Inorganic Chemistry for Bio - Part 2 (Chapter 5-7) PDF

5. The Chemistry of Hydrogen  Hydrogen is the first element in the table.  It is the smallest element on the table.  It has atomic number one, which means it has only one electron orbiting it its shell.  In fact, Hydrogen has only one shell.  It is also the lightest element on the periodic tabl...

5. The Chemistry of Hydrogen  Hydrogen is the first element in the table.  It is the smallest element on the table.  It has atomic number one, which means it has only one electron orbiting it its shell.  In fact, Hydrogen has only one shell.  It is also the lightest element on the periodic table. What is the position of Hydrogen the periodic table?  The position of elements on the periodic table largely depends on their electronic configuration.  Hydrogen has the electronic configuration of 1.  Difficult to attain noble gas configuration.  This characteristic of hydrogen matches those of alkali metals.  But the hydrogen atoms can also gain one electron like halogens. 1  Cont… Position of hydrogen in periodic table  Lightest element known having atomic number 1  Dihydrogen  It resembles both alkali metals and halogens, and therefore its position is anomalous  In modern periodic table it is located separately Resembles with alkali metals 1. Electronic configuration: both hydrogen and alkali metals have ns1valence shell electronic configuration. 2. Electropositive character 3. Oxidation state: +1 4. Combination with electronegative elements: both hydrogen and alkali metals can form binary compounds with electronegative elements. E.g., HCl, NaCl, KCl, H2S, Na2S, K2S, etc. 2  Cont… Resembles with halogens 1. Electronic configuration: both contains one electron less than the nearest noble gas configuration. 1H = 1s1 (2He = 1s2), 9F = 1s22s22p5 (10Ne = 1s22s22p6), 17Cl = 1s22s22p63s23p5 (18Ar = 1s22s22p63s23p6) 2. Non-metallic character: both have non-metallic nature. 3. Atomicity: both are diatomic in nature. 4. Formation of similar types of compounds: CCl4,SiCl4, GeCl4, CH4,SiH4, GeH4 5. Oxidation state: -1, Na+1H-1, Na+1Cl-1 3  Cont… Difference from alkali metals: 1. Ionization enthalpy: Na has an atomic radius of 186 pm and hydrogen has a much smaller atomic radius of 37 pm, as a result, there is more effective nuclear charge for the H atom than Na, which causes high IE. 2. Non-metallic character: alkali metals are typical metals while hydrogen is non-metal in nature. 3. Atomicity: hydrogen is diatomic while alkali metals are monoatomic. 4. nature of compounds: compounds of hydrogen are predominantly covalent while those of alkali metal compounds are ionic. 4  Cont… Difference from halogens: 1. Less tendency of hydride formation: due to the electronegativity difference between hydrogen and halogens, hydrogen has less tendency to form hydride (H-) as compared to the tendency of halogens to form halides (X-) 2. Absence of unshared pairs of electrons: there is no unshared electron around the hydrogen nucleus in the compounds of hydrogen, while there are unshared electrons around the halogen's nucleus in the halide compounds. 3. nature of oxides: oxides of halogens are acidic while hydrogen oxide is neutral. 5 Isotopes of Hydrogen Cpds of Protium and Deuterium 6  Occurrence of hydrogen  It is the most abundant element in the universe, and the third most abundant element on the surface of the globe, it being visualized as the major future energy source. Relative abundancies of hydrogen isotopes Preparation of hydrogen 1. Electrolysis of water: 2H2O (l) 2H2(g) + O2(g) 7  Cont…  The hydrogen prepared by this method is highly pure. However, this method is not commonly used because it is very expensive. The method is only available on the place of cheap electricity. 2. By the reaction between steam and cock: C + H2O (g) CO(g) + H2(g)  Since the mixture of CO and H2 is used for the synthesis of methanol and hydrocarbon, it is also called synthesis of gas or syn of gas  The process of producing gas from cock or coal is called coal gasification. 