Coordination cpds. and complexation

1. Resonance:the dynamic motion of e(s) can represented by various electronic (pictures) knowns as canonical str.s or forms.Each of ese structures make a certain contribu to  actual structure is said to be a resonance hybride of its canonical forms.**  resonance: indicates a tendency for  e(s) to be somewhat delocalized.e.g. HCl (canonical str.s)

2. str.a ..is ionic w- more electronegative chlorine taking  bonding pair of e(s). str. b ..is  covalent situa w-  bonding pair being shared (.)  2 atoms,but  greater electronegativity of  chlorine make is structure a polar covalent form.str.c… Is not likely to bo str. b_ of  relatively low e affinity of H for  bonding pair of e(s).

3. ** for all practical purposes, str.of HCl can represented resonance hybrid of str.s a & b depending on some mathematical contribu to a valence bond or quantum mechanical descrip of  molecule.

4. Unsaturated cpds.= greater mobility of  e(s) yield more possibilities for canonical forms.***using CO2*is str.s not in equilibrium.

5. c…. Actual str. Is a resonance hybrid in w_ each of  oers makes a weighted contribu to  overall electronic picture.Note:str. a&e & str. b&c r- equivalent on  basis of symmetry.Simillargrp.ofstr.s can be drown for  sulfate ion (single & double s-o bonds).

6. Coordination cpds:- include trans. metal cpds.* variable oxid. #* colored* often contain covalent cpds. bonded to  metal *hydrate* lewis acid-base adduct* coordinate covalent bonds* unusual magnetic properties

7. Coordination cpds. and complexation *trans. Series & period 2(outside trans. Element) After normal valence requirement have additional anions or molecules Additional bonding species ligands (w_ bond directly to  metal cation in accordance w- maximum coordina #)

8. If  complex bears a charge,it is known complex ion.(metal associated w- ligands).**complex ion w- counter ions known as a coordina cpd. * some complexes r- stable in crystalline form & decompose in solution, while oers r- stable only in solu.

9. Ligands occupy space about  metal known as coordination sphere, also can displaced by other ligands.* coordination sphere: is  central metal & surrounding ligands.  sequar bractes separate  complex from counter ions such as

10. For e.g. forma some coordina cpds. can be seen w- :Iron(III) & chlorine. wn is cpd. Is dissolved in water and/or HCl,  followig coordina cpds. r- formed.

11. Wernerr theory * Central metal ion has 2 types of valency: 1- 1ry valence(principle valence) (ionizable) 2- 2ry valence(nonionizable): # of molecules / ions bond to metal atom, called coordination #. e.g. has 1ry valence of 3 and coordination # of 6.

12. * 1ry valences of metal r- filled by anions, but 2ry valence( in coordination #) by anions and /or neutral molecules known as ligands (cation rarly p-)*- ligands r- arranged around  metallic ion in certain cc geometry.

14. * most stable complexes r- formed by cations of  trans. series & particulary  trans.elem.s in grps.VIII.Ben grps.VIB,VIIB,IB,IIB &IIIA also form stable coordination cpds.  major criteria for max. stability of metal in a complex involve:* high (+)ve charge.* small cationic radius* unoccupied d orbitals.

15. **Neutral atoms r- not usually found as coordinating agents. * Important feature that all ligands have is possession of at least one nonbonded pair of e(s)w_ is used to form a coordinate covalent bond w- metal ion. Properties of ligands:the ligands species in complexes r- generally anion or neutral molecules.

16. Ligand bonds to the metal through coordinate covalent interaction in which the ligand is the donor species & metal ion is acceptor.* the more stable complexes are formed which anionic or molecular ligands involving the elements of grps. VA, VI A,VIIA.

17. * Order of stability of a ligand in a complex follows the order of basicity of the ligand. this refers to the strength of the electrostatic field emanating from an anion or ,in case of a neutral molecule, the ‘availability’ of lone pair of e(s). Basicity related to lewis base.

