aa = a ee bis(tetraaqua-p2-hydroxo iron (I1)) cation COORDINATION NUMBER OF METAL COMPLEXES ‘The coordination number of a metal ion ii complex is the number of ligand donor atoms to: Tt is determined by counting the number of the donor atoms or site directly attached to the metal. Coordination number varies from | to 8, though the two extremes are rare, The structure of a ligand strongly depends on the coordination number as it determines the number of spatial orientation possible in any given complex. The various coordination numbers will be considered and the possible structures discussed. Coordination Number: complexes having coordination number one are rare and little is known of their chemistry. “Coordination Number 2: The complex with coordination number 2 well established are silver 7 eg. where the electronie configuration of Ag* is d'°. The hybridization of 180° and the possible shape is linear structure. es are [He(CN)s], [Au(CN)s] ete. N—Age-NH)J*. The arrows point from the donors to the acceptor. ination Number 3: Complexes with coordination number 3 are few. Metals with ition are. commonly found with this coordination number. The hybridization is angle 120%, which gives rise to trigonal planar structure. [Hels], Phi)z}Au(PPhs)s]* ete. Coordination number three is favoured by bulky ligand that can aric hindrance and prevent further coordination.m which can give rise to two Is of the central Coordination Number 4: This is a common coordination Tetrahedral and square planar, depe different geometric metal that received the donor pairs. Divalent jons such as Zn, electronic configuration and zero crystal field stabilization energy will give rise to tetrahedral ‘complexes with sp3 hybridization with bond angle 109° examples [CdCL]™, [2n(OH)s}*and (Hg(BF)s)* Similarly, few metals with d? and d° are known to form tetrahedral complexes MnOs’, MnCl, Ti of tetrahedral complexes e.g. [NiCl] and [Ni(CO):Jwith d* configuration 1s. Metals with other d-configurations have very limited number Square Planar Complexes are common with metals d8 electronic configuration. Examples are [PtCl(NHs)3], [Ni(CN)s}* and PdCL. The hybridization in these complexes is dsp? with bond angle of 90°, 7 ee nN Coordination number pyramidal and trigonal bipyramidal with the metal having spd hybridization. In square planar, dx2-y? orbital in the metal will receive one of the donated pairs while in trigonal bipyramidal; dz? orbital of the metal will receive one of the donated pairs. The energy difference between the two configurations is small hence they are interconvertible. Examples are (Fe(CO)s] and [Cu(bipy)aI]*, [VO(acac)2] and [VO(SCN).}*. two possible structures with coordination number five are square based NCS, ee . \L oe rr Coordination Number 6: This is the most common coordination number with two possible ‘geometries i.e. octahedral and trigonal prismatic, Octahedral is the most common with metal center having sp'd? or d°sp? hybridization with bond angle 90°. Examples; [Cu(H:0)s]*,[Co(en)sP* and [Fe(CN)s]*. 8STEREOISOMERISM Stereolsomerism occurs in complexes due to difference in spatial arrangement of ligands round the central metal. The atom-to-atom bonding sequence is, however, identical in these compounds. Amongst complexes, coordination number four and six are most frequently encountered Stereoisomerism is divided into geometric and. ‘optical isomerism. Coordination ‘Geometry Geometrical isomers | Optical isomers number 4 Tetrahedral Not possible Possible but rare ‘Square planar Possible ‘Not possible 6 ‘Octahedral Possible Possible Geometric isomerism ‘Geometrical isomers are also known as cis-trans isomers. The term cis means ‘adjacent to’, while means ‘opposite to’. Geometrical isomers are distinguished by dipole moment studies. The ‘has zero or nearly zero dipole moment, while cis analog possess a positive dipole ‘The most common type of geometrical isomerism involves cis and trans in square planar and octahedral complexes. In square planar ‘complex (P((NHs)Cl], the trans isomers are shown below. Note that in the érans- form, identical ligands are bond angle of 180° while in cis- the bond angle between identical ligands is 90°,Higher coordination numbers are possible but not common e.g. coordination seven [ZF] and [H#F;}*, coordination number eight [ZrFs}* and [Mo(CN)s}* ISOMERISM IN COMPLEXES Just as an infinite number of ways exist to arrange large number of different coloured shoes, there exists different ways on the molecular scale in which a given number of ligands can be arranged in three dimensional array round a central metal. This is the basic concept of isomerism in coordination chemistry. Different coordination numbers, shapes and points of attachment of ligands provide an almost infinite number of three-dimensional structurally related complexes ‘generally called isomers. Isomerism does not exist in complexes with identical monodentate and ‘atom ligands, However, wherever more than one type of ligand is bound, and even, the. chelating ligands bind, the possibility of isomerism needs to be considered. It is O note that the number of isomers increases with increase in coordination. The concept is well developed in complexes with coordination numbers four and six while no of isomerism is associated with coordination numbers one, two and three. Thecoordination chemistry, Isomers are of different