BCHY101L Reference Material I - Fall 2024-2025 PDF

Module-2: Metal Complexes and Organometallics 1  Contents…..(6 hours) Inorganic complexes - structure, bonding and application; Organometallics – introduction, stability, structure and applications of metal carbonyls and ferrocene Metals in biology: h...

Module-2: Metal Complexes and Organometallics 1  Contents…..(6 hours) Inorganic complexes - structure, bonding and application; Organometallics – introduction, stability, structure and applications of metal carbonyls and ferrocene Metals in biology: haemoglobin and chlorophyll- structure and property). 2  Inorganic Complexes: Structure, Bonding and Applications ❖ Double Salt: Double Salts and Coordination Compounds ❖ A double salt is a salt that contains more than ❖Co-ordination Compounds one different cation or anion o Coordination compound is one which contains a central metal atom Ferric alum (NH4)2SO4.Fe2(SO)4.24H2O or ion Surrounded by a number of opposite charged or neutral molecules Double salts completely dissociate in o Formed by donation of lone pairs of electrons by ions or neutral molecules aqueous medium to the central metal ion o For ex: central Fe is linked to six CN by coordinate bond by donation In water: NH4+, SO42-, Fe3+ of lone electron pair Fe(CN)2 + 4KCN Fe(CN)2.4KCN 3Cl– (counterion) 4K+ + [Fe(CN)6]4- Other Ex: ligand M Ni(CO)4 Ag(CN)2, (coordination sphere) Cu(NH3)4 N forms a coordinate covalent bond Coordination compound called complex is one which contains complex ions to the metal Ligands ▪ Molecule or ion having a lone electron pair that can be used to form a bond to a metal ion (Lewis base). ▪ coordinate covalent bond: metal-ligand bond ▪ monodentate : one bond to metal ion ▪ bidentate : two bond to metal ion ▪ polydentate : more than two bonds to a metal ion possible 4 Chelating Agents  Coordination compounds containing polydentate ligands are called chelates  A chelating agent which forms several bonds to a metal without unduly straining its own structure  A chelate is an organic compound formed when a polydentate ligand bonds to a central metal atom.  For example, ethylenediamine as polydentate ligand can readily replace ammonia to form chelates Ex: Important biomolecules like heme and chlorophyll are porphyrins Ex: Phosphates are used to tie up Ca2+ and Mg2+ in hard water to prevent them from interfering with detergents. Bonding in coordination compounds To know the most common structures observed for metal complexes. To predict the relative stabilities of metal complexes with Werner Coordination Theory different ligands Werner's Theory: Alfred Werner,1892 Swiss chemist put forward a theory to explain the formation of complex compounds. Werner's theory states that 1. Metals possess two types of valencies called primary / ionizable and secondary / non - ionizable valency. 2. Every metal atom in coordination compounds have a tendency to satisfy both its primary and secondary valencies. 3. The ligands satisfying secondary valencies are always directed towards fixed positions in space thereby giving a definite geometry to the complex but primary valencies are non - directional. Limitations: 1. Bonding within coordination sphere. 2. Square planar (or) Tetrahedral 3.It does not explain the colour, and the magnetic First reaction indicates that in the coordination compound (COCl3. 6NH3), and optical properties shown by coordination compounds. all the three chlorine atoms are in the ionisation sphere and six ammonia molecules are in the coordination sphere. Lewis Acid Base Theory - Gilbert N. Lewis, 1920s ❖ Lewis Acid/Base reactions: Base: electron pair donor; Acid: electron pair acceptor ❖ Ligands: Lewis bases ; Metals: Lewis acids ; Coordinate covalent bonds ❖ Metal Complexes - Formation of a complex was described as an acid - base reaction according to Lewis Sidgwick’s Rule ❖ Sidgwick’s Effective atomic number (EAN) rule is based on the octet theory of Lewis and this is the first attempt to account for the bonding in complexes. Valence Bond Theory (Linus Pauling, 1931) Valence bond theory predicts that the bonding in a metal complex arises from overlap of filled ligand orbitals and vacant metal orbitals. Metal ligand bonds arise by the donation of electron pair to metal atom/ion which must possess requisite number of vacant orbitals of nearby equal energy. These orbitals undergo hybridization to give a set of same number of hybrid orbitals of equal energy. 