Fe-Ni Clusters
Encyclopedia
Metal clusters are structures displaying polyhedral frameworks held together by two or more metal-metal bonds per metal atom, where the metal atoms are located at the vertices of closed, triangulated polyhedra. The metal cluster chemistry
discussed here is that of Fe-Ni clusters.
Individually, iron
(Fe) and nickel
(Ni) generally form metal clusters with π-acceptor ligands. Π acceptor ligands are ligands that remove some of the electron density
from the metal.
Figure 1 contains pictures of representative cluster shapes. Clusters take the form of closed, triangulated polyhedral
.
Corresponding bulk systems of Fe
and Ni
atoms show a variety of composition-dependent abnormalities and unusual effects. Fe-Ni composites are studied in hopes to understand and utilize these unusual and new properties.
Fe-Ni clusters are used for several main purposes. Fe-Ni clusters ranging from single to hundreds of atoms are used in catalysis
, depending on the reaction mechanism. Additionally, Fe-Ni clusters, usually of one or two metal atoms, are used in biological systems. These applications are discussed below.
Metal-Metal bonds, being d-orbital interactions, happen at larger distances. More stable metal-metal bonds are expected to be longer than unstable bonds. This is shown by the fact that the Ni-Fe bond length is in between Ni-Ni and Fe-Fe bond lengths. For example, in Fe-Ni four atom clusters (FeNi)2 which are most stable in a tetrahedral structure, the bond length
of metal-metal Ni-Fe bond is 2.65Å and Fe-Fe bond is 2.85 Å. When bonding in these structures is examined, it follows that lowest energy cluster structures of iron and nickel are given by geometries with a maximum number of Fe-Fe bonds, and a small number of Ni-Ni bonds.
The simplest Fe-Ni clusters are of one iron atom and one nickel atom bonded together. More complex clusters can be added through the addition of another atom. Some pictures of sample geometries are shown in Fig. 2.
All Fe-Ni clusters exhibit some degree of distortion from usual geometry. This distortion generally becomes more pronounced as the number of Fe atoms increases.
Notice how in the above cluster diagrams, as calculated by Rollmann and colleagues, the symmetry of the cluster changes from a pure octahedron
(D3h) to a square pyramid
(C4v) as more iron atoms are added.
, or how much energy is required to break the bonds between two atoms. The larger the binding energy, the stronger the bond. Binding energies of Fen-xNix clusters are found to generally decrease by successive substitutions of Ni atoms for Fe atoms.
The average magnetic moment
(μav) increases in a Fe-Ni cluster through the replacement of more and more Fe atoms. This is due to fact that magnetic moments of Fe atom/ Fe bulk are more than that of Ni atom/ Ni bulk values. The local magnetic moment of Ni (μatom,local) decreases by a proportional increase of Fe atoms. This is due to charge transfer from nickel’s 4s orbital and iron atoms to nickel’s 3d orbitals.
Below is a table of the bond length (Re, in Å), binding energy (Eb, in eV), and magnetic moment (M, in μa) of the small clusters Fe2, Ni2, and FeNi from two authors. Notice how both authors show that Fe2 has the smallest bond length, the lowest binding energy, and the largest magnetic moment of the cluster combinations.
Below is another table of bond length (Re), binding energy (Eb), and magnetic moment (M) of Fe-Ni clusters containing five atoms.
per atom in Ni clusters was found to be 0.7-0.8 μB, as compared with 0.6 μB for bulk Ni. This is explained by longer metal-metal bonds in cluster structures than in bulk structures, a consequence of a larger s character of metal-metal bonds in clusters. Magnetic moments approach bulk values as cluster size increases, though this is often difficult to predict computationally.
Magnetic quenching is an important phenomenon that is well documented for Ni clusters, and represents a significant effect of ligands on metal cluster magnetism
. It has been shown that CO ligands cause the magnetic moments of surface Ni atoms to go to zero and the magnetic moment of inner Ni atoms to decrease to 0.5 μB. In this case, the 4s-derived Ni-Ni bonding molecular orbitals experience repulsion with the Ni-CO σ orbital, which causes its energy level to increase so that 3d-derived molecular orbitals are filled instead. Furthermore, Ni-CO π backbonding leaves Ni slightly positive, causing more transfer of electrons to 3d-derived orbitals, which are less disperse than those of 4s. Together, these effects result in a 3d10, diamagnetic character of the ligated Ni atoms, and their magnetic moment decreases to zero.