8  Properties of hydrogen Physical properties  Hydrogen has the second lowest boiling point and melting points of all substances, second only to helium.  Pure hydrogen is odorless, colorless and tasteless.  Hydrogen is non-toxic but can act as a simple asphyxiant by displacing the oxygen in the air.  It is almost insoluble in water (only about 2%)  It is highly combustible and therefore handled carefully.  It is the lightest substance( one litter of hydrogen at STP weighs 0.0088 g) 9  Properties of hydrogen chemical properties  Hydrogen is non-toxic but can act as a simple asphyxiant by displacing the oxygen in the air. (oxygen levels below 19.5% are biologically inactive for humans).  The hydrogen molecule (H2) is not highly reactive due the high bond dissociation energy (435.88 KJ/mol.)  But the atomic form of hydrogen is highly reactive.  The half life period of the hydrogen atom is 0.3 sec, and therefore, it immediately get converted into the molecular form liberating a large amount of energy which used for cutting and welding purpose. 10  Reaction of hydrogen Combustion: it burns with pale blue flame 2H2(g) + O2(g) → 2H2O(l) Reactions with metals: reactive metals like Na, K, Ca can react with H to form hydrides. E.g., Ca(s) + H2(g) → CaH2(s). Hydrogen can also form interstitial (non-stoichiometric) compounds with different transition metals like Ni, Pt, Pd, etc. Reaction with metal oxides: hydrogen reduces oxides of less active metals e.g., Fe2O3 + 3H2→ 2Fe + 3H2O CuO + H2 → Cu + H2O Reaction with non-metal: 3H2(g) + N2(g)→ 2NH3(g) (with the presence of transition metal catalysts like Fe, Mo at 673 K and 200 atm) called Haber process. 11  Cont… Reactions with carbon monoxide: CO + H2 → CH3OH (in the presence of ZnO or Cr2O3 catalysts at 700 K and 200 atm) Reaction with unsaturated hydrocarbon: H2C ═ CH2 + H2 → H3C ═ CH3 (in the presence of Ni, Pd or Pt catalysts at 473 K) Hydrogenations of oil: it involves the addition of hydrogen into the unsaturated fat with Ni catalyst. 12  Uses of hydrogen  It used as a reducing agent.  It used for the manufacturing of vanaspati fat, ammonia, metal hydrides, methanol, fertilizers such as urea, etc. 3H2(g) + N2(g)→ 2NH3(g) CO2(g)+ NH3(g)→ CH4N2O(s) (urea)  It used in the manufacture of synthetic petrol.  Welding – especially of steel, with oxyhydrogen welding.  In the fuel cell for generating electrical energy.  It is used to produce hydrogen peroxide. (H2O2 has medical applications on the skin to prevent infection of minor cuts, scrapes, and burns).  etc. 13  Hydrogen compounds (hydrides)  Under certain conditions hydrogen can combine with almost all elements except noble gases to form compounds called hydrides.  There are three types of hydrides. These are: i. Ionic / saline hydrides – compounds containing hydrogen & most s-block elements which are highly electropositive. ii. Covalent / molecular hydrides - compounds containing hydrogen & most p-block elements. These group of hydride can also be classified as electron deficient (e.g., BH3, AlH3, etc.), electron precise (e.g., CH4, SiH4, GeH4, SnH4, etc.) and electron rich hydrides (e.g., NH3, PH3,H2O H2S, etc.). 14  Cont… iii. Metallic / non-stoichiometric hydrides  These type of hydrides are formed by many d-block and f-block elements  Can conduct heat and electricity though not efficient  They exhibit metallic properties and are powerful reducing agent.  Their composition varies with temperature and pressure. In these hydrides, the law of constant composition does not hold good.  Examples: LaH2.87, TiH1.73, ZrH1.75 15  Hydrogen Bonding.  Hydrogen bonding is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom.  It results from the attractive force between a hydrogen atom covalently bonded to a very high electronegative atom such as a N, O, or F atom and another very electronegative atom. Water 16  Hydrogen bonding in biological systems  Hydrogen bonding is of immense importance in biological systems.  One of the best-known examples is the formation of the double helical structure of DNA (deoxyribonucleic acid).  