The Chemistry of d- and f- Block: Introduction to d- and f- Block, and Complex Chemistry
In this section, we will have a discussion about the d- and f-block chemistry which in this part is the introduction about d- and f- block elements and compounds. The elements of d- and f-block in this section will include the definition of transition metal or d-block element and how the pattern is fit on the periodic table. Besides that, for the compounds of d- and f-block will mainly discuss about the complex compounds.
To begin with, we will start from the how this area (group 3 - 12) is defined, some of the case is called as d-block and the others called as transition metal. The definition with the name of d-block is unambiguous and it allows us to talk about Zn(II), Cd(II), and Hg(II). Meanwhile, there are 2 definitions for transition metal which are:
The second definition would allow us to include Cu(II), Ag(II), and Au(III), but both definitions exclude Zn(II), Cd(II), and Hg(II). Furthermore in f-block, the first row is known as lanthanoids and the second row is known as actinoids (sometimes it is called lanthanides and actinides). However, the f-block elements cause an ambiguity as which one should be put in group? Are La and Ac or Lu and Lr?
Based on several trends in the neighbouring group (group 4: Ti, Zr, and Hf) shows that the pattern for group 3 is fit where the Lu and Lr is placed in group 3 rather than La and Ac as shown in figure above for the melting point. Moreover, this pattern also works for other properties (e.g. electronegativity or atomic radius).
After a brief overview about the elements, we can move to the complex compounds in d-block. This type of compound was observed for the first time by Alfred Werner, a Swiss Chemist, who won the Nobel Prize for chemistry in 1913. Werner formulated three different Co complexes as shown below with its colour.
Besides that, he reacted the Co compounds with AgNO3 and he found that not all chloride reacted with AgNO3 as shown on the reaction below, which is fit with the current formulation.
Moreover, as you noticed from the table earlier, the square brackets are used to indicate metal complex.
The understanding of metal complexes is very important in our life as many our daily life is related to them. The first thing is vitamin B12 or cyanocobalamin is a cobalt complex, and this compound is abundant in such as marmite. Besides that, another bioinorganic complex compound is haemoglobin which has an important role to carry oxygen in our body.
Metals can also react with CO to form Metal-CO complex, for example Fe reacts with CO to form Fe(CO)5 which is a yellow oily liquid. Another example is Ni reacts with CO to form Ni(CO)4 which is a colourless gas and this reaction is used for Ni metal extraction.
Gemstones are also the example of metal complexes such as ruby and sapphire. The red colour of ruby comes from Cr oxide impurity, while the blue colour of sapphire comes from Ti and Fe oxide impurities. Both gemstones are known as the corundum where it has an octahedral-based geometry; Octahedral is one of the most common geometries of complex compounds.
Back to the Werner's work, he also realised that in metal complexes, the metal is surrounded by 6 groups. Later, he recognised that sometimes the metal is surrounded by 4 groups. This term was called the "secondary" valencies by Werner as now referred to as the coordination number. Meanwhile, the "primary" valencies in Werner's term is now called oxidation states. Following the coordination number, this term is related closely to the chemical formulae, but sometimes it can be confusing.
For example, as most of us acknowledge that CH4 has 4 coordination number, OH2 has 2 coordination number, and HCl has 1 coordination number. However, NaCl, CaCl2, and FeCl2 have 6 coordination number (instead of 1 for NaCl, and 3 for CaCl2 and FeCl2), CrO3 has 4 coordination number (instead of 3). Those coordination numbers can be determined from their lattice structure not simply from the formulae. The other examples are KMnO4 where there is no Mn - K bond, so it should be written as K[MnO4]; and BaCuF4 should be written as Ba[CuF4] as there is no Ba - Cu bond and the coordination number of Cu-F is 6.
A complex compound can be defined as a metal surrounded by a set of ions or molecules as ligands and each metal-ligand interaction is a two-electron interaction (covalent bond). Ligands are usually electron pair donors (dative bonding) and metals are usually electron acceptors, so we can say that metal-ligands system is a lewis acids-bases system. Besides that, all ligands bind through a σ-bond where the metal provides the empty hybrid orbital so the electron from the ligands can fill it in. The bonding of complex compounds can be seen as two point of view, ionic view and covalent view.
For example is TiCl4, if it is seen from the ionic view, so TiCl4 is Ti4+ ion surrounded by 4 Cl- ions. In the other sides, from covalent view it is seen as Ti bound to 4 Cl through shared electron covalent bonds. Another example is PtCl2(NH3)2, where Pt2+ ion surrounded by 2 Cl- ions and 2 NH3 molecules. Meanwhile from covalent view as seen as Pt bound to 2 Cl through shared electron covalent bonds and to 2 NH3 molecules through dative bonds from NH3. In some cases, the ionic view and covalent view has the same result such as [Co(NH3)6]2+.