8  Formation of some complexes Tetrahedral Geometry [CuCl4]2- In this complex ion, Cu atom has +2 Tetrahedral copper complex oxidation state, it loses two electrons. 4s and 4p is available for hybridization gives sp3 3d 4s 4p Cu ground state 3d94s2 Cu2+ sp3 4 e– pairs by Cl– ions One unpaired electron - paramagnetic and attracted by magnets – High spin complexes SP3 hybridization-Tetrahedral geometry Square Planar Geometry Square planar nickel complex [Ni(CN)4]2- 3d 4s 4p Ni (3d84s2) Ni2+ [Ni(CN)4]2- dsp2 All paired electrons – diamagnetic - weakly repelled by magnets –Low spin complexes CN– Strong ligand Note: Since CN- is strong ligand, 3d orbitals paired up against to Hund’s rule in Nickel atom in (Ni (CN)4)2- Octahedral sp3d2 Geometry Gives [CoF6]3– four unpaired electrons, which makes it paramagnetic and is called a high-spin complex. Ground state Co= (3d74s2) Note: Since F- is Weak ligand, so the Hund’s rule is obeyed in 3d orbitals of Cobalt atom in (CoF6)3- Octahedral d2sp3 Geometry [Fe(CN)6]3- Fe: (3d64s2) 3d 4s 4p Fe+3 [Fe(CN)6]3- CN– Strong ligand d2sp3 13 Bonding in Coordination Compounds – Contd……… ❖ Many of the properties of metal complexes are dictated by their electronic structures. Crystal field theory (CFT) ❖ Electronic structure can be explained by an ionic model ❖ Ligands are treated as point charges ❖ There is no interaction between metal orbitals and ligand orbitals ❖ Bonding between the metal cation and ligand is purely electrostatic in the complex ❖ When complex is formed, the ligands destroy the degeneracy of d-orbitals i.e. the d-orbitals have different energies and grouped as two sets (eg and t2g) ❖ In octahedral complex two types electrostatic interaction are possible. ▪ There is a attraction between the positive metal ion and negative charged ligand, this force holds the ligand to the metal in complex. ▪ Electrostatic repulsion between the lone pairs in the ligands and the electrons in d-orbitals of the metal. Even, the magnitude of this repulsion depends on the particular d-orbitals Orbital occupancy for high- and low-spin complexes of d4 through d7 metal ions high spin: low spin: low spin: strong-  As the ligand approach to central metal, high spin: strong- weak-field field ligand the repulsion between negative (lone pair weak-field field ligand ligand of) ligands and electrons of d orbitals ligand increases  Weak field ligands (Halide ions and OH- ions cause small degree of crystal field d- orbital splitting  strong field ligands (CO and CN-) cause large degree of splitting of d-orbital energy levels  Energy difference between two sets of d- orbitals is called crystal field splitting (CFT)  CFT value can be measured easily from UV-Vis spectrum of the complex  For ex. [Ti (H2O)6]3- Ti3+ has one d- electron, reddish violet in solution – d-d transition will undergo because electron transfer from t2g to eg The crystal field stabilization energy (CFSE) is the stability that results from placing a transition metal ion in the crystal field generated by a set of ligands. CFSE [Co(NH3)6]3+ > [Co(NH3)6]2+ [Co(NH3)6]3+ = 23,000 cm-1 (3d) [Rh(NH3)6]3+ = 34,000 cm-1 (4d) [Ir(NH3)6]3+ = 41,000 cm-1 (5d) Spectrochemical Series ▪ For a given ligand, the color depends on the oxidation state of the metal ion. I- < Cl- < F- < OH- < H2O < SCN- < NH3 < en < NO2- < CN- < CO WEAKER FIELD STRONGER FIELD ❖ Complexes of cobalt (III) show the shift in color due SMALLER Δ LARGER Δ to the ligand. ❖ (a) CN–, (b) NO2–, (c) phen, LONGER λ SHORTER λ (d) en, (e) NH3, (f) gly, (g) H2O, (h) ox2–, (i) CO3 2– For a given metal ion, the color depends on the ligand. