Chemistry
Coordination Compounds
Coordination compounds are molecules that consist of a central metal atom or ion bonded to surrounding molecules or ions, known as ligands. These ligands donate electron pairs to the metal, forming coordinate covalent bonds. Coordination compounds often exhibit unique properties and are widely used in fields such as catalysis, medicine, and materials science.
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eBook - PDFEngineering Chemistry
Fundamentals and Applications
- Shikha Agarwal(Author)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
20.1 Introduction Coordination chemistry is the branch of chemistry that deals with the study of chemical, structural, magnetic and spectral properties of Coordination Compounds. Before having an insight into the properties of Coordination Compounds, it is essential to understand what are Coordination Compounds. Consider the example of potash alum K 2 SO 4 .Al 2 (SO 4 ) 3 .24H 2 O. When this salt is dissolved in water, it dissociates into individual ions and the aqueous solution gives the test of all the constituent ions, that is, it gives the test of K + , Al 3+ and SO 4 2– . Similarly aqueous solution of Mohr’s salt FeSO 4 .Al 2 (SO 4 ) 3 .6H 2 O gives the test of Fe 2+ , Al 3+ and SO 4 2– ions. Hence in these salts the individual ions do not lose their identity. Such compounds are called double salts or lattice compounds. On the other hand, if solutions of Fe(CN) 2 and KCN are mixed together and evaporated, potassium ferrocyanide Fe(CN) 2 .4KCN is formed. The aqueous solution of this salt does not give the test of Fe 2+ ions and CN – ions but gives the test for K + and ferrocyanide ion, Fe (CN) 6 4– . Fe(CN) 2 + 4KCN → Fe(CN) 2 .4KCN 4K + + Fe(CN) 6 4– Thus we see that in the above compound the individual ions lose their identity. Such compounds are termed as Coordination Compounds or complex compounds. Their aqueous solution does not contain simple ions; instead the complex ion remains intact. A coordination compound may consist of a • Simple cation and complex anion, for example, K 4 [Fe(CN) 6 ] • Complex cation and simple anion [Pt(NH 3 ) 5 Cl]Br 3 • Complex cation and complex anion, for example, [Ag(NH 3 ) 2 ][Ag(CN) 2 ]. The ions [Fe(CN) 6 ] 2– , [Pt(NH 3 ) 5 Cl] 4+ , [Ag(NH 3 ) 2 ] + and [Ag(CN) 2 ] – are the complex ions. A complex ion is an electrically charged radical formed by the union of a metal cation with one or more neutral molecules or anions. COORDINATION CHEMISTRY Chapter 20
- Takashiro Akitsu(Author)
- 2018(Publication Date)
- IntechOpen(Publisher)
For proper molecular organization, there is preference to use directional bonds (coordinate covalent bond and hydrogen bond) [5]. Coordination com-pounds are formed by dual aggregation of metal and multidentate ligands. In these Coordination Compounds, molecules and atoms may be considered as specific points. In these, network is con -nected, and metal and ligand are connected with each other to form a coordination compound. If inferred impulsions are applied to coordination number of metal through beating its coordina-tion sphere by its counterions, then prediction of formation of final network should increase. The chorography of coordination network is expected to follow from geometry of its constituent parts [6]. The connection of ligand toward metal atom is controlled by covalent synthesis. The proph-ecy of network structure depends upon coordination properties of metal atom. The coordination of metal is effected by counter ion, duplicity of ligand, and solvent [ 7]. 2. Coordination polymers Polymers can be described as molecules of high molecular weight which are formed by repetition of their monomeric subunits connected by covalent bonds [8]. However, coor-dination polymers are formed by central metal atom connected with organic ligands via coordination bonds and weak chemical bonds. These compounds are also named as metal organic frameworks [9 ]. The organization of different factors in coordination polymers exists in solid form most of the time [10]. Coordination polymers are completely regular in shape, having high porosity and designable frameworks. Synthesis of these networks is done under mild conditions by using discrete subunits and this method is commonly known as bottom-up method. Components of these polymers are blocking ligands, coun -teranions, and template molecules. Transition metal ions are often used as functional con-nectors in the formation of coordination polymers.
