (0.05 moles) of NiCl2.
    Figure 7 - Table On Raw Data Of Absorbance Against Concentration Of Ni(II) For Standard Calibration Curve

    Absorbance versus concentration (standard calibration curve) of Ni(II)

    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)

    Determining the stability constant

    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
    Figure 8 - Table On Calculation Of Volume Of Ligands To Be Used

    *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.

    • A 100 cm3 glass beaker was taken.
    • Following this, 1.29 ± 0.01 g (0.01 moles) of NiCl2 (green solid) was weighed on a watch glass using a top pan digital mass balance.
    • After that, 50.0 ± 0.10 cm3 of distilled water was added to the beaker using a burette.
    • A glass rod was used to stir the solution, dissolve the solid and obtain a clear green colored solution.
    • A 10.00 cm3 graduated pipette was used to add 5.10 ± 0.05 cm3 of liquid ammonia (NH4OH) to the same beaker.
    • A glass rod was used to stir the reaction mixture and mix the ligand homogenously.
    • The stop-watch was started.
    • As soon as the stop-watch reads 30.00 ± 0.01 mins, a 1.00 cm3 graduated pipette was used to fill one of the cuvettes and the other cuvette was filled with distilled water. The color of the reaction mixture was noted down.
    • The UV-Visible spectrophotometer was set at (insert the value of wavelength you get in Graph-1)
    • The absorbance of the sample was noted.
    • Steps 8-10 were repeated for two more times.

    The same process was repeated for other ligands. Refer to Table - 2 for the volume of the ligand to be used in Step-5.

    Figure 9 - Table On Raw Data For Absorbance Of Ni (II) At Equilibrium

    Data processing

    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
    Figure 10 - Table On Determination Of Thermodynamic Stability Constant

    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.

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