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Spectrochemical series (strength of ligand interaction) Effect of ligand on splitting energy Increasing Δ Cl- < F- < H2O < NH3 < en < NO2- < CN- Increasing Δ 2 Low spin – color variations shown with increasing CFSE (Cr3+ = 24-3-18 = d3) 19 Applications of Coordination Compounds ❖Coordination compounds are of great importance. ❖Play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine. Catalysis Extraction of metal ions Analytical chemistry (development of numerous analytical methods) Hardness estimation - Biological importance Medicinal application Industrial application Extraction / Purification of metal ⮚ Extraction ❖ processes of metals, like those of silver and gold, make use of complex formation. ❖ These noble metals are extracted from their ore by the formation of cyanide complexes - dicyanoargentite(I) - [Ag(CN)2]– and dicyanoaurate (I) - [Au(CN)2]– in the presence of oxygen and water, from which the metallic forms can be separated by the addition of zinc. ▪ ❖ Purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds. For example, impure nickel is converted to [Ni(CO)4], which is decomposed to yield pure nickel. Analytical chemistry ❖ In the qualitative methods of analysis, complex formation is of immense importance in the identification and separation of most inorganic ions. ❖ Familiar colour reactions given by metal ions with a number of ligands (especially chelating ligands), as a result of formation of coordination entities, form the basis for their detection and estimation by classical and instrumental methods of analysis. Examples of such reagents include EDTA, DMG (dimethylglyoxime), α–nitroso, β– naphthol, cupron, etc. ❖ Since Cu is more stable than Cd. Therefore, on passing H2S only CdS is precipitated. Thus Cd2+ ion easily detected in the presence of Cu2+ ions. Cu2+ + 4CN- [Cu(CN)4]2- Cd2+ + 4CN- [Cd(CN)4]2- ❖ Presence of Co and Fe can be detected by the formation of blue and blood red color thiocyanate complexes respectively 22 Detection of Complex formation ❖ Formation of Precipitate Ni2+ + 2 HDMG [Ni(DMG)2] + 2H+ ❖ Ni2+ and Pd2+ form insoluble colored precipitates with dimethyglyoxime Biological Importance ❖ Pigment responsible for photosynthesis, chlorophyll, is a coordination compound of magnesium. ❖ Haemoglobin, the red pigment of blood which acts as oxygen carrier is a coordination compound of iron. ❖ Vitamin B12, cyanocobalamine, the anti- pernicious anaemia factor, is a coordination compound of cobalt. ❖ Other compounds of biological importance with coordinated metal ions are the enzymes like, carboxypeptidase A and carbonic anhydrase (catalysts of biological Heme B: Heme B is a porphyrin (four linked pyrrole rings) systems) that readily binds iron, as shown. This is an example of a ❖ Metalloprotein with the metal ion cofactor biomolecule that contains non-protein ligands for a transition metal. have many diverse functions including transport, storage, and signal transduction. Medicinal Application  To treat problems caused by the presence of metals in toxic proportions in plant/animal systems, chelation therapy is used.  Excess of copper and iron are removed by the chelating ligands D– penicillamine and desferrioxime B via the formation of coordination compounds.  D-penicillamine - Used in the treatment for poisoning by heavy metals, including Wilson's disease (build-up of copper in the body).  Naturally occurring compounds such as desferrioxamine B, are used by bacteria to assist in the uptake of iron, can also be used to remove unwanted iron by chelation therapy. ❖ EDTA is used in the treatment of lead poisoning. ❖ Coordination compound of platinum effectively inhibit the growth of tumours. cisplatin - cis [PtCl2(NH3)2], and related compounds. Industrial applications Coordination compounds are used as catalysts for many industrial processes. Examples rhodium complex, [(Ph3P)3RhCl], a Wilkinson catalyst - hydrogenation of alkenes. Articles can be electroplated with silver and gold much more smoothly and evenly from solutions of the complexes, [Ag(CN)2]– and [Au(CN)2]– than from a solution of simple metal ions. In black and white photography, the developed film is fixed by washing with hypo solution which dissolves the non decomposed AgBr to form a complex ion, [Ag(S2O3)2]3– (Argentothiosulphate ion). Prussian blue – Mixture of hexacyano Fe(II) and Fe(III) - Fe4[Fe(CN)6]3 inks, blueprinting, cosmetics, paints (commercial coloring agents) Hardness of water ❖ Hardness of water is estimated by titration with the sodium salt of EDTA. ❖ During titration, the calcium and magnesium ions in hard water form the stable complexes, Calcium EDTA and Magnesium EDTA. ❖ Hardness of water is estimated by simple titration with Na2EDTA. ❖ The selective estimation of these ions can be done due to difference in the stability constants of calcium and magnesium complexes. Representative Metal Complexes in Catalysis The researchers have found that the compound strongly Zeise’s Salt : K[Pt(C2H4)Cl3] inhibits cyclooxygenase (COX) enzymes, but it shows low Magnus Green Salt : [Pt(NH3)4][PtCl4] cytotoxicity against cancer Edman’s Salt : K[Co(NH3)2(NO2)4 cells. Reinecke’s Salt : NH4[Cr(NH3)2(NCS)4] Vaska’s Complex : [Ir(CO)(PPh3)2Cl] Wilkinson’s Catalyst : [Rh(PPh3)3Cl] Wilkinson catalyst is widely used for the hydrogenation reaction of unsaturated hydrocarbons (olefins) Organometallics – Introduction, stability, structure and applications of metal carbonyls and ferrocene 29 What are Organometallics? ❖ An area which bridges organic and inorganic chemistry. ❖ A branch of coordination chemistry where the complex has one or more metal-carbon bonds. C always is more electronegative compared to M. ❖ The leading journals of the field define an "organometallic" compound as one in which there is a bonding interaction (ionic or covalent, localized or delocalized) between one or more carbon atoms of an organic group or molecule and a main group, transition, lanthanide, or actinide metal atom (or atoms) ❖ Following longstanding tradition, organic derivatives of metalloids such as boron (B), silicon (Si), germanium (Ge), arsenic (As), tellurium (Te) are also included in this definition. Zeise’s Salt- The first transition metal ❖ Discovery 1827 organometallic compound: ❖ Structure ~ 150 years later First σ-bonded Organometallic Compound- Diethyl Zinc: 3 C2H5I + 3 Zn → (C2H5)2Zn + C2H5ZnI + ZnI2 30 potassium trichloro(ethylene)platinate(II)  First organometallics in homogeneous catalysis- The Hydroformylation (1938) Otto Roelen First Industrial plant- Pioneer in Industrial homogeneous catalysis hydroformylation (1897-1993) *A plasticizer is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity DEHP is the most common phthalate plasticizer in medical devices such as intravenous tubing and bags, nasogastric tubes, dialysis bags and tubing, blood bags and transfusion tubing, and air tubes. 31 Some Important Ligand Nomenclature “eta-x” was originally developed to indicate how many carbons of a π-system were ηx coordinated to a metal center. Hapticity is another word used to describe the bonding mode of a ligand to a metal center. For Ex: η5-cyclopentadienyl (CP) ligand, has all five carbons of the ring bonding to the transition metal center. ηx values for carbon ligands where the x value is odd usually indicate anionic carbon ligands (e.g., η5-Cp, η1-CH3, η1-allyl or η3-allyl, η1-CH=CH2) The no of electrons donated (ionic method of electron counting) by the ligand is usually equal to x + 1 Even ηx values usually indicate neutral carbon π-system ligands (e.g., η6-C6H6, η2- CH2=CH2, η4-butadiene, η4-cyclooctadiene) Number of electrons donated by the ligand in the even (neutral) case is usually just equal to x. η5-Cp η3-Cp η3-allyl η1-allyl 32 ❖Organometallic compounds are classified into three classes. (i) Sigma (σ) bonded organometallic compounds: In these complexes, the metal atom and carbon atom of the ligand are joined together with a sigma bond, For Example: (a) Grignard reagents, R–Mg–X where R is an alkyl or aryl group, and X is a halogen. (b) Zinc compounds of the formula R2Zn such as (C2H5)2Zn Other similar compounds are (CH3)4Sn, (C2H5)4Pb, Al2(CH3)6, Al2(C2H5)6, Pb(CH3)4 etc. Al2(CH3)6 is a dimeric compound and has a structure similar to diborane, (B2H6). It is an electron deficient compound, and two methyl groups act as bridges between two aluminium atoms. (ii) Pi (π) bonded organometallic compounds:  These are the compounds of metals with alkenes, alkynes, benzene and other ring compounds.  In these complexes, the metal and ligand form a bond that involves the π-electrons of the ligand.  