eBook - PDFFundamentals of Inorganic Chemistry
An Introductory Text for Degree Studies
- J Barrett, M A Malati(Authors)
- 1997(Publication Date)
- Woodhead Publishing(Publisher)
10 Coordination Complexes 10.1 INTRODUCTION Alfred Werner (Nobel Prize for Chemistry, 1913) recognized in 1893 that compounds such as C0CI3.6NH3 should be more correctly formulated as coordination complexes, in this case as [Co(NH 3 ) 6 ]Cl 3 with the central metal ion retaining its normal or primary valency of three (with regard to the three chloride ions), but possessing a secondary valency of six (with regard to the six ammonia molecules). The ammonia ligand molecules are bonded to the central cobalt(III) ion by coordinate or dative bonds in which the electrons are provided by the ligands in the production of a complex ion, [Co(NH 3 ) 6 ] 3+ . Coordination complexes play a dominant role in modern inorganic chemistry with those with central transition metal ions representing a large area of chemistry. The material in this chapter is restricted to the theoretical treatment of the three main types of complex—those in which the coordination number of the metal ion (the number of ligand atoms attached to the metal) is six (octahedral, O h ) or four (square planar, £> 4A , or tetrahedral, T d ). The point group symbols are used to denote the symmetry of the central metal ion and its immediate environment of donor ligand atoms, rather than necessarily expressing the true symmetry of the complex as a whole. In all complexes the metal to ligand bonding relies mainly upon pairs of electrons which are donated from the ligand to the metal. In some instances reverse donation from metal to ligand is important. The conventional view of the bonding describes it as coordinate bonding in which the electrons of the metal are not allowed any importance. This is a great oversimplification of the apparent situation where a particular oxidation state of a metal accepts a number of pairs of ligand electrons to allow complex formation.
- Chris J Jones, John R Thornback, Peter J Sadler, Jon R Dilworth, David R Williams, David M Taylor(Authors)
- 2007(Publication Date)
- Royal Society of Chemistry(Publisher)
These polyhedra are usually shown in their most regular form as if all the edges were of equal length, although in reality this is an idealised representation of a structure which is usually not fully regular. Coordination Compounds, or complexes, of metals typically contain inor-ganic donor atoms such as O, N, S, P, F or Cl. In the particular case where one or more donor atoms in the complex is carbon, and there is a direct metal to carbon bond, the metal complex is said to be an organometallic compound. In order for such compounds to be unreactive towards air and moisture it is necessary for their metal-ligand bonding to have significant covalent character. Organometallic compounds in which the bonding is ionic in character are usually rather reactive. As an example diethyl zinc, Zn(C 2 H 5 ) 2 , is spontane-ously flammable in air. 2.4.2.2 Isomerism In some cases different structural arrangements of atoms are possible for a particular metal complex. As an example the ligands in a complex [ML 4 ] (L is a monodentate ligand, i.e. bound to the metal through only one donor atom) containing a metal ion with CN four might be arranged at the vertices of a 12 When the formula of a complex is written in square brackets, these enclose the full list of ligands directly attached to the metal centre. The symbol for the metal appears first in the formula followed by those of the, ligands usually in alphabetical order. If the complete set of ligands bound to the metal is not listed, square brackets should not be used. 55 The Chemistry of Metals in a Nutshell
eBook - PDFChemistry
The Molecular Nature of Matter
- Neil D. Jespersen, Alison Hyslop(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
With all of these concepts available to us now, we will direct our emphasis in the opposite direction and apply the concepts to examine some of the physical and chemical properties of the compounds of metals. This chapter is devoted to an in-depth look at complex ions of metals, a topic first introduced in Chapter 17. These substances—which have applications from food pre- servatives to catalyzing biochemical reactions, such as vitamin B 12 , which is involved in metabolism—have a variety of structures and colors. LEARNING OBJECTIVES After reading this chapter, you should be able to: • describe the different kinds of ligands and the rules for writing formulas for metal complexes containing them • use the rules for naming metal complexes • show the different geometries associated with each of the common coordination numbers, and draw the structures • write structural formulas for all the isomers of a given metal complex • describe how crystal field theory explains the properties of metal complexes • explain the biological function of several common metal complexes in the body 1004 Chapter 21 | Metal Complexes A ligand atom that donates an electron pair to the metal is said to be a donor atom and the metal is the acceptor. Thus, in the preceding AgNH 3 + example, the nitrogen of the ammonia molecule is the donor atom and the silver ion is the acceptor. Because of the way the metal–ligand bond is formed it is a coordinate covalent bond, and compounds that contain metal complexes are often called Coordination Compounds. The complexes themselves are sometimes referred to as coordination complexes. Types of Ligands Ligands, the Lewis bases in these metal complexes, can range from monatomic ions to com- plex ligands that can bind metals at more than one site, and they can be neutral or anionic. Anions that serve as ligands include many simple monatomic ions, such as the halide ions (F - , Cl - , Br - , and I - ) and the sulfide ion (S 2- ).
