Coordination Compounds and Ligands

Coordination Compounds and LigandsIn order to explain the pass pissulae and structures of the science laboratoryyrinthine abstr designs, or difficultes, organise by renewal alloy salts with molecular species such(prenominal) as ammonia, Werner coined the terms primary valence and secondary valence, as explained in Chapter 1. These concepts remain valid today except that the term oxidation extract has replaced primary valence and the term coordination sum has replaced secondary valence. Werner had recognized that a transition alloy salt could form a decomposable compound in which the surface ion became bonded to a number of groups which need not inevitably be the counter anions originally present in the salt. The orientations in home of these admixture- parachute groups would lead to the interlacing having a particular geometric structure. In this chapter the structures of transition element mingledes be examined in to a greater extent detail and nigh definiti ons of rouge terms argon provided.One definition of a admixture analyzable or coordination compound is a compound formed from a Lewis virulentic and a Brnsted base, a Lewis acid being an negatron mates acceptor and a Brnsted base a proton acceptor. Thus the interaction of the Lewis acid alloy centre in Ni(ClO4)2 with the Brnsted base ammonia to form a multiform fit in to equation 4.1 Ni(ClO4)2 + 6NH3 Ni(NH3)6(ClO4)2 (4.1) provides an example of the formation of a coordination compound. In writing the formulae of metal Gordianes it is conventional to include the complete coordination complex within lusty brackets, an example being provided by Co(NH3)5ClCl2, in which the coordination complex is Co(NH3)5Cl2+ with two chloride counterions. The Brnsted bases attached to the metal ion in such compounds atomic number 18 called ligands. These may be simple ions such as Cl-, dispirited molecules such as H2O or NH3, erectr molecules such as H2NCH2CH2NH2 or N(CH2CH2NH2)3, or ev en macromolecules, such as proteins. The coordination number (CN) of a metal ion in a complex stool be defined as the number of ligand sponsor atoms to which the metal is directly bonded. In the case of Co(NH3)5Cl2+ this give be 6, the sum of bingle chloride and flipper ammonia ligands each donating an electron pair. Although this definition usually works tumefy for coordination compounds, it is not always enchant for organometallic compounds. An alternative definition of CN would be the number of electron pairs arising from the ligand donor atoms to which the metal is directly bonded. To apply this definition, it is necessary to assume an ionic formulation and a particular oxidation state for the metal ion, so that charges kitty be assigned to the ligands as appropriate and the number of electron pairs determined.Types of LigandWhere a ligand is bound to a metal ion through a champion donor atom, as with Cl-, H2O or NH3, the ligand is said to be unidentate (the ligand bind s to the metal through a single point of attachment as if it had whiztooth). Where two donor atoms can buoy be utilise to bind to a metal ion, as with H2NCH2CH2NH2, the ligand is said to be bidentate, and where several donor atoms ar present in a single ligand as with N(CH2CH2NH2)3, the ligand is said to be polydentate. When a bi- or polydentate ligand uses two or more donor atoms to bind to a single metal ion, it is said to form a chelate complex (from the classical for claw). Such complexes t can to be more fixed than similar complexes containing unidentate ligands. A huge compartmentalization of ligands appear in coordination complexes, Any of a variety of elements may function as donor atoms towards metal ions, alone the closely commonly encountered be probably nitrogen, phosphorus, oxygen, sulfur and the halides. In addition, a large number of compounds ar known which contain carbon donor atoms these be known as organometallic compounds. Bidentate ligands may be cla ssified according to the number of atoms in the ligand which separate the donor atoms and hence the size of the chelate ring formed with the metal ion. Thus 1,1-ligands form a four-membered chelate ring when bound to a metal ion, 1,2-ligands a five membered ring, and so on. Cyclic compounds which contain donor atoms oriented so that they can bind to a metal ion and which are large ample to encircle it are known as macrocyclic proligands. Bicyclic proligands are in addition known which can completely encapsulate a metal ion. both(prenominal) of these systems have given the names cryptand or sepulchrate, Certain polydentate ligands are especially good at linking together several metal ions and are refered to as polynucleating ligands.GeometryIn coordination chemistry, a