Examples are Zeise’s salt, ferrocene and dibenzene chromium.  (iii) Sigma and π-bonded organometallic compounds Metal carbonyl compounds formed between metal and carbon monoxide, belong to this class. These compounds possess both σ-and π-bonding. Generally, oxidation state of metal atoms in these compounds is zero. Carbonyls may be mononuclear, bridged or polynuclear. *Net result is the number of electron pair accepted by metal is equal to number of electron pair donated to ligands in backbonding. Hence in carbonyls oxidation state of metal is zero. Stability of Organometallic Compounds ❖ In general terms, the stability of an organometallic compound may refer to either its ❖ Thermal stability, or ❖ Resistance to chemical attack (by air and moisture). ❖ These different types of stabilities would depend both on thermodynamic as well as kinetic factors. The organometallic compounds are generally hydrolysed via nucleophilic attack by water, which is facilitated by: (1) the presence of empty low-lying (vacant) orbitals on the metal (2) the polarity of metal-carbon bonds. (3) Rate of hydrolysis is dependent on M-C bond polarity – greater the polarity, faster will be the rate *Where metal atoms form covalent bonds with carbon atoms, the electrons are usually shared unequally. As a result, the bond is polarized—one end is more negative than the other. The extent of polarization depends on the strength of which the metal atom binds 35 electrons.  The 18-electron Rule or Effective atomic number (EAN) ❖ The 18e rule is a way to help us decide whether a given d-block transition metal organometallic complex is likely to be stable. Not all the organic formulas we can write down correspond to stable species. Recall: Second row elements (B, C, N, O, F) have 4 valence orbitals (1s + 3p) so they can accommodate up to 8 valence electrons--the octet rule. ▪ For example, CH5 requires a 5-valent carbon and is therefore not stable. Stable compounds, such as CH4, have the noble gas octet, and so carbon can be thought of as following an 18e rule. ▪ The 18e rule, which applies to many low-valent transition metal complexes, follows a similar line of reasoning. The metal now has one s, and three p orbitals, as before, but now also five d orbitals. We need 18e to fill all nine orbitals; some come from the metal, the rest from the ligands. Therefore, we can expect that the low-lying MOs can accommodate up to 18 valence electrons-- The 18-Electron Rule. ❖ The rule states that “thermodynamically stable transition metal organometallic compounds are formed when the sum of the metal d electrons and the electrons conventionally considered as being supplied by the surrounding ligands equals 18” 36 Counting electrons for metal complex To count the electrons of a metal complex, one must: a) note any overall charge on the metal complex b) know the charges of the ligands bound to the metal center (ionic ligand method) c) know the number of electrons being donated to the metal center from each ligand (ionic ligand method) Similarly for a transition metal complex, the electron count is the sum of the metal valence electrons + the ligand centered electrons. ❖ Covalent Model: No of e = No. of metal electrons (zero valent) + No. ligand electrons - complex charge Metal: The number of metal electrons equals its column number (i.e., Ti = 4e, Cr = 6e, Ni = 10e) Ligands: In general L donates 2 electrons, X donates 1 electron. ❖ Ionic Model: No of e = No. of metal electrons (dn) + No. of ligand electrons Metal: Determined based on the number of valence electrons for a metal at the oxidation state present in the complex Ligands: In general and L and X are both 2 e donors. ❖ Complexes with 18 e- counts are referred to as saturated. 37 ❖ Complexes with counts lower than 18e- are called unsaturated. Electron counting: Example 1  Please note that we are using the Ionic Method of electron-counting.  95% of inorganic/organometallic chemists use the ionic method.  The ionic method assigns formal charges to the metal and ligands in order to keep the ligands with an even no of electrons and (usually) a filled valence shell. 