eBook - PDF- Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2019(Publication Date)
- Openstax(Publisher)
This figure shows, from left to right, solutions containing [M(H 2 O) 6 ] n+ ions with M = Sc 3+ (d 0 ), Cr 3+ (d 3 ), Co 2+ (d 7 ), Ni 2+ (d 8 ), Cu 2+ (d 9 ), and Zn 2+ (d 10 ). (credit: Sahar Atwa) Remember that in most main group element compounds, the valence electrons of the isolated atoms combine to form chemical bonds that satisfy the octet rule. For instance, the four valence electrons of carbon overlap with electrons from four hydrogen atoms to form CH 4 . The one valence electron leaves sodium and adds to the seven valence electrons of chlorine to form the ionic formula unit NaCl ( Figure 19.13). Transition metals do not normally bond in this fashion. They primarily form coordinate covalent bonds, a form of the Lewis acid-base interaction in which both of the electrons in the bond are contributed by a donor (Lewis base) to an electron acceptor (Lewis acid). The Lewis acid in coordination complexes, often called a central metal ion (or atom), is often a transition metal or inner transition metal, although main group elements can also form Coordination Compounds. The Lewis base donors, called ligands, can be a wide variety of chemicals—atoms, molecules, or ions. The only requirement is that they have one or more electron pairs, which can be donated to the central metal. Most often, this involves a donor atom with a lone pair of electrons that can form a coordinate bond to the metal. FIGURE 19.13 (a) Covalent bonds involve the sharing of electrons, and ionic bonds involve the transferring of electrons associated with each bonding atom, as indicated by the colored electrons. (b) However, coordinate covalent bonds involve electrons from a Lewis base being donated to a metal center. The lone pairs from six water molecules form bonds to the scandium ion to form an octahedral complex. (Only the donated pairs are shown.) The coordination sphere consists of the central metal ion or atom plus its attached ligands.
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2019(Publication Date)
- Openstax(Publisher)
(b) However, coordinate covalent bonds involve electrons from a Lewis base being donated to a metal center. The lone pairs from six water molecules form bonds to the scandium ion to form an octahedral complex. (Only the donated pairs are shown.) The coordination sphere consists of the central metal ion or atom plus its attached ligands. Brackets in a formula enclose the coordination sphere; species outside the brackets are not part of the coordination sphere. The coordination number of the central metal ion or atom is the number of donor atoms bonded to it. The coordination number for the silver ion in [Ag(NH 3 ) 2 ] + is two ( Figure 19.14). For the copper(II) ion in [CuCl 4 ] 2− , the coordination number is four, whereas for the cobalt(II) ion in [Co(H 2 O) 6 ] 2+ the coordination number is six. Each of these ligands is monodentate, from the Greek for “one toothed,” meaning that they connect with the central metal through only one atom. In this case, the number of ligands and the coordination number are equal. 19.2 • Coordination Chemistry of Transition Metals 949 FIGURE 19.14 The complexes (a) [Ag(NH 3 ) 2 ] + , (b) [Cu(Cl) 4 ] 2− , and (c) [Co(H 2 O) 6 ] 2+ have coordination numbers of two, four, and six, respectively. The geometries of these complexes are the same as we have seen with VSEPR theory for main group elements: linear, tetrahedral, and octahedral. Many other ligands coordinate to the metal in more complex fashions. Bidentate ligands are those in which two atoms coordinate to the metal center. For example, ethylenediamine (en, H 2 NCH 2 CH 2 NH 2 ) contains two nitrogen atoms, each of which has a lone pair and can serve as a Lewis base ( Figure 19.15). Both of the atoms can coordinate to a single metal center. In the complex [Co(en) 3 ] 3+ , there are three bidentate en ligands, and the coordination number of the cobalt(III) ion is six.
