Science Makes Sense- Week 22:Transition elements, complex chemistry, d-orbitals, chelation

Have you noticed how chemists are usually depicted in magazines or advertisements? You will see serious-looking men or women in lab coats surrounded by brightly colored yellow, emerald-green or purple liquids in beakers or other containers. Nine times out of ten, most chemists pour and mix colorless, boring solutions and rarely do they create exciting-colored chemicals! The exception to this rule are those chemists who study complex chemistry and work with the transition elements.

The transition elements are the three rows or periods in the Periodic Table that are in the center between the alkaline earth metals on the left and the non- metals on the right. These are a bunch of almost 30 elements where in each row the ‘d’ shell containing ten electrons are being filled. (Ref. 1)

Most of the elements where the ‘d’ shells are being filled exhibit some interesting properties. They are metals (sometimes they are referred to as transition metals instead of transition elements), brightly colored, show different ratios in which they combine with other elements (which means they have different oxidation states) and some catalytic activity. They also combine in complex ways with metals and non-metals. (Ref. 2). In fact, the study of transition elements is a special branch of chemistry called complex chemistry. We will study why the filling up of the ‘d’ shells leads to these exceptional properties.

The definition of a transition metal is one which forms one or more stable ions which have incompletely filled ‘d’ orbitals. According to this definition, zinc, Zn, and scandium, Sc, are not really part of the transition metals, since their ions do not have any ‘d’ shell electrons.(Ref. 3)

Transition metals are good conductors of heat and electricity, less reactive than Group I alkali metals, but have high melting points, (exception is mercury,Hg, which is a liquid at room temperature), and high densities. (Ref. 4) They also form complex compounds, exhibit chelation, are good catalysts and as we already mentioned, form many colored compounds.

Let us look at four properties when we study these elements. Unlike most other metals, these transition metals do not just form simple ionic compounds like the alkali and alkaline metals. (Group I and II) Apart from the simple ionic compounds like nickel, copper, iron or cobalt chloride, they also form some complex/ coordination compounds. Many atoms or molecules called ligands, can attach themselves to these transition metals These ligands can be electron pair donor in a coordinate covalent bond. Common ligands are water, ammonia, carbon monoxide and anions (negatively charged ions) like cyanide ions, chloride ions and hydroxide ions. Sometimes these ligands are cations (positively charged ions) and electron acceptors. No wonder this branch of the study of transition metals is sometimes called complex chemistry. (Ref. 5)
Second, sometimes these ligands can be five identical carbon monoxide molecules that attach to iron, Fe, or six water ligands to nickel,Ni. These multi-dentate ligands attached to a central metal ion are called a chelating agent. ‘ Chelate’ means ‘crab-like’ and it feels like these five or six or however many ligands are like the pincers of a crab. These chelating agents have significant biological applications.(See nuggets)(Ref. 6)

 

 

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Thirdly,transition metals are good catalysts. One of the reasons is that they can exist in a variety of oxidation states. This has the property of providing or withdrawing electrons from the intermediate stage of a reaction. For example, if a particular intermediate stage of a reaction is electron rich, the transition element could hold some of the electron density and facilitate the reaction. Or the transition metal might undergo oxidation- reduction reaction to achieve electron transfer to a substrate; this again allows the reaction to take place. In other words, a transition metal, existing in a variety of oxidation states and undergoing easy transitions between these oxidation states, can coordinate with a substrate, and also be a source or a sink for electrons, making it a good catalyst. (Ref. 7)

Finally, let us look at the reason for bright colors for transition metals. The colors of the transition metals are best described by the crystal field theory. Electrons in the d- orbital or shell are not at the same energy when there are molecules or ions attached to the metal ion. These molecules or ions are called ligands, as we already know. Ligands split the ‘d’ levels into two or more different energy levels. We see colors because the energy level differences in the ‘d’ levels is in the visible light range. When light strikes ions in solution, some of the energy with visible wavelength is absorbed by the electrons moving from a lower to a higher energy level. We see different colors because of a slight difference in energy levels that the ‘d’ levels split into. If the energy difference is greater, electrons will absorb light closer to the blue-violet end of the spectrum, and the solution will appear yellow. If the difference in the ‘d’ energy levels is less,the electrons absorb closer to the red end of the spectrum and the solution appears green; anywhere in between you will cover all the other colors. This explains why transition metals are colored. (Ref.8)

Activities for Middle School Teachers:
Before studying transition metals, students should be very familiar with the Periodic Table. They should also be able to express most elements using the electronic configuration, especially the filling up of ‘s’,’p’, and ‘d’ orbitals.
Students should understand that visible light is part of the electromagnetic spectrum and is a form of energy. They should also do research on how we perceive color in any object.
What is the role of a catalyst in any reaction? Do a comparison of enzymes and catalysts.

Nuggets of Information:
Ethylenediamine tetra acetic acid, EDTA, has 4 acetic acids,(tetra acetic acid) 2 nitrogens(diamine) on a 2 hydrocarbon chain.( Ref. 9). Chelates are biologically very signifiant. EDTA oral fluid is made up of several compounds containing transition metals and these are chelates used for curing many health issues. (Ref. 10)
Metal poisoning is treated using chelation therapy. Transition metals captures the toxic metals from the body, chelates them so that they are passed out of the excretion system of the body. (Ref. 10)
Most transition metals like iron, Fe, and chromium Cr, are key ingredients in automobiles, bridges, usually as alloys. One of the greatest applications is in cookware. They are also used in mercury thermometers, colored paints and photo reactive eye glasses. Titanium, Ti, is used to detect underwater sound, barium titanate is piezoelectric. (generates electric charge when slightly distorted)(Ref.11) Iron, Fe, along with cobalt, Co and Nickel, Ni, are magnetic and used for magnetic devices, cassette tapes, computer discs. Copper, Cu and cadmium, Cd, are used in photovoltaic cells. (Ref.11)
Human beings need two transition metals that are very essential: one is iron that is a complex compound in hemoglobin and cobalt as a complex compound in Vitamin B12.
Low iron leads to anemia and Vitamin B12 is crucial in the human diet. ( Ref.11)

REFERENCES:
1.chem-guide.blogspot.in(put in transition metals in search site)
2.chemistry-assignment.com (put in transition metals in search site)
3.hyperphysics.phy-Astro.gsu.edu
4.bbc.co.uk/schools/gcse….ansitionmetalsrev1.html
5.Britannica.com (put in ligand chemistry in search site)
6.sci-fun.chem.wisc.edu
7. madsci.org (ask the question ” why are transition metal compounds good catalysts?”)
8.answers.yahoo.com/question/index?qid=9.
9. articlesfactory.com (put EDTA in search site)
10. articlesfactory.com (put biologically significant chelates)
11.chem.uwec.edu/Chem115_F02/bat

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