1) There is no overall charge on the complex 2) There is one anionic ligand (CH3−, methyl group) 3) Since there is no overall charge on the complex (it is neutral), and since we have one anionic ligand Now we can do our electron counting: present, the Re metal atom must have a +1 charge to compensate for the one negatively charged ligand. Re(+1) 5d5 6s2 d6 The +1 charge on the metal is also its oxidation state. 2 PR3 4e- So the Re is the in the +1 oxidation state. We denote 2 CO 4e- this in two different ways: Re(+1), Re(I), or ReI. CH3− 2e- CH2=CH2 2e- Total: 18e- 38 Electron Counting : Example 2 ❖ Step 2: Determine the d electron count. Recall: subtract the metal's oxidation state from ❖ Step 1: Determine the oxidation state of the its group #. metal. To do this, balance the ligand charges with an equal opposite charge on the metal. This is the metal's formal oxidation state. ❖ Step 3: Determine the To determine ligand charges, create an ionic electron count of the model by assigning each M-L electron pair to the complex by adding the # more electronegative atom (L). This should result of electrons donated by in stable ligand species or ones known as Rh = 4d85s1 each ligand to the metal's reaction intermediates in solution. d electron count. 39 Methods of counting electrons 40  Examples HMn(CO)5 [ Ar ] 3 d 5 4 s 2 [Xe] 4f145d26s2 Mn(+1) d6 6e- 5 CO 10e- H− 2e- Total: 18e- Charge: [Kr] 4d 5s [Ar]4s13d5 Pt= 5d9 6s1 Ru = 4d7 5s1 W=5d4 6s2 41 Examples Ru = 4d7 5s1 42 Other examples 16 e- Ni= 3d8 4s2 Ni(+2) =8 +4 (alkyl) +4 (CO) Pd= 4d10 16 e- Pd(+4) =6 +4 (µBr-) +4 (C=C2-) +2 (CH2 18 e- 16 e- Pt= 5d9 6s1 Rh=[Kr] 4d8 5s116 e- 43 Pt(+2) =8 +2 (Cl) +4 (PEt3) +2 (H) Rh(+1) =8 +2 (Cl) +8 (PPh3)  Metal-Carbonyls  Metal carbonyls are important class of organometallic compounds  Unlike the alkyl ligands, the carbonyl (CO) ligand is unsaturated thus allowing not only the ligand to σ−donate but also to accept electrons in its π* orbital from dπ metal orbitals and thereby making the CO ligand π−acidic.  Other difference lies in the fact that CO is a soft ligand compared to the other common σ−and π−basic ligands like H2O or the alkoxides (RO−), which are considered as hard ligands  Being π−acidic in nature, CO is a strong field ligand that achieves greater d−orbital splitting through the metal to ligand π−back donation.  A metal−CO bonding interaction thus comprises of a CO to metal σ−donation and a metal to CO π−back donation ❖ Standard Bonding Modes  As one goes from a terminal CO- bonding mode to μ2-bridging and finally μ3-bridging, there is a relatively dramatic drop in the CO stretching frequency seen in the IR. 2e- neutral donor 2e- neutral donor 3e- neutral donor 44 Types of CO-Metal bonding interactions ❖Formation of σ-bond: ▪ The overlapping of empty hybrid orbital on metal atom with the filled hybrid orbital on carbon atom of carbon monoxide molecule through lone pair electrons results into the formation of a M←CO σ-bond. ❖Formation of π-bond by back donation: ▪ This bond is formed because of overlapping of filled dπ orbitals or hybrid dpπ orbitals of metal 45 atom with antibonding pi orbitals on CO molecule. Structure of Ni(CO)4 46  Applications ❖Metal carbonyls are used in several industrial processes. Perhaps the earliest application was the extraction and purification of nickel via nickel tetracarbonyl by the Mond process. ❖Fe(CO)5 is used for the preparation of inductors, pigments, as dietary supplements in the production of radar-absorbing materials in the stealth technology (Low observable technology), and in thermal spraying coating technology. ❖Metal carbonyls are used in a number of industrially important carbonylation reactions. In the oxo process, an alkene, hydrogen gas, and carbon monoxide react together with a catalyst (such as HCo(CO)4) to give aldehydes (hydroformylation). ❖ Several other Metal-Carbonyl complexes have been employed in the hydrocarboxylation and hydrogenation reactions. Dicobalt octacarbonyl Co2(CO)8 can be used for hydrocarboxylation of olefins ❖Many organometallic complexes are the sources for the pure metal particles/metal 47 coatings using chemical vapur deposition (CVD) process Structure and Bonding Ferrocene Mössbauer spectroscopy indicates that the iron center in ferrocene should be