- Julian A Davies, C M Hockensmith, Yu N Kukushkin(Authors)
- 1996(Publication Date)
- World Scientific(Publisher)
Chapter 1. SYNTHESIS OF Coordination Compounds: THEORETICAL CONSIDERATIONS The reaction chemistry of Coordination Compounds is governed by a number of general principles. These principles are based largely upon experimental observations and much of the underlying theoretical basis of coordination chemistry remains essentially qualitative in nature. Nevertheless, even qualitative guidelines are valuable in designing new synthetic methods. This Chapter deals with the basic principles used in planning syntheses of Coordination Compounds. Complete discussion of the underlying basis of coordination chemistry may be found in a number of texts. 1 1.1 Labile and inert Coordination Compounds Because syntheses of Coordination Compounds frequently involve ligand substitution reactions, an understanding of the relative lability (or its opposite, kinetic inertness) of metal complexes is important in the design of experimental procedures. Geometric isomers of labile complexes pose a special problem in synthesis because facile routes for interconversion may allow isomerization, favoring formation of the thermodynamically more stable isomer. The reactions of thermodynamically stable, homoleptic, anionic metal cyanide complexes with 14 CN illustrate the fundamental difference in reactivity between labile and kinetically inert complexes, 11 * Eq. 1.1: [M(CN) 6 ] 3 -+ 6 14 CN ^ [M(14CN) 6 ] 3 + 6CN- (1.1) where M = Mn, Cr With manganese, the half life of the reaction is on the order of one hour. In contrast, with chromium, the half-life is approximately 24 days. A simple consideration of the relative ligand field stabilization energies (LFSE) of d 3 (Cr 3 +) and d 4 (Mn 3 +) systems and how these are affected by passage along the reaction coordinate allows rationalization of the experimental results.
eBook - PDFNomenclature of Inorganic Chemistry
Inorganic Chemistry Division Commission on Nomenclature of Inorganic Chemistry
- Sam Stuart(Author)
- 2013(Publication Date)
- Pergamon(Publisher)
* In these and in some of the following examples the central metal atom has been omitted from the formulae for the sake of clarity. 57 7.512 Coordination Compounds 7.513 The first ligand to be mentioned in the name is given the lowest possible designator and the second ligand, the next lowest possible designator. The assignments to the remaining ligands follow from their position in the complex as lettered above. By the choice of a specific order and direction for assigning locants, it is possible to designate a specific optical isomer and to distinguish between enantiomers. However, in contrast to the practice for organic enantiomers (with tetrahedral atoms) there are differences in the names for the enantiomers other than some symbol designating the chirality. There is objection to this practice by some who prefer that the assignment of locants for one optically active form (say the levo) be the mirror image of that for the dextro. This matter of the choice of a system of assignment of locants for enantiomers is under careful consideration by the Commission. Examples: 0 2 N ^ N H 3 Pt p y ^ H 2 OH a-arnmine-6Hhydroxylamine)-i/-nitro-c-(pyriciine)platinum(l -f ) chloride a-ammine-Mhydroxylamine)-^-nitro-c-(pyridine)platinum(ii) chloride There are two more isomers of this composition. 2. û(/-diainmîne-6c-diaqua-i/e-bis(pyridine)cobalt(3 -f ) ion û/-diammine-6c-diaqua-i/e-bis(pyridine)cobalt(iii) ion There are four more possible isomers of this composition, one of which exists in enantiomeric forms. 7.513—The application of these locant designators to chelate ligands is based on the following rules* : (a) For symmetrical linear ligands of the type A A, the position in the coordination sphere of the coordinating atom at one end of the ligand shall be given and then, successively, that of each atom of the ligand through which coordination takes place.