The structures of adenine and thymine are exactly matched to permit hydrogen bonding between them, and they are referred to as complementary bases.  Guanine and cytosine form the second base pair.  The hydrogen bonding between these base pairs in the strands of DNA.  The DNA double helix is held together by two types of bonds, covalent and hydrogen. 17  Covalent bonds occur within each linear strand and strongly bond the bases, sugars, and phosphate groups.  Hydrogen bonds occur between the two strands and involve a base from one strand with a base from the second in complementary pairing. These hydrogen bonds are individually weak but collectively quite strong. 18 19  6. The chemistry of main group elements Group assignment Group 1- the chemistry of alkali metals Group 2- the chemistry of alkaline earth metals Group 3- the chemistry of boron family elements Group 4- the chemistry of carbon family elements Group 5- the chemistry of nitrogen family elements Group 6- the chemistry of chalcogens Group 7- the chemistry of halogens Group 8- the chemistry of noble gases 20  Unit 7. Chemistry of transition elements Objectives: By the end of this topic students will be able to:  Explain the general physical and chemical properties of d-block elements  Explain the magnetic properties of transition metal complexes  Explain how transition metal complexes can be colored  Understand why transition elements have variable oxidation state  Recognize why transition elements tend to form nonstoichiometric compound  Able to name different coordination compounds 21  Cont.… Introduction  The elements in the modern periodic table that fall between the 3rd and 12th groups are known as d- block elements.  These elements' valence electrons are located in the d orbital.  D-block elements are also called transition elements/metals, however there is a subtle difference between the two terms.  D-block elements: are all those whose valence electrons have entered the d orbital.  Transition elements: are the d-blocks having at least one stable cation with partly filled d orbital.  E.g. Sc (stable oxidation state = +3, but there is no partially field d orbital) and Zn (stable oxidation state = +2, but there is no partially field d orbital) are not transition metals. 22  General physical properties of d-block elements In general, d-block elements have:  high melting and boiling point (due to the presence of strong metallic bond)  A tendency to form colored complex (due to the presence of unpaired d electrons and vacant d- orbital in order for a d-d transition, LMCT & MLCT transitions take place)  Magnetic properties (due to the presence of unpaired electrons in (n-1) d)  Very high densities compared to alkali and alkaline earth metals  Except Hg which is liquid at room temperature, all transition elements are solid/ have a typical metallic structure.  The general electronic configuration of transition element is (n-1)d1-9ns0-2 23  Magnetic property of d-block elements  Understanding the orbital arrangement for complexes with tetrahedral, square planar, octahedral, and other geometries using crystal field theory is crucial in addition to ligand field strength in order to explain the magnetic behavior and color of metal complexes. Order of ligand field strength obtained from spectrochemical data I- < Br- < [NCS]- < Cl- < F- < [OH]- < [ox]2- ≈ H2O < [NCS]- < NH3 < en < bpy < phen < [CN]- ≈ CO 24  Cont.…  A substance that repelled by a strong magnetic field is termed as diamagnetic, and that attracted by a strong magnetic field is termed as paramagnetic.  Transition metals and their complex have a tendency to exhibit magnetic property.  The d-block element that shows a paramagnetic property contains unpaired electron in the d orbital.  The d-block element shows a diamagnetic property if all electrons in the d orbital are spin paired.  Most of the d-block metal atoms and ions have unpaired electrons, so that they have paramagnetic property.  