assigned the +2 oxidation state. Each cyclopentadienyl (Cp) ring should then be allocated a single negative charge. Thus ferrocene could be described as iron(II) bis(cyclopentadienide) Fe2+[C5H5- ]2. The number of π-electrons on each ring is then six, which makes it aromatic according to Hückel's rule. These twelve π-electrons are then shared with the metal via covalent bonding. Since Fe2+ has six d-electrons, the complex attains an 18-electron configuration, which accounts for its stability. In modern notation, this sandwich structural model of the ferrocene molecule is denoted as Fe(η5-C5H5)2. *Each Cp ring contains 4 electrons due to double bonds and Crystallography reveals that the cyclopentadienide one extra pair of electrons makes rings are in staggered conformation. the total as six Hybridization: d2sp3 Magnetic Nature: Diamagnetic 48 Applications of Ferrocene 1. Fuel additives: Ferrocene and its derivatives could be used as antiknock agents in the fuel for petrol engines. They are safer than previously used TEL. Ferrocene powder Ferrocene crystals 2. Pharmaceutical: Ferrocene derivatives have been investigated as drugs e.g. one drug has entered clinic trials, Ferroquine is an antimalarial. Ferrocene-containing polymer-based drug delivery systems have been investigated. Ferrocene as fuel suppressant 3. Solid rocket propellant: Ferrocene and related  Ferox Gas & Diesel Fuel Additive derivatives are used as powerful burn rate catalysts in is a catalyst that is an eco- ammonium perchlorate composite propellant. friendly fuel additive and 4. As a ligand scaffold: Chiral ferrocenyl phosphines are horsepower booster. employed as ligands for transition-metal catalyzed  It allegedly increases mileage reactions. Some of them have found industrial applications from between 10 and 20% while in the synthesis of pharmaceuticals and agrochemicals. also significantly reducing 49 harmful emissions.  Metals in biology Contents……Metals in biology (haemoglobin, chlorophyll- structure and property) 50 Chlorophyll- Structure and Property ❖ Structure of Chlorophyll Chlorophylls are green pigments with polycyclic, planar structures resembling the protoporphyrin system methyl present in haemoglobin In chlorophyll, Mg2+ is the metal centre Principal pigment in The four inward-oriented nitrogen atoms of the photosynthesis porphyrin ring in chlorophyll are coordinated with the Mg2+ All chlorophylls have a long phytol side chain, esterified to a carboxyl-group substituent in ring IV aldehyde Chlorophylls also have a fifth five membered ring not present in heme Accessary pigment in The heterocyclic five-membered ring system that photosynthesis surrounds the Mg2+ has an extended polyene structure, with alternating single and double bonds Such polyenes characteristically show strong absorption in the visible region of the electromagnetic spectrum Chlorophylls have unusually high molar extinction coefficients (higher light absorbance) and are therefore particularly well-suited for absorbing visible light during 51 photosynthesis ❖ Chloroplasts always contain both chlorophyll a and chlorophyll b ❖ Both are green, their absorption spectra are sufficiently different that they complement each other’s range of light absorption in the visible region ❖ Both chlorophyll a & b absorb in the blue (short wavelength) and red region (longer wavelength) so that the remaining green region is transmitted – hence chlorophylls are green in colour ❖ Most plants contain about twice as much chlorophyll a as chlorophyll b ❖ Chlorophyll is always associated with specific binding proteins, forming light-harvesting complexes (LHCs) in which chlorophyll molecules are fixed in relation to each other, to other protein complexes, and to the membrane. 