Theory of coordination_compounds[1]

This presentation gives information about coordination compounds and its theory and nomenclature

1. Theory of coordination compounds By Mohammed Ismail A

2. Coordination compound A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent bonds.

3. Coordination bond A coordinate bond(dative covalent bond) is a covalent bond in which both elctrons come from same atom A coordinate bond is shown by an arrow.the arrow points from the atom donating the lone pair to the accepting atom A B

4. terminologies Lewis Acid All electron acceptors are lewis acids. Lewis Base All electron donors are lewis base. Central metal ion In the complex ion an acceptor accepts a pair of electrons from the donor atoms. The acceptor is usually a metal / metal ion to which one (or) more of neutral molecules (or) anions are attached. The acceptor metal cation is referred to as central metal cation . Hence, central metal cation in a complex serves as a lewis acid.

5. Terminologies contd . Oxidation state This number denotes the charge, explaining the number of electrons it has lost to form the cation . It is oxidation number that denotes the charge, if the central metal atom would have if all the ligand in the complex were removed along with their electron pairs that were shared with the central atom. It is usually represented by Roman Numeral. Ligand (Latin word meaning to bind) A ligand is an ion (or) a molecule capable of functioning as an electron donor. Therefore the neutral molecules or ions which are directly attached to the central metal ion are called as ligand (or) coordination groups. These coordination groups or ligands can donate a pair of electrons to the central metal ion (or) atom. Hence, in a complex compound ligands act as Lewisbases .

6. Terminologies contd. Coordination sphere In a complex compound, it usually, central metal ion and the ligands are enclosed with in square bracket is called as coordination sphere. This represents a single constituent unit. The ionisable species are placed outside the square bracket. These ions do not ionise to give the test for constituent ions. Coordination number The coordination number of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded. Numerically coordination number represents the total number of the chemical bonds formed between the central metal ion and the donor atoms of the ligands. For example in K4[Fe(CN)6] the coordination number of Fe(II) is 6 and in [Cu(NH3)4]SO4 the coordination number of Cu(II) is 4.

7. Types of ligand When a ligand is bound to a metal ion through a single donor atom, as with - Cl , H2O or NH3, the ligand is said to be unidentate . Whenever a single coordinating group (or) ligand occupies two (or) more coordination position on the same central metal ions, a complex possessing a closed ring is formed. Such ligands are called polydentate ligands. When a single ligand has two coordinating positions,it is called bidentate ligand and when there are three coordinating positions available, it is called a tridentate ligand and so on. For example, ethylenediamine is a bidentate ligand because it has two amino groups each of which can donate a pair of electrons.

8. Name of ligand Positive ligands The positive ligands are named with an ending - ium .   Neutral ligands The neutral ligands are named as such without any special name. But water is written as 'aqua : Ammonia is written as ammine. Note that two m's to distinguish from organic amine  CO-Carbonyl, NO- Nitrosyl , NH2 - CH2 - CH2 - NH2-ethylenediamine (en), Pyridine C5H5N. Negative Ligands Negative ligands end in suffix 'O'. Example F-- Fluoro , Cl -- Chloro , C2O42-- Oxalato , CN-- Cyano , NO2--Nitro, Br-- Bromo , SO42-- Sulphato , CH3COO-- acetato CNS-- thiocyanato , NCS-- isothiocyanato , S2O32-- thiosulphato .

9. continued Chelates If a ligand is capable of forming more than one bond with the central metal atom (or) ion then the ring structures are produced which are known as metal chelates. Hence the ring forming group are described as chelating agents (or) polydentate ligands.

10. Theory of coordination complex W erner’s theory : Alfred Werner studied the structure of coordination complexes and put forward his ideas in the year 1893 which were known as 'Werner's coordination theory . Experimental verification : when silver nitrate was added to CoCl 3 ·6NH 3,  all the three chloride ions were converted to silver chloride. However, when silver nitrate was added to CoCl 3 ·5NH 3 , only two mole of silver chloride was formed. When CoCl 3 ·4NH 3  was treated with silver nitrate, one mole of silver chloride was formed. Based on his observations, Werner postulated the following theory

11. Postulates of werner’s theory 1.Every metal atom has two types of valencies i ) Primary valency or ionisable valency ii ) Secondary valency or non ionisable valency 2.The primary valency corresponds to the oxidation state of the metal ion. The primary valency of the metal ion is always satisfied by negative ions. 3.Secondary valency corresponds to the coordination number of the metal ion or atom. The secondary valencies may be satisfied by either negative ions or neutral molecules. 4.The molecules or ion that satisfy secondary valencies are called ligands. 5.The ligands which satisfy secondary valencies must project in definite directions in space. So the secondary valencies are directional in nature whereas the primary valencies are non-directional in nature. 6.The ligands have unshared pair of electrons. These unshared pair of electrons are donated to central metal ion or atom in a compound. Such compounds are called coordination compounds .

12. Werner’s representation Werner represented the first member of the series [Co(NH3)6]Cl3 as follows. In this representation, the primary valency (dotted lines) are satisfied by the three chloride ions. The six secondary valencies (solid lines) are satisfied by the six ammonia molecules.

13. Limitations of werner’s theory It failed to explain why all elements don’t form coordination compounds. It failed to explain the directional properties of bonds in coordination compounds. It does not explain the colour, and the magnetic and optical properties shown by coordination compounds.

