7 COMPLEX FORMATION 7.1 WHAT IS A METAL COMPLEX?

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1 7 COMPLEX ORMATION 7.1 WHAT IS A METAL COMPLEX? What is your picture of copper (II) nitrate dissolved in water? Most likely, it would be that the ions are floating around individually at random. This is not the case for many salts: your perception of ionic species in solution has been simplified in the past. In fact, each of the copper ions has four water molecules linked to it in a definite way: if you imagine a copper atom at the centre of a square, the water molecules are at each corner. If ammonia is added to the solution, the solution goes a deep blue, as the water molecules are replaced by ammonia molecules. The result is an ion that is more than just the metal ion, since it also contains four neutral ammonia molecules. The product is a species which has new properties, which are different to those of the individual components. or example, if the complex ion was crystallised from solution (with sulfate anion), it would not smell of ammonia. The structures and reaction are shown in igure 7.1. H 2 O OH 2 H 3 N NH 3 Cu NH 3 Cu H 2 O H 2 O OH 2 H 3 N NH 3 IGURE 7.1 Copper complexes Such an ion is known a metal complex: a species (can be neutral, cationic or anionic) containing a metal ion bonded to a set number of neutral or anionic species arranged in a particular geometry. The name given in general to the species that links to the metal ion is ligand. Thus, water and ammonia act as ligands in the example above. Ligands are characterised by having at least one pair of non-bonded electrons. They may be neutral molecules, such as water and ammonia, or anions, such as chloride, fluoride and cyanide. Not all metals ions form complexes: the greater the charge and the larger the ion, the more likely that a metal will form a complex. One of the significant properties of the transition metals is their ability to form complexes. On the other hand, alkali metal ions do not form complexes at all. The number of metal-ligand bonds formed in a complex is known as the coordination number (abbreviated as CN). In igure 7.1, the coordination number for the two copper complexes is four in each case. The coordination number is determined by the size and charge of each partner in the complex. Iron (III) forma a complex with six fluoride ions, but only four chloride ions. This is due to the larger size of the chloride ion. Some metal ions have constant coordination numbers, e.g. chromium (III) six and platinum (II) four. Some ligands have more than one bonding site. They are known as chelating agents. Ligands may classified by the number of bonding sites, using the Latin word for teeth: dentate. Thus, ammonia is monodentate, while the organic nitrogen compound commonly known as ethylenediamine is bidentate. A very important ligand in analytical chemistry is known as EDTA (EthyleneDiamineTetraAcetic acid), which is hexadentate: it has six bonding sites. The structures of these are given below in igure 7.2.

2 7. Complex ormation CH 2 CH 2 HOOCCH 2 CH 2 COOH H 2 N NH 2 HOOCCH 2 NCH 2 CH 2 N CH 2 COOH (a) (b) IGURE 7.2 Important multidentate ligands (a) ethylenediamine (b) EDTA; bonding sites shown by arrows Ligands, such as ethylenediamine and EDTA, when complexed to a metal, form a type of ring structure, since the ligand loops around to bind to the metal in at least two places. This type of complex is shown in igure 7.3, and is known as a chelate. IGURE 7.3 Chelate complex of calcium ion with EDTA (solid lines indicate carbon chains linking bonding atoms, dotted lines are the metal-ligand bonds) Charges of complexes The charges held by the individual components of the complex - the cation and the ligand(s) - determine the overall charge on the complex. It is a simple matter if adding up these charges. EXAMPLE What are the charges on the following complexes: (a) copper (II)-ammonia, (b) copper (II)- EDTA, (c) copper (II)-fluoride (CN 4)? (a) Ammonia is a neutral ligand, and therefore, doesn t affect the 2+ charge of copper. The charge of the complex is 2+. (b) When EDTA forms a complex, it loses its four acid protons, thus attaining a 4- charge. When combined with the 2+ charge of copper, the complex gains a 2- charge. (c) our fluoride ions have a total charge of 4-, so the complex has a charge of 2-. The structural formula for the complex ion is often shown in large square brackets, with the overall charge outside the brackets, rather than next to the cation, as shown in igure e IGURE 7.4 Structural formula for e 6 3- Chemistry 2 7.2

3 7. Complex ormation PRACTICE QUESTIONS 1. Explain why ethane could not act as a ligand. 2. Draw the structure of a complex of copper (II) with (a) cyanide ion and (b) ethylenediamine as ligands. The coordination number in each case is 4. What is the charge on the complex ion in each case? 3. What is the coordination number of the complex in igure 7.3? 7.2 HOW DO THE BONDS ORM? The exact way that a ligand bonds to a metal is quite complex (pun intended), and is not entirely understood by chemists. One theory suggests it is similar to a very polar covalent bond, but where one atom (in the ligand) contributes both electrons to the bond. The other theory, which better explains experimental results, proposes a bond more like ionic attraction. It is not critical for you to understand either of these theories: a picture of the metalligand bond as being somewhere between the two is all that is necessary. It is most important that you realise that the bond is not as strong as a covalent bond, but does not break apart in water (like an ionic bond). 7.3 ACTORS AECTING COMPLEX ORMATION The reaction of a metal ion (M) with a ligand (L) is an equilibrium reaction, as shown in Equation 7.1, where n is the number of ligands involved in the complex. M + nl ML n Eqn 7.1 An equilibrium constant can be written for the reaction, as shown in Equation 7.2. [ML] K = Eqn 7.2 [M][L] n This equilibrium constant is given a special name to indicate that it refers to the formation of a complex: hence the name, formation constant (K f). It has also been called the stability constant (K s). Being formed in an equilibrium process, the complex can be destroyed - the equilibrium shifted towards the separate metal ion and free ligand. The value of the formation constant is a guide to the strength (or stability) of the complex: the larger the number, the less likely that the complex will break up. Table 7.1 lists the formation constants of some complexes. Look at the data, and note the effect of the different ions and ligands on the value of K f. The formation constants in Table 7.1 indicate that both the nature of the metal ion and the ligand affect the strength of the complex. In general, the strength of a complex increases with: increasing charge on the metal ion (3+ ions form stronger complexes than 1+ ions), increasing size of the metal ion, more bonding sites per ligand molecule (multidentate ligands form stronger complexes than monodentates). Chemistry 2 7.3