structure is first described by its coordination number, the number of ligands attached to the metal (more specifically, the number of -type bonds between ligand(s) and the aboriginal atom). Usually one can count the ligands attached, besides sometimes even the counting can become ambiguous. Coordination numbers are normally between two and nine, but large numbers of ligands are not uncommon for the lanthanides and actinides. The number of bonds depends on the size, charge, and electron configuration of the metal ion and the ligands. Metal ions may have more than one coordination number.Typically the chemistry of complexes is dominated by interactions between s and p molecular orbitals of the ligands and the d orbitals of the metal ions. The s, p, and d orbitals of the metal can accommodate 18 electrons (see 18-Electron rule for f-block elements, this extends to 32 electrons). The maximum coordination number for a authentic metal is thus related to the electronic configuration of the metal ion (more specifically, the number of empty orbitals) and to the ratio of the size of the ligands and the metal ion. Large metals and small ligands lead to high coordination numbers, e.g. Mo(CN)84-. Sma ll metals with large ligands lead to low coordination numbers, e.g. PtP(CMe3)2. cod to their large size, lanthanides, actinides, and early transition metals tend to have high coordination numbers. variant ligand structural arrangements result from the coordination number. Most structures follow the points-on-a-sphere pattern (or, as if the central atom were in the middle of a polyhedron where the corners of that shape are the locations of the ligands), where orbital overlap (between ligand and metal orbitals) and ligand-ligand repulsions tend to lead to certain regular geometries. The or so observed geometries are listed below, but there are many cases which influence from a regular geometry, e.g. payable to the use of ligands of unalike types (which results in crooked bond lengths the coordination atoms do not follow a points-on-a-sphere pattern), due to the size of ligands, or due to electronic effects (see e.g. Jahn-Teller distortion)Linear for two-coordination,Trigonal pl anate for three-coordination,Tetrahedral or square planar for four-coordinationTrigonal bi pyramidal or square pyramidal for five-coordination,Octahedral (orthogonal) or trigonal prismatic for six-coordination,Pentagonal bipyramidal for seven-coordination,Square antiprismatic for eight-coordination, andTri-capped trigonal prismatic (Triaugmented triangular prism) for nine coordination. most exceptions and provisions should be notedThe reckon descriptions of 5-, 7-, 8-, and 9- coordination are often indistinct geometrically from alternative structures with slightly different L-M-L (ligand-metal-ligand) angles. The classic example of this is the difference between square pyramidal and trigonal bipyramidal structures.Due to special electronic effects such as (second-order) Jahn-Teller stabilization, certain geometries are stabilized comparative to the other possibilities, e.g. for some compounds the trigonal prismatic geometry is stabilized relative to octahedral structures for six- coordination.IsomerismThe arrangement of the ligands is fixed for a given complex, but in some cases it is mutable by a reaction that forms some other stable isomer.There exist many kinds of isomerism in coordination complexes, just as in many other compounds.StereoisomerismStereoisomerism occurs with the same bonds in different orientations relative to one another. Stereoisomerism can be further classified intoCis-trans isomerism and facial-meridional isomerismCis-trans isomerism occurs in octahedral and square planar complexes (but not tetrahedral). When two ligands are mutually adjacent they are said to be cis, when opposite each other, trans. When three identical ligands conduct one face of an octahedron, the isomer is said to be facial, or fac. In a fac isomer, any two identical ligands are adjacent or cis to each other. If these three ligands and the metal ion are in one plane, the isomer is said to be meridional, or mer. A mer isomer can be considered as a faction of a tra ns and a cis, since it contains both trans and cis pairs of identical ligands.Optical isomerismOptical isomerism occurs when the reflect image of a compound is not superimposable with the original compound. It is so called because such isomers are optically active, that is, they rotate the plane of polarized light. The symbol (lambda) is used as a prefix to describe the left-handed propeller twist formed by three bidentate ligands, as shown. Similarly, the symbol (delta) is used as a prefix for the right-handed propeller twist.7Structural