14. Valence bond theory Valence bond theory, primarily the work of Linus Pauling regarded bonding  as characterized by the overlap of atomic or hybrid orbitals of individual atoms. The postulates of valence bond theory   The central metal atom/ion makes available a number of vacant orbitals equal to its coordination number. These vacant orbitals form covalent bonds with the ligand orbitals. A covalent bond is formed by the overlap of a vacant metal orbital and filled ligand orbitals. This complete overlap leads to the formation of a metal ligand,  (sigma) bond. A strong covalent bond is formed only when the orbitals overlap to the maximum extent. This maximum overlapping is possible only when the metal vacant orbitals undergo a process called 'hybridisation'. A hybridised orbital has a better directional characteristics than an unhybridised one .

15. The following table gives the coordination number, orbital hybridisation and spatial geometry of the more important geometrics Coordination number Type of hybridisation Geometry 2 sp Linear 4 sp3 Tetrahedral 4 dsp2 Square planar 6 sp3d2 Octahedral 6 d2sp3 Octahedral

16. Magnetic moment  

17. Applications of valence bond theory

18. Continued. Octahedral complex

19. Defects of Valence bond theory Although VB theory was the principal way in which chemist visualized coordination compounds until the 1950s, it has fallen into disfavour due to its inability to account for various magnetic, electronic and spectroscopic properties of these compounds .

20. Nomenclature of coordination compounds The coordination compounds are named in the following way.  To name a coordination compound, no matter whether the complex ion is the cation or the anion,  always name the cation before the anion . (This is just like naming an ionic compound .) In naming the complex ion: 1 .  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

21. continued 2 . The names of some common ligands are listed in Table 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  -o; -ate  - ato ; - ite   - ito For neutral ligands, the common name of the molecule is used e.g. H 2 NCH 2 CH 2 NH 2  ( ethylenediamine ).  Important exceptions : water is called ‘aqua’, ammonia is called ‘ammine’, carbon monoxide is called ‘carbonyl’, and the N 2  and O 2  are called ‘ dinitrogen ’ and ‘ dioxygen ’.

22. tables Anionic Ligands Names   Neutral Ligands Names Br - bromo   NH 3 ammine F - fluoro   H 2 O aqua O 2- oxo   NO Nitrosyl OH - Hydroxo   CO Carbonyl CN - cyano   O 2 dioxygen C 2 O 4 2- oxalato   N 2 dinitrogen CO 3 2- carbonato   C 5 H 5 N pyridine CH 3 COO - acetato   H 2 NCH 2 CH 2 NH 2 ethylenediamine Table 1. Names of Some Common Ligands

23. continued 3 .  Greek prefixes are used to designate the number of each type of ligand in the complex ion. If the ligand already contains a Greek prefix (e.g. ethylene di amine ) or if it is polydentate ligands ( ie . can attach at more than one binding site) the prefixes bis -, tris -, tetrakis -, pentakis -, are used instead . 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. If the complex ion is an anion, the name of the metal ends with the suffix – ate. 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 ).

24. Number Prefix Number Prefix Number Prefix 1 mono 5 penta (pentakis) 9 nona (ennea) 2 di (bis) 6 hexa (hexakis) 10 deca 3 tri (tris) 7 hepta 11 undeca 4 tetra (tetrakis) 8 octa 12 dodeca Table 2. Numerical Prefixes

25. Name of Metal Name in an Anionic Complex Iron Ferrate Copper Cuprate Lead Plumbate Silver Argenate Gold Aurate Tin Stannate Table 3: Name of Metals in Anionic Complexes

26. continued 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) catiom   BEFORE  the (possibly complex) anion. For historic reasons, some coordination compounds are called by their common names. For example, Fe(CN) 6 3-  and Fe(CN) 6 4-  are named ferricyanide and ferrocyanide respectively, and Fe(CO) 5  is called iron carbonyl.

27. examples 1. [Cr(NH 3 ) 3 (H 2 O) 3 ]Cl 3 Answer: triamminetriaquachromium(III) chloride 2. [ Pt (NH 3 ) 5 Cl]Br 3 Answer: pentaamminechloroplatinum (IV) bromide 3. [ Pt (H 2 NCH 2 CH 2 NH 2 ) 2 Cl 2 ]Cl 2 Answer: dichlorobis ( ethylenediamine )platinum(IV) chloride 4. [Co(H 2 NCH 2 CH 2 NH 2 ) 3 ] 2 (SO 4 ) 3 Answer: tris ( ethylenediamine )cobalt(III) sulfate 5. K 4 [Fe(CN) 6 ] Answer: potassium hexacyanoferrate (II)

28. Contined . 6. Na 2 [NiCl 4 ] Answer: sodium tetrachloronickelate (II) 7.   Pt (NH 3 ) 2 Cl 4 Answer: diamminetetrachloroplatinum (IV) 8. Fe(CO) 5 Answer: pentacarbonyliron (0) 9. (NH 4 ) 2 [Ni(C 2 O 4 ) 2 (H 2 O) 2 ] Answer: ammonium diaquabis(oxalato)nickelate(II) 10.[Ag(NH 3 ) 2 ][Ag(CN) 2 ] Answer: diamminesilver(I) dicyanoargentate(I)

29. Can you give the molecular formulas of the following coordination compounds ? 1 . hexaammineiron (III) nitrate 2. ammonium tetrachlorocuprate (II) 3. sodium monochloropentacyanoferrate (III) 4. potassium hexafluorocobaltate (III)

30. answers 1. [Fe(NH 3 ) 6 ](NO 3 ) 3 2. (NH 4 ) 2 [CuCl 4 ] 3. Na 3 [FeCl 1 (CN) 5 ] 4. K 3 [CoF 6 ]