52 Role of Mg in chlorophyll ❖ Without Mg2+ the chlorophyll is fluorescent – i.e. the absorbed light energy is emitted back immediately ❖ In the case of fluorescence, the absorbed light energy is lost immediately – will not be used for chemical reaction ❖ With Mg2+ chlorophyll becomes phosphorescent ❖ In the case of phosphorescence, there will be excited state of finite life time and the energy can be used for chemical reactions ❖ The Mg2+ coordination increase the rigidity of the planar chlorophyll ring: The energy loss as heat due to vibration of the ring during light absorption is prevented 53  Photosynthesis Reaction  Photosynthesis, Process by which green plants and certain other organisms transform light into chemical energy  In green plants, light energy is captured by chlorophyll in the chloroplasts of the leaves and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds (simple and complex sugars) that are the basis of both plant and animal life  It occurs in two stages. During the light-dependent stage chlorophyll absorbs light energy, which excites some electrons in the pigment molecules to higher energy levels and reacts with two hydrogen atoms in water molecules, generates NADPH (an enzyme) and high-energy ATP molecules. Oxygen, released as a by-product  NADPH is a powerful reducing agent that is used to convert carbon dioxide into glucose during the dark reactions of photosynthesis (also called the Calvin Cycle) Two types of photosystems cooperate in the light reactions 54 *NADP (nicotinamide adenine dinucleotide phosphate) A Photosynthesis Road Map 55 Hemoglobin Hb Hb is not an exact Four units of Hb tetramer of Mb 3 major types of Hb Hb A (Adult) Hb F (Fetal) Hb S (Sickle cell) 56 ❖ Each of these subunit polypeptides contains a heme group—an iron atom at the center of a poryphyrin ring—which reversibly binds a single O2 molecule in the ferrous state (Fe2+) of heme. ❖ Whereas free heme (Fe2+) binds O2 irreversibly and is converted to the ferric state (Fe3+) in the process, Hb can reversibly bind O2 because the valence state of the iron atom is protected by encapsulating the heme in the globin protein fold ❖ Each tetrameric (α2β2) Hb can therefore reversibly bind four O2 molecules. ❖ Oxygenation changes the electronic state of the Fe2+ heme iron, The colors of arterial and venous blood are different. Oxygenated (arterial) blood is bright red, while dexoygenated (venous) blood is dark reddish-purple. The difference in color results from the electronic state of the iron ion (ferrous vs ferric), which in turn influences the π → π* and n → π* electronic transitions of porphyrin and hence its 57 optical characteristics reddish purple of venous blood to the brilliant scarlet of arterial blood. ❖ The organic component of the heme group— the protoporphyrin—is made up of four pyrrole rings (A, B, C & D) linked by methine bridges to form a tetrapyrrole ring. Four methyl groups, two vinyl groups, and two propionate side chains are attached. ❖ The iron atom at the center of the protoporphyrin is bonded to the four pyrrole atoms. ❖ Under normal conditions the iron is in the ferrous (Fe2+) oxidation state. The iron atom can form two additional bonds, one on each side of the heme plane, called the fifth and sixth coordination sites. ❖ The fifth coordination site is covalently bound by the imidazole side chain of the globin chain (the “proximal histidine,” α87 and β92). ❖ The sixth coordination site of the iron ion can bind O2 or other gaseous ligands (CO, NO, ❖ CN−, and H2S 58  Role of distal histidine: ▪ The distal histidine amino acid from the hemoglobin protein molecule further stabilizes the O2 molecule by hydrogen-bonding interactions and acts as a Gate for Ligand Entry in both subunits of adult human Hemoglobin ▪ Makes O2 to bind in a bent fashion and makes it difficult for CO to bind in a linear fashion. ▪ An isolated heme binds CO 25000 times as strongly as O2 in solution. In the living system binding affinity for oxygen is reduced considerably. For CO to Tense (T) state Relaxed (R) state bind strongly, it has to bind linearly which is made difficult by distal histidine 59  Reference Books J.D. Lee, Concise Inorganic Chemistry, Oxford University Press, 5th Edition, 2014. F.A. Cotton, G. Wilkinson, C.A. Murillo and M. Bochmann. Advanced Inroganic Chemistry, 6th Edition, John Wiley, 2007 J.E. Huheey, E.A. Keiter, R.L.. Keiter and O.K. Medhi Inorganic Chemistry: Principles of Structure and Reactivity, 4th Edition, Pearson Education, 2006. T. Overton, F. Armstron, J. Rourke and M. Weller. Inorganic Chemistry, 6th Edition, Oxford University Press, 2015. 60