22. **Wn polydentate ligands complex a metal ion a ring str.is produced composed of  metal &  ligand molecule.ese ring str.s have special significance ,r- termed chelates. str. below show a simple chelate(.) a metal ion & ethylenediamine

23. In general, more stable chelate r- ose where  total # of atoms in  ring including  metal r- 5,6 or 7. 4 & 8-membered rings r- usually unstable.***polydentate ligands used for chelate forma -chelating agent [w_ use in pharmaceutical & in drug therapy].

24. Bonding in complexes:ere r- several approaches to discussion of bonding in metal complexes:1-Valence bond theory: developed by Pauling,w_ is a useful qualitative picture of bonding in coordination cpds. [predicts metal complex bonding arises from overlap of filled ligand orbitals & vacant orbitals].*Resulting bond is a coordinate covalent bond.

26. 2-crystal field theory(CFT):developed by Bethe & Van Vleck.*  coordination cpds depend on electrostatic interaction.[(.)(-) L& (+)ion metal]** is theory completely neglects covalent bonds formation.Van Vlek…..introduced 2 more comprehensive approaches:1) Ligand Field Theory(LFT) : involve modifica of CFT w_ includes covalent bond forma through orbital overlap.

27. 2) Molecular Orbital Theory(MOT):w̠ give  most complete qualitative &quantitative picture of bonding, but has many computational problems.We know  orienta of d orbitals in space only 2 along  axes. oer 3 orbitals r- directed (.) axes.

28. *1ry goal of is to obtaining a qualitative picture of bonding in complexes,  valence bond approach will be used.one attrective feature of this theory that lends itself to an easily formed picture of hybridization & bonding, although this picture not entirely correct.

29. valence bond problem can expressed, Cr(III) complexing w̅ cyanato CN̅ ions to form [Cr(CN̅)6]‾³. Cr(III) is dᶟ ion.(3 e)ese e(s) r- unpaired & occupy 3 off-axis d orbitals(dxy,dyz,dxz),thus leaving 2 d ,1 s,3 p orbitals empty for bonding w̅ 6 cyanato grps.

30. If ese 6 orbitals r̄ hybridize, a valence bond picture of complex ion can be drawn:

31. Similar valence bond reasoning can be applied to oer complexes formed w̅ metal ions having 1 to 3 e(s) in eir d orbitals. but  problem arises wn 4 or more e(s) in ese orbitals on  metal. e.g. complexes formed w̅ iron(III)(a d⁵ ion) according to is theory must alter  normal ground state arrangement of e(s),use different orbitals for bonding,or bond in a noncovalent manner.

32. Unpaired e(s) → r̅ attracted to a magnetic field, paramagnetic.Paired e(s) → r̅ repelled by a magnetic field, diamagnetic.*wn complexed w̅ water molecules  hexaaquoiron (III)ion is formed, and has  same magnetic properties as would be expected from  electronic str. above.

33. Magnetic moment is 5.9 B.M.(Bohr magnetons) w̠ is consistent w̅ 5 unpaired e(s).is referred to as a high-spin complexb̠ e(s) r̅ unpaired as much as possible. w̅in  framework of valence bond theory, problem of finding 6 empty atomic orbitals to overlap w̅  donor water molecules can be accomplished by assuming at  4d orbitals r- ofapproperiate energy to hybridize w̅  4s &4p orbitals. hybridiza of is type is termed outer orbital hybridza.Still provides 6 Oh arranged hybrids designated .

34. Wn replace water by cyanato grps.CN, hexacyanoferrate(III) ion w̠ has distinctly different magnetic properties.magnetic moment is about 2 B.M. w̠ is indicative of one unpaired e, ion is termed a low-spin complex.

35. ** electronic configura of is complex involves  3d orbitals an inner orbital hybrid.

36. * Metal ions w̅ 7,8,9 e(s) in d orbitals ,generally have a Coordination # of 4 w̠ lead to either Th or square planar arrangement of  ligands. strength of ligand &  forma of high-and low-spin complexes may be predictive of type of hybridiza & en geometry of  complex.

37. The valence bond approach to bonding and the use of magnetic criteria to predict hybridization and geometry r̅ not successful in many cases.one of the more comprehensive theories (e.g. ,ligand field or molecular orbital) would actually give a more theoritical satisfying account of bonding.

38. Λ Oh arrangement of hexacoordinate complexes in the following fig.s: