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Abstract

Jobs Method was used to the determine the empirical formulas of the coordinate complexes formed under the specific conditions and various concentrations of the ligands used. These were examined with a spectrometer for the concentration ratio with the highest concentration of the coordinate complex needed. The results of this were EDTA was the better ligand for the strength of hold of the ligand to the metal ion. Then En was the next best, by being a bidentate ligand and SCN- coming in last due to the ligand only having one electron share site. An IR spectra was used to determine the actuality of the metal ion complexes being considered hard or soft. In turn, whether the binding mode was S-bound or N-bound determined the hard or soft consideration. Ag+ was the only S-bound metal complex making this the only soft acid of the complexes tried.

Introduction

Coordination complexes, also known as metal complexes, are a class of compounds that consist of a central metal ion surrounded by ligands. These ligands are typically electron-rich species, such as ions or molecules, that donate electrons to the metal ion to form a coordinate covalent bond. Coordination complexes are an important class of compounds due to their widespread use in many areas of chemistry, including catalysis, bioinorganic chemistry, and materials science.(1)

One of the key features of coordination complexes is the presence of different isomers, including ionization, hydration, and linkage isomers. These isomers result from different arrangements of ligands around the central metal ion, and they can have different chemical and physical properties. For example, linkage isomers have ligands bonded to the metal ion through different atoms, resulting in different bond strengths and reactivities. (2)

Ligand denticity is another important feature of coordination complexes. This refers to the number of donor atoms in a ligand that are available to bond with the metal ion. The chelate effect, which is the increased stability of a complex due to the presence of a chelating ligand, is an important consequence of ligand denticity. (3)

Furthermore, coordination chemistry is intimately connected with hard-soft acid-base theory (HSAB). According to this theory, molecules can be classified as "hard" or "soft" based on their electronic properties. Hard acids prefer to bind to hard bases, while soft acids prefer to bind to soft bases. The interactions between hard and soft acids and bases can have important consequences for the stability and reactivity of coordination complexes. (4)

A Jobs plot identifies the ratio of ligand to metal at stoichiometric proportions and thus allows one to determine the formula of the coordination complex, (5). A Jobs Plot experiment can be compared along the lines with a titration experiment. In a titration experiment the starting material will be that of a reactant and thus a titrant is added to measure the effect that this addition of substance has on the reaction. For this titration experiment there is only one solution as the addition of the titrant occurs thus creating a two process system, (reaction with the titrant and dilution). Comparing this to a Jobs Plot experiment, a Jobs Methods of Continuous Variations has multiple solutions being measured along with a constant volume of reagents mixed to measure the proportions of this reaction. (5)

The Jobs Plot was chosen instead of a titration because spectroscopy allows separation of color where in the experiment, there were colored complexes and metal solutions involving colorless ligands. HSAB theory was used to determine a metal should considerably be hard or soft. In this report, exploration of the properties and behavior of coordination complexes in two experiments, one of which strongly relies on HSAB theory and the other using Jobs Methods of Continuous Variations.

Discussion/Results

Bidentate is explained by sharing two electrons to a transition metal whereas monodentate only shares one electron with the transition metal. The more spaces where electrons can attach themselves on a transition metal, the stronger the hold on the transition metal. (6)

SCN- is labeled as monodentate ligand with only one electron shared with Fe compared to En which is a bidentate ligand with two electrons shared with Ni, meaning that En is the better ligand to use for the transition metals, because there is two spots that the electrons are able to combine and hold on to on the transition metal. This can be shown in comparison to figure 1 and figure 2 Jobs Plots. Compared to the bidentate and monodentate ligands, EDTA was hexadentate meaning that EDTA has 6 donor atoms available making EDTA the strongest hold of the three ligands shown in figure 3.

According to the IR stretching frequencies of the unknowns shown in table 1, it can be seen that Ag+ is the only one whose v was smaller for the Co reference than the Hg making Ag+ the only soft acid. Also from this table it can be seen that Ag+ also is S-bound rather than N-bound compared to the other 6 unknowns. This can be found through the size of the element as well. As charge/size increases so does the hardness of the acid. (Reference compounds of the S-bound and N-bound shown in figure 4.)

Unknown K2Co(NCS)4 K2Hg(SCN)4 Mn2+ 57.06 138.0 Fe2+ 47.6 132.2 Co2+ 41.68 130.57 Ni2+ 5.96 111.2 Cu2+ 26.04 132.36 Zn2+ 121.28 173.62 Ag+ 10.3 16.5 .

Conclusion

Jobs Method was used to determine the strongest hold of the metal ion complexes. The more electron sharing ability was the best for strength of the coordination complexes. For example, the hexadentate metal ion complex EDTA, has the strongest hold of the metal ion Cu2+. From hexadentate to bidentate En ligand being the next best, making the monodentate the weakest hold of the metal ion Fe3+ to SCN- ligand due to the amount of electron sharing atoms that the ligand had. Determining whether the ligand is soft or hard, an IR spectra was used and the IR stretching frequencies was then compared to unknown complexes. The Ag+ was the only soft acid due to the difference of IR stretching frequencies that was concluded along with it being bound to S.