Density Functional Theory
(DFT) calculations have shown that these ligand-induced electronic effects are limited to only surface Ni atoms, and inner cluster atoms are virtually unperturbed. Experimental findings have described two electronically distinct cluster atoms, inner atoms and surface atoms. These results indicate the significant effect that a cluster’s size has on its properties, magnetic and other.
. A primary source of energy in bacteria is the oxidation and reduction
of H2 which is performed by hydrogenase
enzymes.
These enzymes are able to create a charge gradient across the cell membrane
which serves as an energy store. In aerobic environments, the oxidation and reduction
of oxygen
is the primary energy source. However, many bacteria are capable of living in environments where O2 supply is limited and use H2 as their primary energy source . The hydrogense enzymes which provide energy to the bacteria are centered around either a Fe-Fe or Fe-Ni active site
. H2 metabolism is not used by humans or other complex life forms, but proteins in the mitochondria of mammalian life appear to have evolved from hydrogenase enzymes, indicating that hydrogenase is a crucial step in the evolutionary development of metabolism.
The active site of Fe-Ni containing hydrogenase enzymes often is composed of one or more bridging sulfur
ligands, carbonyl
, cyanide
and terminal sulfur ligands. The non-bridging sulfur ligands are often cystine
amino acid residues that attach the active site to the protein backbone. Metal-metal bonds between the Fe and Ni have not been observed. Several oxidation states of the Fe-Ni core have been observed in a variety of enzymes, though not all appear to be catalytically relevant.
The extreme oxygen and carbon monoxide
sensitivity of these enzymes presents a challenge when studying the enzymes, but many crystallographic studies have been performed. Crystal structures for enzymes isolated from D. gigas, Desulfovibrio vulgaris, Desulfovibrio fructosovorans, Desulfovibrio desulfuricans, and Desulfomicrobium baculatum have been obtained, among others. A few bacteria, such as R. eutropha, have adapted to survive under ambient oxygen levels.
These enzymes have inspired study of structural and functional model complexes in hopes of making synthetic catalysis for hydrogen production (see Fe-Ni and Hydrogen Production, below, for more detail).
In the search for a clean, renewable energy source to replace fossil fuels, hydrogen has gained much attention as a possible fuel for the future. One of the challenges that must be overcome if this is to become a reality is an efficient way to produce and consume hydrogen. Currently, we have the technology to generate hydrogen from coal
, natural gas
, biomass
and water
. The majority of hydrogen currently produced comes from natural gas reformation, and hence does not help remove fossil fuel as an energy source. A variety of sustainable methods for hydrogen production are currently being researched, including solar, geothermal and catalytic hydrogen production
.
Platinum
is currently used to catalyze hydrogen production, but as Pt is expensive, found in limited supply, and easily poisoned by carbon monoxide during H2 production, it is not a practical for large-scale use. Catalysts inspired by the Fe-Ni active site of many hydrogen producing enzymes are particularly desirable due to the readily available and inexpensive metals.
The synthesis
of Fe-Ni biomimetic catalytic complexes has proved difficult, primarily due to the extreme oxygen-sensitivity of such complexes. To date, only one example of a Fe-Ni model complex that is stable enough to withstand the range of electronic potential required for catalysis has been published.
When designing model complexes, it is crucial to preserve the key features of the active site of the Fe-Ni hydrogenases: the iron organometallic moiety with CO or CN- ligands, nickel coordinated to terminal sulfur ligands, and the thiolate bridge between the metals. By preserving these traits of the enzyme active site, it is hoped that the synthetic complexes will operate at the electrochemical potential necessary for catalysis, have a high turnover frequency and be robust.
Cluster chemistry
In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...
discussed here is that of Fe-Ni clusters.
Individually, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
(Fe) and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
(Ni) generally form metal clusters with π-acceptor ligands. Π acceptor ligands are ligands that remove some of the electron density
Electron density
Electron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds...
from the metal.
Figure 1 contains pictures of representative cluster shapes. Clusters take the form of closed, triangulated polyhedral
Polyhedron
In elementary geometry a polyhedron is a geometric solid in three dimensions with flat faces and straight edges...
.
