Theories of Chemical Bonding of Coordination Compounds Blomstrand-Jorgensen’s Chain Theory This theory is only of historical importance. Blomstrand along with his student Jorgensen developed the chain theory in order to explain the existence of metal complexes. At that time, there was a belief that elements had only one kind of valence. Hence, considering CoCl,.6NH, they proposed that there could be only three bonds to cobalt(III) in its complexes. As a consequence, a chain structure was suggested to account for the additional six ammonia molecules in CoCl,.6NH.NH,-Cl c NH, - NH, - NH, - NH, - Cl NH,-Cl CoCl,.6NH, Since the three chlorides are separated by some distance from cobalt, they are assumed to be less tightly bound to cobalt. Hence all the three chlorides are readily precipitated as AgCI on the addition of AgNO; solution. This is in agreement with the experimental fact. On the basis of this theory, CoCl;-5NH, is represented as:cl Cr NH, - NH, - NH, - NH, - Cl NH,-Cl In this structure, the chloride which is directly attached to cobalt is very tightly bound to it. Hence only 2/3 of its chloride content should get precipitated as AgCI on treatment with AgNO, solution. This is also in agreement with the experimental fact. Structure for CoCl,.4NH, is also in accordance with experiments which show that two of thecl c NH, - NH, - NH, - NH; - Cl cl Chlorides are more tightly held than the third one. They did not succeed in preparing the next member of this series, namely, CoCl,.3NH . However, they were able to prepare the analogous iridium complex. IrCl,.3NH;, it was represented as: cl Ir NH, - NH, - NH, - Cl ClThe chloride that is not directly linked to iridium should be less tightly bound to the central metal according to this theory. This means that 1/3 of its chloride content should be precipitated as AgCl on the addition of AgNO, solution. But a solution of this compound did not give any precipitate of AgCl. Thus, this chain theory was proved to be incorrect in this case and also in several other metal complexes.Werner’s Coordination Theory In 1893, Alfred Werner, at the age of 26, proposed a theory which is now referred to as Werner’s Coordination Theory: His theory is a guiding principle in the concept of valence in inorganic chemistry. The well-known concept of "primary" and "secondary" valencies in metal complexes has been developed by him. Postulates of Werner’s theory (1) Generally, elements exhibit two types of valencies, namely primary valence and secondary valence. The primary valence is also known as ionisable valence and secondary valence is otherwise known as nonionisable valence. Anions can satisfy primary valence whereas anions or neutral molecules can satisfy secondary valence. In modern terms, the primary valence corresponds to the oxidation number and the secondary valence corresponds to the coordination number.+ (2) All elements tend to satisfy both primary and secondary valencies, But in every case the fulfillment of secondary valence appears to be more essential. For example, in CoCl;.4NHg, two of the three chloride ions are attached by secondary valence and hence it is represented as [Co(NH,),CLIC1 . + (3) The secondary valences are directed towards some fixed positions in space. For example, in 4-coordinated complexes the four valences are arranged in either a planar or a tetrahedral manner and in 6-coordinated complexes the 6 valences are directed towards the six corners of an octahedron. + Werner extensively studied the chloramminecobalt(III) complexes to substantiate his theory. The experimental observations collected by him on these complexes are tabulated in the following Table:Compl No. of Crions No. of Cr ions No. of ions Molar precipitated as in solution _ conductivity Agel (Ohm) CoCl,.6NH, 3 [Co.(NH),ICl, 4 404 CoCl,.5NH, 2 [Co(NH,),CIICL 3 229 CoCl,.4NH, A [Co.(NH,),CLICL 2 97 CoCl;.3NH, 0 [Co.(NH),Cls] 0 0 The data in the above Table can be explained by Werner's coordination theory. According to Werner's theory, the first member of the series is formulated as [Co(NH,),]C1,. In this complex the primary valences (longer dotted lines) are satisfied by the three chloride ions. ci Hy! Ny mM | cM a | re NH, cl “clThe six secondary valences (shorter solid lines) are satisfied by the six ammonia molecules. These ammonia molecules are very tightly bound to cobalt and hence they do not dissociate in solution. But the three chloride ions are far away from the central cobalt and hence they are less firmly held by the metal. Therefore, all the three chloride ions dissociate in solution giving [Co(NH,)}* and 3CI ions, with a total of four ions. Thus, Agt precipitates all the three chloride ions as AgCI and the molar conductivity of the complex corresponds to that of a tetra-univalent electrolyte (404 Ohmr).The second member of the series, CoCl;.5NHg is formulated as [Co(NH,);CICI,-Since there are only five ammonia molecules to satisfy the secondary valences, one chloride ion must play the double role of satisfying both a primary and a secondary valence. This is because the fulfillment of all secondary valences is essential according to one of the postulates of Werner.Werner represented the bond between such a ligand and the central metal by a combined solid and dotted line as shown in the previous figures. The chloride playing the double role is very firmly held by the central metal and hence is not precipitated as AgCl by Agt ions in solution. Two-thirds of its chloride content of the complex is present in the second coordination- sphere, therefore, it is formulated as [Co(NH,)sCIJCl,-In solution it gives a total of 3 ions, [Co(NH,),Cl}** and 2CT. Its molar conductivity corresponds to that of bi- univalent electrolyte (229 Ohm-'). The extension of this theory to the next member of the series CoCl,.4NH,, leads to the following structure with the Werner formulation [Co(NH,),CLICI.The two chloride ions in this complex satisfy both the primary and secondary valences. Hence, both of them are tightly bound to the central metal. In solution the complex dissociates into two ions, [Co(NH,),Cl,]* and Cl.Only 1/3 of its chloride content gets precipitated as AgCI and its molar conductivity corresponds to that of a uni-univalent electrolyte (97 Ohnr'), (+1, -1). Werner's theory predicts structure shown in for the next member of the series, CoCl,.3NH, and it is formulated as [Co(NH,);Clj]. This theory also predicts that This complex will not yield any chloride ion in solution. Actually, no chloride ion is precipitated as AgCI on cl treating it with AgNO, solution. a E a Co It is a non-electrolyte in solution because no ions are produced in solution. va HN “Cl NH,CoCl,.6NH, Original Ions per “Free” CI” Ions. Modern Formulation Color Formula Unit per Formula Unit Formulation CoCl;6 NH3 Orange CoCl'5NH3; Purple CoCl-4NH3 Green CoCl;-4NH3 Violet [Co(NH5)s]Cls [Co(NH)sClIClo trans-[Co(NH),Cla|Cl 4 3 2 2 cis-[Co(NH3),ChICl reneWerner’s Theory * This approach correctly predicts there would be two forms of CoCl,-4NH,. — The formula would be written [Co(NH;),CLICI. — One of the two forms has the two chlorines next to each other. — The other has the chlorines Ecven | opposite each other.Defect of Werner’s Theory (1) Werner's theory describes the structures of many coordination compounds successfully; however, it does not explain the nature of bonding within the coordination sphere. (2) More than 90% of the known complexes at Werner's time were either 4- coordinated or 6-coordinated. Werner's theory is unable to account for the preference for 4- and 6-coordination among complexes. (3) Werner's theory fails to account for the fact that certain 4- coordinated complexes are square-planar whereas some others are tetrahedral.