isomerismStructural isomerism occurs when the bonds are themselves different. Linkage isomerism is only one of several types of structural isomerism in coordination complexes (as well as other classes of chemical compounds). Linkage isomerism occurs with ambidentate ligands which can bind in more than one place. For example, NO2 is an ambidentate ligand it can bind to a metal at either the N atom or at an O atom. http//t2.gstatic.com/images?q=tb nANd9GcRKxYHqV_eczrlInNE3ZAbZOBh-Q1JBpMbyWoRehkKI8y1KEukt=1usg=__PClvZyGR5yoOsiA5HEgW1Zjyvko= name Coordination CompoundsA complex is a substance in which a metal atom or ion is associated with a group of neutral molecules or anions called ligands. Coordination compounds are neutral substances (i.e. uncharged) in which at least one ion is present as a complex. You will learn more about coordination compounds in the lab lectures of experiment 4 in this course.The coordination compounds are named in the following way. (At the end of this tutorial we have some examples to show you how coordination compounds are named.)A. To name a coordination compound, no matter whether the complex ion is the cation or the anion, always name the cation sooner the anion. (This is just like naming an ionic compound.)B. In naming the complex ion1. Name the ligands first, in alphabetical order, then the metal atom or ion. Note The metal atom or ion is written before the ligands in the chemical formula.2. The names of some common ligands are listed in delay 1.For anionic ligands end in -o for anions that end in -ide(e.g. chloride), -ate (e.g. sulfate, nitrate), and -ite (e.g. nirite), change the endings as follows -ide http//www.chemistry.wustl.edu/edudev/LabTutorials/arrow.jpg-o -ate http//www.chemistry.wustl.edu/edudev/LabTutorials/arrow.jpg-ato -ite http//www.chemistry.wustl.edu/edudev/LabTutorials/arrow.jpg-itoFor neutral ligands, the common name of the molecule is used e.g. H2NCH2CH2NH2 (ethylenediamine). Important exceptions water is called aqua, ammonia is called ammine, carbon monoxide is called carbonyl, and the N2 and O2 are called dinitrogen and dioxygen.3. Greek prefixes are used to target the number of each type of ligand in the complex ion, e.g. di-, tri- and tetra-. If the ligand already contains a Greek prefix (e.g. ethylenediamine) or if it is polydentate ligands (ie. can attach at more than one binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are use d instead. (See examples 3 and 4.) The numerical prefixes are listed in Table 2.4. After naming the ligands, name the central metal. If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is call cobalt and Pt is called platinum. (See examples 1-4). If the complex ion is an anion, the name of the metal ends with the suffix -ate. (See examples 5 and 6.). For example, Co in a complex anion is called cobaltate and Pt is called platinate. For some metals, the Latin names are used in the complex anions e.g. Fe is called ferrate (not ironate).5. Following the name of the metal, the oxidation state of the metal in the complex is given as a Roman numeral in parentheses.C. To name a neutral complex molecule, follow the rules of naming a complex cation. Remember Name the (possibly complex) cation BEFORE the (possibly complex) anion.See examples 7 and 8.For historic reasons, some coordination compounds are called by their common names. For e xample, Fe(CN)63 and Fe(CN)64 are named ferricyanide and ferrocyanide respectively, and Fe(CO)5 is called iron carbonyl.Examples Give the imperious names for the following coordination compounds1. Cr(NH3)3(H2O)3Cl3Answer triamminetriaquachromium(III) chlorideSolution The complex ion is inside the parentheses, which is a cation.The ammine ligands are named before the aqua ligands according to alphabetical order.Since there are three chlorides binding with the complex ion, the charge on the complex ion essential be +3 ( since the compound is electrically neutral).From the charge on the complex ion and the charge on the ligands, we can calculate the oxidation number of the metal. In this example, all the ligands are neutral molecules. Therefore, the oxidation number of chromium essential be same as the charge of the complex ion, +3.K4Fe(CN)6Answer grand hexacyanoferrate(II)Solution potassium is the cation and the complex ion is the anion.Since there are 4 K+ binding with a complex ion, the charge on the complex ion mustiness be 4.Since each ligand carries -1 charge, the oxidation number of Fe must be +2.The common name of this compound is potassium ferrocyanide.Applications of Co-ordination Compounds(1) Estimation of hardness in water, as Ca++ and Mg2+ ions form complexes with EDTA.