Corresponding bulk systems of Fe
FE
Fe or FE may refer to:* Iron * Fe , the f-rune of the Younger Futhark* Fe * Fe * "Fe" , a song by Jorge González...
and Ni
Ni
- Geography :* Ni River, a river of New Caledonia* Ni River , a tributary of the Mattaponi River* Mount Ni, a hill in Shandong, China- Letters :* Ni , romanisation of the Japanese kana に and ニ...
atoms show a variety of composition-dependent abnormalities and unusual effects. Fe-Ni composites are studied in hopes to understand and utilize these unusual and new properties.
Fe-Ni clusters are used for several main purposes. Fe-Ni clusters ranging from single to hundreds of atoms are used in catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
, depending on the reaction mechanism. Additionally, Fe-Ni clusters, usually of one or two metal atoms, are used in biological systems. These applications are discussed below.
Structure and Geometry
Several general trends are recognized in determining the structure of Fe-Ni clusters. Larger clusters, containing both iron and nickel, are most stable with Fe atoms located in the inner parts of the cluster and Ni metals on outside. In other terms, when iron and nickel form body-centered cubic structures the preferred position of Ni atoms is at the surface, instead of at the center of the cluster, as it is energetically unfavorable for two nickel atoms to occupy nearest-neighbor positions.Metal-Metal bonds, being d-orbital interactions, happen at larger distances. More stable metal-metal bonds are expected to be longer than unstable bonds. This is shown by the fact that the Ni-Fe bond length is in between Ni-Ni and Fe-Fe bond lengths. For example, in Fe-Ni four atom clusters (FeNi)2 which are most stable in a tetrahedral structure, the bond length
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
of metal-metal Ni-Fe bond is 2.65Å and Fe-Fe bond is 2.85 Å. When bonding in these structures is examined, it follows that lowest energy cluster structures of iron and nickel are given by geometries with a maximum number of Fe-Fe bonds, and a small number of Ni-Ni bonds.
The simplest Fe-Ni clusters are of one iron atom and one nickel atom bonded together. More complex clusters can be added through the addition of another atom. Some pictures of sample geometries are shown in Fig. 2.
All Fe-Ni clusters exhibit some degree of distortion from usual geometry. This distortion generally becomes more pronounced as the number of Fe atoms increases.
Notice how in the above cluster diagrams, as calculated by Rollmann and colleagues, the symmetry of the cluster changes from a pure octahedron
Octahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
(D3h) to a square pyramid
Square pyramid
In geometry, a square pyramid is a pyramid having a square base. If the apex is perpendicularly above the center of the square, it will have C4v symmetry.- Johnson solid :...
(C4v) as more iron atoms are added.
Reactivity and Stability
As mentioned previously, the relative bonding between Ni atoms in (FeNi)n clusters is weak and the stability of these clusters could be enhanced by increasing the number of Fe-Fe and Fe-Ni bonds. One measure of stability in Fe-Ni clusters is the binding energyBinding energy
Binding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
, or how much energy is required to break the bonds between two atoms. The larger the binding energy, the stronger the bond. Binding energies of Fen-xNix clusters are found to generally decrease by successive substitutions of Ni atoms for Fe atoms.
The average magnetic moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
(μav) increases in a Fe-Ni cluster through the replacement of more and more Fe atoms. This is due to fact that magnetic moments of Fe atom/ Fe bulk are more than that of Ni atom/ Ni bulk values. The local magnetic moment of Ni (μatom,local) decreases by a proportional increase of Fe atoms. This is due to charge transfer from nickel’s 4s orbital and iron atoms to nickel’s 3d orbitals.
Below is a table of the bond length (Re, in Å), binding energy (Eb, in eV), and magnetic moment (M, in μa) of the small clusters Fe2, Ni2, and FeNi from two authors. Notice how both authors show that Fe2 has the smallest bond length, the lowest binding energy, and the largest magnetic moment of the cluster combinations.
| Re | Eb | M | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Author | Fe2 | Ni2 | FeNi | Fe2 | Ni2 | FeNi | Fe2 | Ni2 | FeNi |
| Nakazawa | 2.15 | 2.38 | 2.34 | 0.64 | 0.80 | 2.04 | 9 | 3 | 5 |
| Rao | 2.02 | 2.14 | 2.08 | 1.70 | 2.83 | 2.33 | 6 | 2 | 4 |
Below is another table of bond length (Re), binding energy (Eb), and magnetic moment (M) of Fe-Ni clusters containing five atoms.