4 7. Complex ormation TABLE 7.1 ormation constants for selected complexes Metal Ligand Coordination Number ormation Constant Cu 2+ NH x Cd 2+ NH x 10 7 Cd 2+ CN x Ag + EDTA 6 2 x 10 7 Ca 2+ EDTA 6 5 x e 2+ EDTA 6 2 x Al 3+ EDTA 6 1 x Cd 2+ EDTA 6 3 x Cu 2+ EDTA 6 6 x e 3+ EDTA 6 1 x Some metal ions have particular attraction for a single ligand, which cause exceptions to the generalisations above. or example, aluminium and fluoride form a complex - Al with a stability constant of 5 x This is stronger than the complex formed by aluminium and EDTA. What do K f values tell us? In any aqueous solution of a metal ion (other than the alkali metals), there must be a concentration of the balancing anions (which are potential ligands), and lots of water - which is a weak ligand. What does this mean? Any reaction of the metal ion with an added ligand involves the breaking up of one complex (metal-water or metal-anion) and the formation of a new, more stable one. Equation 11.3 summarises this process, where S is the solvent or anion which forms a weak complex with the metal ion, M. MS + L ML + S Eqn 11.3 This also extends to stronger complexes when another ligand is added, which has the capacity to form a more stable complex with the metal. To summarise: a ligand whose complex has a higher K f will cause an existing complex to become unstable. The difference in K f values will determine how complete the exchange of ligands would be. EXAMPLE A copper (II) solution is mixed with ammonia to form the complex. EDTA is added to this solution? What happens? The K f value for Cu-EDTA is significantly higher (300,000 times greater) than that of copperammonia. So, in the presence of EDTA, Cu(NH 3 ) 4 2+ would fall apart, and the copper would be complexed with the EDTA. If two different ligands are added to a solution of a metal ion, most of the cations will bond to the ligand which forms the more stable complex. The proportion of the two complexes formed is determined by the difference in the two K f values. Of course, this is also affected by the relative concentrations of the ligands. Chemistry 2 7.4

5 7. Complex ormation EXAMPLE What will happen to a solution of copper (II) ions if EDTA and ammonia are both added? The copper-edta complex is far more stable than copper-ammonia, so the great majority of the copper ions will bond to EDTA. PRACTICE QUESTIONS 4. Explain why the effects of a cyanide spill could be minimised by the addition of e Predict what would happen if: (a) ammonia was added to a solution of cadmium nitrate (b) EDTA was added to a solution of cadmium-ammonia complex (c) ammonia was added to a solution of cadmium-edta complex (d) a small concentration of ammonia was added to a solution containing cadmium and copper (II) ions 7.4 IMPORTANCE O COMPLEXES Metal complexes are extremely important across a variety of areas, from our blood to medical treatment of certain illnesses to chemical analysis of water. Haemoglobin, which is the red part of our blood, is a complex between iron (III) and large organic ligands. It is this complex, which rearranges itself to incorporate oxygen when we breathe, and to release it to the cells. A large hexadentate ligand - known as desferrioxamine B - is used to treat patients who cannot excrete iron from their bodies after blood transfusions cause a build up of the metal ion in the body. The ligand forms a very stable chelate complex with iron, and helps to remove it. The ligand EDTA is a most important titrant in chemical analysis, since it has the capacity to react with many different metals. One of its most significant applications is the determination of water hardness (caused by Ca 2+ and Mg 2+ ), which is a problem in natural waters, causing soaps not to work, and boilers to be clogged up with ionic salts. EDTA is used as a last resort first aid treatment for someone who has swallowed some heavy metal ions. The EDTA will mop them up, but also runs the risk of stripping important biochemical complexes (e.g. co-enzymes) of their metal ions. Soils contain a number of important ligands called humic acids, which are complex organic compounds, the structures of which aren t fully known. However, it is known that they are significant in controlling the transport of many metals through the soil, in solution as a soluble complex or precipitated out as an insoluble one. Iron and aluminium are very strongly bound, while magnesium is not. These are natural processes, but can be disrupted by the presence of unnatural ligands, which may release certain metals from their humic complexes. One major problem associated with unnatural complexes is the ability of ligands to remove heavy metals from insoluble forms, and therefore, allow their transport in solution. PbX (s) + L (aq) PbL (aq) + X (aq) Once mobilised, the heavy metals can then be ingested by organisms at various levels in the food chain, potentially causing health problems. Chemistry 2 7.5

6 7. Complex ormation WHAT YOU NEED TO BE ABLE TO DO define terminology associated with complex formation list the structural feature required for a ligand briefly describe the formation of the metal-ligand bond draw the structures of important multidentate ligands, and their complexes with metal ions outline factors that affect the stability of a complex describe some significant applications of complexes Chemistry 2 7.6