(2) Animal and plant world e.g. chlorophyll is a complex of Mg2+ and haemoglobin is a complex of Fe2+ vitamin B12 is a complex of Co2+.(3) Electroplating of metals involves the use of complex salt as electrolytes e.g. KAg(CN)2 in silver-tongued plating.(4) Extraction of metals e.g. Ag and Au are extracted from ores by dissolving in NaCN to form complexes.(5) Estimation and detection of metal ions e.g. Ni2+ ion is estimated using dimethyl glyoxime.(6) Medicines e.g. cis-platin i.e. cis PtCl2(NH3)2 is used in treatment in cancer immenseness and Applications of Coordination CompoundsImportance and applications of coordination compounds find use in many qualitative and quantitative chemical analyses. The familiar seeming reactions given by metal ions with number of ligands. Similarly purification of metal can be achieved through formation and sub sequence decomposition of their coordination compounds.Inflexibility of water is predictable by simple titration with Na2EDTA.the Ca2+ and Mg2+ ions form stable complex with EDTA. The selective estimation of these ions can be done due to difference in the stability constants of calcium and magnesium complexes. Some master(prenominal) extraction processes of metals like those of silver and silver, make use of complex formation.Importance and applications of coordination compounds are of great importance in biological system. The pigment accountable for photosynthesis chlorophyll is a coordinated compound of magnesium. Haemoglobin, the red pigment of declivity which acts as oxygen carrier is a coordination compound of iron. Coordination compounds are used as accelerators for many industrial processes.Applications of articles can be electroplating with the silver and gold much more smoothly and evenly from the solution of the complexes. In depressed and white photography, the developed film is fixed by washing with hypodermic solution which dissolves the unrecompensed AgBr to from a complex ion Ag9S2O3)23-There is growing interest in the user of chelate therapy in medicinal chemistry. An example is the treatment of puzzle caused by the presence of metal in toxic proportion in plant and animal. Thus, excess of copper and iron are removed by chelating ligands D-penicillamine and desferrioxime B via the formation of the coordination compounds. EDTA is use in the conduct of guide poisoning. Some coordination compounds of platinum effectively inhibit the growth of tumours.Sonochemical Asymmetric Hydrogenation with PalladiumEnantioselective hydrogenation is one of the most versatile methods of asymmetric synthesis, with intricate catalysis, using chiral auto-changers, rapidly fit an alternat ive to the .traditional. homogeneous methods. The role of modifiers in asymmetric hydrogenations is to elevate catalysis, with the bind mode and geometry of adsorption being important, as well as the modifier concentration and the type and position of the substituent groups in the aromatic ring. Ultrasonic dig (sonication) is known to bebeneficial in catalytic asymmetric hydrogenations. Sonication removes accelerator pedal surface impurities, and gives enhanced adsorption to the chiral modifiers. Now a team from loot Technological University, Houghton, U.S.A. (S. C. Mhadgut, I.Bucsi, M. Trk and B. Trk, Chem. Commun., 2004, (8), 984-985 inside 10.1039/b315244h) has revisited the Pd-catalysed, proline-modified, asymmetric hydrogenation of isophorone (3,3,5-trimethyl-2-cyclohexen-1-one (with a C=C bond)). They examined the catalyst, the modifier and the effects of sonication. Pd/Al2O3 was found to give a better, thoughlow, enantiomeric excess (ee) than Pd/C. Prolineand its derivat ives (isomeric hydroxyl-prolines, prolinols and proline esters) were tested as chiralmodifiers for Pd/Al2O3. Proline was the best modifier, and both enantiomers gave ee 35%. Presonication was found to enhance the enantioselectivity when both the Pd/Al2O3 catalyst and the proline modifier were present. .Modifier-free. presonication and the presence of substrate during pretreatment rock-bottom the enantioselectivity. The reaction was performed at 50 bar pressure sensation and 25C. Presonication for 20 minutes gave the highest optical yields, and increased optical yields across all the H2 pressure range. Maximum ee occurred at a 12 isophoroneproline ratio, and with optimised conditions and presonication, the ee for the Pd/Al2O3-(S)-proline catalytic system was 85%. Ultrasonic cleaning of the catalyst enhanced both the adsorption of the modifier and the modifier- induced surface restructuring of the Pd. The high ee was due to proline adsorption on the Pd surface. New catalysts that can strongly adsorb proline could thus become important in heterogeneous catalysis for C=C double bond hydrogenation of a,b-unsaturated carbonyl compounds.