Chemistery 221Chapter 14: Complex-Formation TitrationsComplex-formation reagents aere used widely for titrating cations.Most metal ions react with electron-pair donors to form coordination compounds or complexes. Many of us have utilized the physical properties (color) of such complexes in the laboratory. Ammonia complexes are widely used to test for cations. During the Electrogravimetric Determination of Copper and Lead in Brass experiment, ammonia was added to an analyte solution to test for the presence of copper (II), which in the presence of ammonia would produce the copper-ammonia complex, Cu(NH3)42+, a bright blue complex. The donor species, or ligand, must have at least one pair of unshared electrons available for bond formations. Ligands are defined as ions or molecules that form covalentbonds with a cation or a nuetral metal atom by donating a pair of electrons, which are thenshared by the two. Ammonia, as mentioned above, along with water and halide ions are common inorganic ligands. The number of covalent bonds that a cation forms with a ligand is referred to as its coordination number. For example, copper (II) has coordination number of four. The species formed from such coordination or complexing, can be electrically positive, nuetral or negative. Copper when complexed with ammonia results in a cationic complex, Cu(NH3)42+, when complexed with glycine, a nuetral complex, Cu(NH2CH2COO)2, and when complexed with chloride, an anionic complex, CuCl42-. A chelate is produced when a metal ion coordinates with two or more donor groups of a single ligand to form a five or six membered heterocyclic ring. The copper complex of glycine, is an example of a chelate:2+HNH2CHCOOHOONH2Cu2+OONH2CuOOOONH2NH2CuThe copper bonds to both the oxygen of the carbonyl group and the nitrogen of the amine group forming a heterocyclic ring.A ligand that has one donor group such as ammonia, is called unidentate. (Dentate is a latin word that means having tooth-like projections, thereby unidentate, single-toothed). Glycine, which has two groups available for covalent bonding, (the carbonyl oxygen and the aminal nitrogen), is called bidentate. As titrants, multidentate ligands, particularly tetradentate and hexadentate chelating agents, those having four or six donor groups, havetow advantages over their unidentate titrants. First, these multidentate titrants, generally react more completely with cations, thereby providing sharper more accurately end points.Second, they ordinarily react with metal ions in a single-step process, whereas wiht unidentate ligands usually involves two or more intermediate species. Again, this would provide a sharper end point, in shorter period of time. This advantage of a single-step reaction is illustrated by the titration curves shown below (taken from Skoog 239):(scan and insert fig 14-1)The titration concerns a reaction that has an overal equilibrium constant of 1020. Curve A is derived for the reaction in which a metal ion M, that has a coordinatioin number of four,reacts with a tetradentate ligand, D, to form the complex MD, a 1:1 complex. Curve B, depicts the reaction of M with a bidentate ligand B to form the complex MB2, a 1:2 complex, in two steps. The formation constant for this reactions would be 1012 and 108 respectively for the first and second reactions. Curve C depicts the reaction of M with a unidentate ligand, A, that forms the complex MA4, a 1:4 complex in four steps. The formation constants for this reaction would be 108, 106, 104, 102 for the first, second, third and fourth reactions. This figure shows how a sharper end point is obtained for a reaction that takes place in a single step.Schwarzenbach, in 1945 first recognized the potential of tetriary amines that also contain carbonyl group as chelating agents.NNO