Ammine-metal complexes

W.W. Wendlandt and J.P. Smith, Thermal Properties of Transition Metal Ammine Complexes, Elsevier, Amsterdam, 1967. 

Coordination chemists are, however, generally interested in those systems with definite coordination numbers. The maximum coordination number is often achieved in a transition-metal ammine complex, especially where synthetic routes involve the use of excess ligand, as the gas, aqueous solution or the anhydrous liquid. Even so, forcing conditions or catalysts are required to form such complexes as Cu(NH3)62+ or Co(NH3)63+. 

Equation 8.9 shows that when NH3 is introduced to an acid solution, it reacts directly with the acid and produces the ammonium ion (NH4) (see Chapter 12). Concurrent with Equation 8.9, NH3 may associate itself with several water molecules (NH3nH20) without coordinating another H+. This hydrated NH3 is commonly referred to as unionized ammonia and is toxic to aquatic life forms at low concentrations. Because NH3 is a volatile gas, some of it may be lost directly to the atmosphere (volatilization) without dissolving in solution. On the other hand, the ammonium ion may undergo various reactions in the soil water that may alter its availability to plants and/or other organisms. These reactions include formation of metal-ammine complexes, adsorption on to mineral surfaces, and chemical reactions with organic matter. 

Metal-Ammine Complexes. All metal ions in water are surrounded by a shell of water molecules (see Chapter 1) ... 

This reaction (between Cd2+ and NH3) involves the formation of a coordinated covalent bond where the element nitrogen of NH3 shares its single unshared electron pair with Cd2+ (see Chapter 1). The number of water molecules that could be displaced from the cation s hydration sphere depends on the concentration of NH3 and the strength by which it associates with the metal ion. In these complex ions, otherwise known as metal-ammine complexes, the metal is called the central atom and the associated molecule or ion is called the ligand. 

The behavior of metal-ammine complexes in water is different from that of the noncomplexed metal ion. For example, if sodium hydroxide is added to a solution containing heavy metals, they would precipitate as metal-hydroxide [M(OH)2]. However, if sodium hydroxide is added to a solution containing heavy metals and excess ammonium (NH4), no metal precipitation takes place because metal-ammine complexes are soluble in alkaline solutions. Consider the reaction... 

Based on the above, total M dissolved (MT) would be described by the sum of all metal-ammine complexes in solution ... 

Replacing the metal-ammine complexes in the denominator as a function of NH3, M2+, and overall formation constants, as demonstrated in Equation 12.24, gives... 

In the same manner we may solve for any a(. Using the equations above, the percent of metal-ammine complexes may be estimated as a function of NH3. The data in Figures 12.20-12.22 represent stability diagrams for Cu2+-ammine, Zn2+-ammine, and Cd-ammine complexes. 

Based on the data presented in Figures 12.20-12.22, it is clear that the potential of NH3 to solubilize heavy metals depends on metal softness and on the concentration of NH3. Soft metals (see Chapter 1) are metals that are electron rich with high polarizability (e.g., Cd2+, Ni2+, Hg2+, Co2, Cu2+, Zn2+, and Ag+. Hard or intermediate metals such as Fe2+, Mn2, Al3+, Fe3+, Ca2+, and Mg2+ do not solubilize in ammoniated waters because of their inability to form metal-ammine complexes. 

To predict the potential concentration of metal-ammine complexes in solution, one needs to understand the relationship between pH and NH3 formation. Consider the equation... 

In the case of hard or intermediate metals whose potential for forming metal-ammine complexes is very low, the precipitates forming because of NH3 addition are those of metal-hydroxides and/or metal-oxyhydroxides. 

Surface Adsorption Behavior of Metal-Ammine Complexes. Metal-hydroxides or oxyhydroxides possess variably charged surfaces. Since pH is expected to be around the PZC, addition of NH3 to the newly formed metal-oxyhydroxide leads to surface adsorption of NH3 by protonation. This is demonstrated below (see also Chapters 3 and 4) ... 

When NHj-treated water contains dissolved hard metals, heavy metals, and clay colloids, a number of reactions may take effect. Many of the metals would be precipitated as metal-hydroxides or oxyhydroxides. Any heavy metals capable of forming metal-ammine complexes would react differently when in the presence of clay colloids or any other charged surfaces. Metal-ammine complexes could form in solution as well as on the colloidal surfaces. The mechanism is shown below ... 

The data above were presented to demonstrate that in treating acid heavy-metal-rich colloidal suspensions with NH3, the latter introduces some undesirable complexities owing to the potential of the heavy metals to form metal-ammine complexes in solution and/or the exchange complex. Disposal of such waters and/or such sludges requires prior knowledge. For example, the formation of metal-ammine complexes in solution would not permit precipitation of heavy metals in the treated water. On the other hand, formation of precipitate-NH4 complexes and/or metal-ammine complexes on the surface of colloidal particles causes disposal problems for such sludges. Nitrification of this NH4 would release NOa and heavy metals owing to acidification. 

Three metals, Zn2+, Cu2+, and Cd2+, were discussed in some detail with respect to their potential to form metal-ammine complexes. Which metal would dissolve the most in an ammoniated solution If this represented areal problem, how would... 

Aluminum hydroxy species, 65,69,160 Stability constants, 69 Stability diagrams, 78 pH of minimum solubility, 65, 71, 72 Ammonium, 326, 331 Volatilization, 330 Oxidation, 334-336,472 Nitrate, 334-336,472 Adsorption, 336,465-466 Metal-ammine complexes, 460—461, 465... 

As described, solid metal ammine complexes can be prepared as very dense materials without any significant internal pore volume, because the porosity needed for ammonia transport is generated as the ammonia release progresses. This allows for ammonia release over large material length scales. Still, the mass transport facilitated by the in situ generated pore structure has to be combined with an understanding of heat transfer resistance, since the ammonia desorption from a metal ammine complex is endothermic. 

For systems based on low-temperature PEMFCs, the optimal metal ammine would be a material that has a suitable ammonia release profile below 80 °C. This would take advantage of heat integration similar to the way in which waste heat is used to release hydrogen from a metal hydride canister. The metal ammine complex should release the ammonia without causing any additional system losses than that of the ammonia cracker (a theoretical efficiency of 86% and typically above 70% in real systems Thomas and Parks, 2006). 

The use of an alkaline fuel cell also involves an ammonia cracker, but since the electrolyte of the AEC is not sensitive to ammonia traces, the system is slightly simpler because of the elimination of the purification step. In addition, alkaline cells can run at slightly elevated temperatures (like the high-temperature PEM or higher), and they do not contain expensive platinum. AFCs could be operated with the same choice of metal ammine complex as the PEM. 

The general use of metal ammine complexes can be further kick-started by the utilization of on-board ammonia storage and delivery systems for NO c aftertreatment on diesel or lean-bum vehicles. The implementation will create experience with vehicle integration, safety and distribution. In addition, bulk production of compact ammine cartridges for DeNO (AdAmmineT ) will drive down the production cost for emerging niche applications, while broad entry into mass markets slowly develops in the future. 

Christensen C H, Sprensen R Z, Johannessen T, Quaade U J, Honkala K, Ehnpe D, Kphler R, Nprskov J K (2005), Metal ammine complexes for hydrogen storage , J. Mater. Chem., 15, 4106-4108. 

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