Early on, we had a brief definition about coordination number; the coordination number is the number of attached atoms, not the number of attached ligands, which is different if any of the ligands are multidentate. One of the most common coordination numbers in complex is 6 where the most common geometry is octahedral, but in some cases can have trigonal prismatic.
Another common geometry in complex compounds is 4 coordination number as most of it is tetrahedral and some very important structures are square planar (usually in compounds d8).
PtCl2(NH3)2 is known as cisplatin which is commonly used for chemotherapy drug.
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| d- and f-block elements |
To begin with, we will start from the how this area (group 3 - 12) is defined, some of the case is called as d-block and the others called as transition metal. The definition with the name of d-block is unambiguous and it allows us to talk about Zn(II), Cd(II), and Hg(II). Meanwhile, there are 2 definitions for transition metal which are:
- those which as elements have partly filled d- or f-shells,
- those elements that have partly filled d- and f-shells in their most common compounds.
The second definition would allow us to include Cu(II), Ag(II), and Au(III), but both definitions exclude Zn(II), Cd(II), and Hg(II). Furthermore in f-block, the first row is known as lanthanoids and the second row is known as actinoids (sometimes it is called lanthanides and actinides). However, the f-block elements cause an ambiguity as which one should be put in group? Are La and Ac or Lu and Lr?
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| The melting point of group 3 and 4 elements |
Based on several trends in the neighbouring group (group 4: Ti, Zr, and Hf) shows that the pattern for group 3 is fit where the Lu and Lr is placed in group 3 rather than La and Ac as shown in figure above for the melting point. Moreover, this pattern also works for other properties (e.g. electronegativity or atomic radius).
After a brief overview about the elements, we can move to the complex compounds in d-block. This type of compound was observed for the first time by Alfred Werner, a Swiss Chemist, who won the Nobel Prize for chemistry in 1913. Werner formulated three different Co complexes as shown below with its colour.
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| Cyanocobalamin (left) may be contained in marmite (centre), and haemoglobin (right) |
Metals can also react with CO to form Metal-CO complex, for example Fe reacts with CO to form Fe(CO)5 which is a yellow oily liquid. Another example is Ni reacts with CO to form Ni(CO)4 which is a colourless gas and this reaction is used for Ni metal extraction.
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| Ruby (left) and sapphire (right) |
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| Corundum, octahedral-based geometry |
Back to the Werner's work, he also realised that in metal complexes, the metal is surrounded by 6 groups. Later, he recognised that sometimes the metal is surrounded by 4 groups. This term was called the "secondary" valencies by Werner as now referred to as the coordination number. Meanwhile, the "primary" valencies in Werner's term is now called oxidation states. Following the coordination number, this term is related closely to the chemical formulae, but sometimes it can be confusing.
For example, as most of us acknowledge that CH4 has 4 coordination number, OH2 has 2 coordination number, and HCl has 1 coordination number. However, NaCl, CaCl2, and FeCl2 have 6 coordination number (instead of 1 for NaCl, and 3 for CaCl2 and FeCl2), CrO3 has 4 coordination number (instead of 3). Those coordination numbers can be determined from their lattice structure not simply from the formulae. The other examples are KMnO4 where there is no Mn - K bond, so it should be written as K[MnO4]; and BaCuF4 should be written as Ba[CuF4] as there is no Ba - Cu bond and the coordination number of Cu-F is 6.
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| Complex formation |
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| σ-bond in complex compound (metal-ligand interaction) |
For example is TiCl4, if it is seen from the ionic view, so TiCl4 is Ti4+ ion surrounded by 4 Cl- ions. In the other sides, from covalent view it is seen as Ti bound to 4 Cl through shared electron covalent bonds. Another example is PtCl2(NH3)2, where Pt2+ ion surrounded by 2 Cl- ions and 2 NH3 molecules. Meanwhile from covalent view as seen as Pt bound to 2 Cl through shared electron covalent bonds and to 2 NH3 molecules through dative bonds from NH3. In some cases, the ionic view and covalent view has the same result such as [Co(NH3)6]2+.
Early on, we had a brief definition about coordination number; the coordination number is the number of attached atoms, not the number of attached ligands, which is different if any of the ligands are multidentate. One of the most common coordination numbers in complex is 6 where the most common geometry is octahedral, but in some cases can have trigonal prismatic.
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| W(CH3)6-trigonal prismatic geometry (left) and W(CO)6-octahedral geometry (right) |
Another common geometry in complex compounds is 4 coordination number as most of it is tetrahedral and some very important structures are square planar (usually in compounds d8).
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| [MnCl4]2- -tetrahedral geometry (left) and PtCl2(NH3)2-square planar geometry (right) |
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