eBook - PDF- Brian W. Pfennig(Author)
- 2021(Publication Date)
- Wiley(Publisher)
534 10 COORDINATION CHEMISTRY Very low coordination numbers are extremely rare among transition metal compounds. Typically, coordination numbers of two will only occur for the +1 cations of the Group 11 metals or for the Hg 2+ ion. All these metals have a spher- ically symmetric, or filled d 10 -subshell, and a low oxidation number. Where sub- stances such as Ag(NH 3 ) 2 + have been characterized, they typically have linear geometries and can easily react to form more stable species having a higher coor- dination number, such as Ag(NH 3 ) 4 + . Low-coordinate geometries can also exist when the ligands are extremely bulky, such as [N(SiMe 3 ) 2 ] - or [P(cyclo-C 6 H 11 ) 3 ]. Three-coordinate transition metal complexes are also uncommon. Again, these spe- cies are only favored by sterically demanding ligands and low oxidation numbers. Some examples include the zero-valent compounds Pt(PPh 3 ) 3 and Cr{N(SiMe 3 ) 2 } 3 . Most of the three-coordinate species have trigonal planar molecular geometries, predominantly as the result of steric repulsions between the bulky ligands. Four-coordinate transition metal complexes, on the other hand, rank second only to six-coordinate species in their prevalence in coordination chemistry. The two limiting four-coordinate geometries are tetrahedral (T d ) and square planar (D 4h ), with a whole spectrum of intermediate and distorted structures in between these two extremes. Tetrahedral species are favored by the Kepert model on the basis of steric interactions. Thus, Coordination Compounds with large ligands, such as Cl - , Br - , and I - , especially tend to form tetrahedral compounds. Among the metals, tetrahedral coordination is favored by small metals in lower oxidation num- bers, metals having noble gas electron configurations (Be 2+ , Al 3+ ), and transition metals with closed d-subshells (such as d 0 or d 10 ), for example, MnO 4 - or Ni(CO) 4 . A large number of Cu(II) compounds are also tetrahedral.
eBook - PDFChemistry
An Atoms First Approach
- Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste, , Steven Zumdahl, Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
Aqueous solutions of copper(II) salts are a characteristic bright blue color due to the presence of the Cu(H 2 O) 6 21 ion. Table 20.11 lists some typical copper compounds. Although trace amounts of copper are essential for life, copper in large amounts is quite toxic; copper salts are used to kill bacteria, fungi, and algae. For example, paints containing copper are used on ship hulls to prevent fouling by marine organisms. Widely dispersed in the earth’s crust, zinc is refined mainly from sphalerite (ZnS), which often occurs with galena (PbS). Zinc is a white, lustrous, very active metal that behaves as an excellent reducing agent and tarnishes rapidly. About 90% of the zinc produced is used for galvanizing steel. Zinc forms colorless salts in the 12 oxidation state. 20.3 Coordination Compounds Transition metal ions characteristically form Coordination Compounds, which are usually colored and often paramagnetic. A coordination compound typically consists of a complex ion, a transition metal ion with its attached ligands (see Section 17.3), and counterions, anions or cations as needed to produce a compound with no net charge. The substance [Co(NH 3 ) 5 Cl]Cl 2 is a typical coordination com- pound. The brackets indicate the composition of the complex ion, in this case Co(NH 3 ) 5 Cl 21 , and the two Cl 2 counterions are shown outside the brackets. Note that in this compound one Cl 2 acts as a ligand along with the five NH 3 molecules. In the solid state this compound consists of the large Co(NH 3 ) 5 Cl 21 cations and twice as many Cl 2 anions, all packed together as efficiently as possible.
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