kinds, they include hydrate or solvent isomers, fonization isomers, and coordination isomers having the same overall formula but with different donor atoms of the sume or diferent Tigands ataced tothe central metal atom or fon. The __Romenclature of different kinds of isomerism is an indication of whether solvent, anions, oF other ‘coordination compounds imposes the isomerism inthe structure. The terms linkage (ambidentate) ism is used when ambidentate ligands impose structural differences in complexes due to tse of different donor atoms on a tigand, Stereoisomers have the same ligands withthe same ___ donor atoms, but differ in the geometric (spatial) arrangement ofthe ligands. Some stereaisomers ee en Sas cated as ial Somes[>| rw | Ne ol ‘fe-tFiamminetrichlorocobalt(Il) mer-triamminetrichiorocobal(HHt) Optical isomerism ‘Those stereoisomers which fulfil the following criteria are designated a optical isomers or ‘enantiomers. These— > Are mirror-images of each other? » Are non-superimposable. > Rotate the plane of polarized light in opposite directions. Optical isomers exist in complexes that are not superimposable on their mirror images. These isomers have ability to rotate the plane of polarised light in opposite direction. A mixture of ‘optical isomers of the same quantity will not rotate the plane of polarized light because the effect. of one isomer is cancelled out by the other. Such a mixture is called racemic mixture, Optical isomerism is possible in tetrahedral and octahedral complexes (cis-isomers) where centre of i symmetry is absent but not in square planar complexes. Optical isomers are called enantiomers. A solution of enantiomer that rotates the plane of polarized light in clockwise direction is designated. i _as positive (+) or dextrorotatory (d) enantiomer while the isomer that rotates the plane of polarized | light in anticlockwise direction is designated as negative (-) or laevorotatory (J) enantiomer. ‘5 dichlorobis(ethylenediammine}eobalt (HI). The optical isomers have. identicalSTRUCTURAL ISOMERISM Also known as constitutional isomers, structural isomers have the same empirical formula but differ in the arrangement of their constituent atoms. This results in difference in physical Properties such as colour. Many different kinds of structural isomerism occur in coordination chemistry. i. Ionization isomerism lonization isomers give different ions in aqueous solution. This is because different anions ‘coordinated to the metal in the coordination sphere. The isomeric pairs differ in that there is an exchange of two anionic groups within and outside the coordination sphere. Examples; [Co(NH;)sBr]SOx (violet) and [Co(NHa)sSO«]Br (red). Note that in [Co(NHs)sBr}SO« the sulphate is the counter ion and can be detected by treating the solution of the complex with BaCl to precipitate the sulphate in the form of BaSO«(qualitative test for sulphate), The bromide ion is coordinated and will not precipitate with silver nitrate because itis not free. In [Co(NHs)sSO4]Br, the test for bromide will be positive since Br is not coordinated to the metal ‘while the test for sulphate will be negative since the sulphate is in the coordination sphere and not free. [Pi(en)2Cl]Br2 and [Pi(en)2Br]Cl2 [Cx(NH):CIBF]NO: and [CreNHS)sCINOAIBr I s)sCIJNO2 and [Cr(NHs)sNO2]CI c and [Co(NHs)\CIBr]Br ii, Hydrate isomerism plexes are prepared in aqueous solutions where water is abundant, complexes can As many com) “prespitte or eqystallize with water of erystalizaton ouside the coordination sphere or wth in er (ligand) inside the coordination sphere. There are many isomers which differ in molecules in their formula. For For example, there are three known hydrate : ee (violet), [Cr(H20)sCI]Ch.H20 (pale green) andOther examples are: [Co(NHs)(HO)CNC: and (Co(NHs).CLICLH,O [Co{NH,)s(H:0)](NOs)s and [Co(NH3)(NO3)](NO)2.H20 iii, Coordination isomerism ‘These isomers contain pairs of ionic complexes that exchange ligands with each other. Many isomeric pairs are possible by redistribution of ligands between two metal centres. Examples are: [Co(NHs)6][Cr(CN)s} and [CrONH5).][Co(CN)] [CofNHs)s(CN)}[Cr(CN)s(NH)] and {CrQNHs)s(CN)}[Co(CN)s(NH3)] [Cofen)s][Cr(CN).] and [Cr(en)s][Co(CN).} [Cu(NHs)«][PtCly] and [Pt(NHs)<}[CuCl,] ‘Note that the cationic complex is written first iv, Linkage isomerism ‘This type of isomerism is observed in complexes containing ambidentate ligands which can coordinate through at least two different binding sites. Example of such ligand is nitrite (NO) which ean coordinate through nitrogen or oxygen. see" (red) and [Co(NHs)s(ONO)}** (yellow) yellow complex, [Co(NHs)s(ONO)}*, is unstable and it is converted into[Co(NHs)s(NO2) > both in solution and the solid state either by heating or by exposure to ultraviolet light. The two isomer can be distinguished through Infrared spectroscopy, for the O-bonded ligand, characteristic absorption bands at 106 and 1470 cm are observed, while for the N-bonded ligand, the corresponding vibrational wavenumbers are 1310 and 1430 cm, Other examples are [Co(CN)s(SCN)]* and [Co(CN)(NCS)}*, where the sulphur and nitrogen of the thiocyanate ligand imposes the observed linkage isomerism, ‘isomers are isomers with the same simplest unit called monomer. The 0 ‘more monomer units results in polymeric complex isomer. An example is (on combination can give; [PY(NH3)s[PtCli{PUNHs)sCs[PtCl]