Some transition metals such as Fe, Co and Ni can contain more unpaired electron in their high spin case and show more paramagnetism (so called ferromagnetism) than the other d-block metals. 25  Cont.…  An orbital moment and a spin moment are produced due to the orbital motion and electron spin respectively.  In order to maintain magnetic characteristics in an atom, ion, or molecule, an interaction between spin magnetic moment and orbital magnetic moment must occur.  The unit of magnetic moment is Bohr magneton (BM).  The higher the magnetic moment value is the more paramagnetic character of the compound.  There are two ways to express the formula for calculating the magnetic moment: one based on the quantity of unpaired electrons (n), and the other on the electron spin quantum number (S). μ= 𝑛(𝑛 + 2) BM or μ= 4𝑠(𝑠 + 1) BM 26  Cont.… The μ of the d1 complex = 𝑛(𝑛 + 2) BM = 1(1 + 2) = 3 = 1.73 BM e.g. Ti3+ complex The μ of the d2 complex = 𝑛(𝑛 + 2) BM = 2(2 + 2) = 8 = 2.83 BM e.g. V3+ complex The μ of the d3 complex = 𝑛(𝑛 + 2) BM = 3(3 + 2) = 15 = 3.88 BM e.g. Cr3+ complex The μ of the d4 low spin octahedral complex = 𝑛(𝑛 + 2) BM = √(2(2+2)) =√8 = 2.83 BM e.g. Cr2+ complex with strong field ligands such as CO and CN- The μ of the d4 high spin octahedral complex = 𝑛(𝑛 + 2) BM = √(4(4+2)) =√24 = 4.90 BM e.g. Cr2+ complex with weak field ligands such as H2O 27  Cont.… Exercise 1. Determine the magnetic moment for each of the following complexes and justify whether they are paramagnetic or diamagnetic? A) [Fe(CN)6]4- B) [Fe(H2O)6]2+ C) [Co(CN)6]3- D) [CoCl4]2- E) [Ni(CN)4]2- 28  The tendency of d-block elements in colored complex formation  A portion of the radiation's energy is absorbed by electrons during the d-d transition, while the remaining energy is released as colored light.  An ion's color is the exact opposite of the color it absorbs.  A colored ion is produced as a result of the d-d transition, which takes place in the visible region for every transition element.  The color of most transition metal complexes is usually caused by the d-d transition, despite the fact that certain transition metal compounds, such as CrO42- (Cr6+, d0, yellow), MnO4- (Mn7+,d0, purple) have color due to charge transfer transition (without d-d transition).  When ligands approach to the metal d orbitals, the five degenerate orbitals are separated into two sets (eg & t2g sets). 29  Cont.…  The gap between eg and t2g is denoted by ΔO (for octahedral complexes)/ΔT (for tetrahedral complexes), which depends on the nature of the ligand, geometry of the complex and the oxidation state of the metal. This means the color of the complex is affected by the above mentioned.  One octahedral complex absorbs photon energy corresponding to ΔO, which excites one electron from t2g to eg and causes the complex to become colored.  When a tetrahedral complex absorbs photon energy equivalent to ΔT, an electron from eg to t2g will be excited and causes the complex to become colored.  Two different complex containing the same metal but different ligands can have different color.  For example, the high-spin complex [Fe(H2O)6]SO4 appears blue-green because it absorbs photons in the red wavelength range, and the low-spin complex K4[Fe(CN)6], on the other hand, absorbs violet photons with more energy, giving it a pale yellow. But both are Fe2+ complexes. 30  Cont.…  Water is a weak field ligand and the Δo for [Fe(H2O)6]SO4 is low, that is why it absorbs the red light (longer wavelength & lower energy) and blue-green colored.  On the other hand, CN- is a strong field ligand and the Δo for K4[Fe(CN)6] is high, which absorbs violet light (shorter wavelength & higher energy) and yellow colored. See the next figure  In [Fe(H2O)6]2+ Δo < PE  In [Fe(CN)6]4- Δo > PE 31  Cont.…  [Ti(H2O)6]3+ is a d1 octahedral complex that have only one spectrum which resulted from d- d transition (t2g1eg0 → t2g0eg1) absorbs yellow light and it is violet colored as can be seen in the next figure.  ∆o increases with increasing oxidation number on the metal, that is why the oxidation state of the metal center can determine the color of the complex. Mn+2