Stereochemistry of Coordination Compounds PDF

# STEREOCHEMISTRY OF COORDINATION COMPOUNDS WITH DIFFERENT COORDINATION NUMBERS The ligands adopt a definite geometry around the central metal ion depending upon the coordination number of the ion. The arrangement of the coordinated ligands is such that that the electrostatic repulsion between them...

# STEREOCHEMISTRY OF COORDINATION COMPOUNDS WITH DIFFERENT COORDINATION NUMBERS The ligands adopt a definite geometry around the central metal ion depending upon the coordination number of the ion. The arrangement of the coordinated ligands is such that that the electrostatic repulsion between them is minimum. We shall consider the stereochemistry of complexes with coordination numbers varying from 2 to 9. It is pertinent to mention here that the lines between metal and ligands in the figures do not show the exact bond order or the type of linkage. These lines merely indicate a chemical association between the metal and the ligands. ## Coordination Number 2 There are only two possible arrangements for compounds with coordination number 2: 1. Linear 2. Bent When the central ion utilizes the two hybrid orbitals for bonding with the ligands and does not contain any lone pair of elections, it forms linear complexes. But, when the central ion utilizes, apart from the two hybrid orbitals, one or more additional hybrid orbitals for accommodating lone pairs of electrons, it forms bent complexes. The linear arrangement is adopted by complexes of Cu(I), Au(I) and Ag(I). Examples are: [CuCl2], [Au(CN)2], [Ag(NH3)2]+, etc. Examples of complexes having non-linear arrangements are rare. ## Coordination Number 3 This coordination number is uncommon, rather rare. Several compounds which from their stoichiometry appear to be 3-coordinate, are found upon examination to have higher coordination numbers. The complex compound K[Cu(CN)2] is an example of true 3-coordination. Some other complex compounds which on X-ray examination have been found to be 3-coordinate are: tris(trimethylphosphinesulphide)copper(I)perchlorate, [Cu(SP(CH3)3)3]+[ClO4], tris(triphenylphosphine)platinum(0), [(C6H5)3P)3Pt] and triiodomercurate (II) anion, [HgI3]. In all these complexes, the geomettry is trigonal planar, as shown in Fig. 11 for [Hgl3] The iodide ions are arranged at the three corners of an approximate equilateral triangle while Hg atom lies at the centre. ## Coordination Number 4 In this case, the two principal geometries encountered are: 1. Tetrahedral 2. Square Planar ### 1. Tetrahedral Geometry This geometry is common amongst complexes of transition as well as non-transition elements. Some of the common examples are: [BeF4]2, [BF4], [ZnCl4]², [Cd(CN)4]², [MnCl4]2, etc. The geometry of [BeF4]2 is shown in Fig. 12. ### 2. Square Planar Geometry This geometry is common amongst complexes of transition elements only. The common examples are [PtCl4]2;, [Ni(CN)4]2, [Cu(NH3)4]2+, [PdCl4]2-, etc. The geometry of [Ni(CN)4]2– is shown in Fig. 13. ## Coordination Number 5 Complexes with coordination number 5 like those with coordination number 3, are rather rare. Several complexes in which the metal ion was considered to be having coordination member 5 were later on shown to have the metal ion with different coordination number. For example, X-ray studies have shown that the compound (NH4)3[ZnCl5], in which the complex ion [ZnCl5] was considered to involve coordination number 5, actually consists of tetrahedral [ZnCl4]2- ions and separate Cl-ions in its lattice. Similarly, the crystalline compound, Cs3 [CoCl5], in which the complex ion [CoCl5]3- was considered to involve coordination number 5, has been found to contain [CoCl4]2-ions and separate Cl-ions in its lattice. Two principal geometries in the case of compounds in which this coordination number has been established are : 1. Trigonal Bipyramidal 2. Square Pyramidal ### 1. Trigonal Bipyramidal Geometry In this case the ligands lie at the vertices of trigonal bipyramid, as shown in Fig. 14. This geometry is shown by complexes such as [Fe(CO)5], [MoCl5]¬, [CuCl5]3-and [SnC15]. ### 2. Square Pyramidal Geometry In this case, the ligands lie at the vertices of a square pyramid, as shown in Fig. 15. An example is furnished by the complex [SbF5]2. Other examples are [VO(acac)2] and [Ni {(C6H5)3P}2Br3]. ## Coordination Number 6 Coordination number 6 is most common and has been extensively studied. The regular geometric arrangement is octahedral although due to distortions the geometry changes to tetragonal. Octahedral geometry is shown by a large number of complexes such as [Cu(NH3)6]2+, [FeF6]3], [TiF6]2-, [PtF6] and [SbF6]. The octahedral geometry of [FeF6]3- is shown in Fig. 16. An example of tetragonally distorted octahedron is furnished by [CoCl2(NH3)4]+ ion. ## Coordination Number 7 There are only a few compounds in which coordination number is 7. Three geometrical forms known in this case are as follows: 1. Pentagonal bipyramidal 2. Distorted Octahedron 3. Trigonal Prism ### 1. Pentagonal Bipyramidal This geometry is found in [UO2F5]3, [UF7]3- and [ZrF7]3. The pentagonal bipyramidal geometry of [UO2F5]3-is shown in Fig. 17. ### 2. Distorted Octahedron The second arrangement is supposed to result from the addition of a seventh atom at the centre of one face of a distorted octahedron. An example is furnished by [NbOF6]3-, as shown in Fig. 18. ### 3. Trigonal Prism The third arrangement is derived by putting a seventh atom above the centre of one of the rectangular faces of a trigonal prism. This arrangement occurs in [NbF7]2~ and [TaF7]2- ions. The structure of [TaF7]2-is shown in Fig. 19. ## Coordination Number 8 This coordination number is most common after coordination numbers 6 and 4. Three types of geometries in the case of such compounds are known. These are : 1. Cubic 2. Square Antiprism 3. Dodecahedral ### 1. Cubic Geometry The most symmetrical arrangement is simple cubic. This occurs, however, only in the case of a few compounds, as, for example, in [UF8]3-, as illustrated in Fig. 20. ### 2. Square Antiprism Geometry This depicts a distortion in the cubic geometry which is adopted to minimise the repulsion between the anions. The common examples are [TaF8]-3, [ReF8]2-and [Zr(acac)4]. The square antiprism structure of [TaF8]3 is shown in Fig. 21. ### 3. Dodecahedral Geometry This type of geometry is exhibited by complexes such as [Mo(CN)8] and [Zr(ox)4]. The geometry of [Mo(CN)8]4-is shown in Fig. 22. ## Coordination Number 9 There is only one symmetrical arrangement known for this coordination number. This is derived from a trigonal prism by placing the three additional atoms outside the centres of the three vertical faces.