| Re | ||||||
|---|---|---|---|---|---|---|
| Cluster | Symmetry | Fe-Fe | Fe-Ni | Ni-Ni | Eb | Mtotal |
| Ni5 | D3h | |||||
| X | 2.40 | 4.00 | ||||
| | C4v | ||||||
| 2.43 | 1.34 | 7.00 | ||||
| |D3h | ||||||
| 2.03, 2.47 | 1.37 | 5 | ||||
| Fe1Ni4 | ||||||
| 2.48 | 8.0 | |||||
| |C4v | ||||||
| 2.49 | 2.43 | 1.5 | 11.0 | |||
| Fe2Ni3 | D3h | |||||
| 2.54 | 11.98 | |||||
| |C2v | 3.56 | 2.49 | 2.46 | 1.54 | 13 | |
| |Cs | 2.46 | 2.49,2.51 | 2.31,2.43 | 1.46 | 11 | |
| Fe3Ni2 | ||||||
| 2.59 | 12.0 | |||||
| |C2v | 2.90 | 2.38,2.59 | ||||
| 1.58 | 15.00 | |||||
| |Cs | 2.48,2.54 | 2.46,2.61 | 2.56 | 1.58 | 9.00 | |
| Fe4Ni1 | C4v | |||||
| 2.57 | 16.00 | |||||
| |C4v | 2.64 | 2.34 | ||||
| 1.69 | 15.00 | |||||
| Fe5 | C4v | |||||
| 2.48 | 16.03 | |||||
| |C4v | 2.52,2.56 | |||||
| 1.72 | 19.00 | |||||
Magnetic Properties
The magnetic properties of metal clusters are strongly influenced by their size and surface ligands. In general, the magnetic moments in small metal clusters are larger than in the case of a macroscopic bulk metal structure. For example, the average magnetic momentMagnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
per atom in Ni clusters was found to be 0.7-0.8 μB, as compared with 0.6 μB for bulk Ni. This is explained by longer metal-metal bonds in cluster structures than in bulk structures, a consequence of a larger s character of metal-metal bonds in clusters. Magnetic moments approach bulk values as cluster size increases, though this is often difficult to predict computationally.
Magnetic quenching is an important phenomenon that is well documented for Ni clusters, and represents a significant effect of ligands on metal cluster magnetism
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
. It has been shown that CO ligands cause the magnetic moments of surface Ni atoms to go to zero and the magnetic moment of inner Ni atoms to decrease to 0.5 μB. In this case, the 4s-derived Ni-Ni bonding molecular orbitals experience repulsion with the Ni-CO σ orbital, which causes its energy level to increase so that 3d-derived molecular orbitals are filled instead. Furthermore, Ni-CO π backbonding leaves Ni slightly positive, causing more transfer of electrons to 3d-derived orbitals, which are less disperse than those of 4s. Together, these effects result in a 3d10, diamagnetic character of the ligated Ni atoms, and their magnetic moment decreases to zero.
Density Functional Theory
Density functional theory
Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...
(DFT) calculations have shown that these ligand-induced electronic effects are limited to only surface Ni atoms, and inner cluster atoms are virtually unperturbed. Experimental findings have described two electronically distinct cluster atoms, inner atoms and surface atoms. These results indicate the significant effect that a cluster’s size has on its properties, magnetic and other.
Fe-Ni Clusters in Biology
Fe-Ni metal clusters are crucial for energy production in many bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. A primary source of energy in bacteria is the oxidation and reduction
Reduction
Reduction, reduced, or reduce may refer to:- Chemistry :* Reduction, part of a reduction-oxidation reaction where oxygen is being removed from a compound.** Reduced gas, a gas with a low oxidation number...
of H2 which is performed by hydrogenase
Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....
enzymes.
These enzymes are able to create a charge gradient across the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
which serves as an energy store. In aerobic environments, the oxidation and reduction
Reduction
Reduction, reduced, or reduce may refer to:- Chemistry :* Reduction, part of a reduction-oxidation reaction where oxygen is being removed from a compound.** Reduced gas, a gas with a low oxidation number...
of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
is the primary energy source. However, many bacteria are capable of living in environments where O2 supply is limited and use H2 as their primary energy source . The hydrogense enzymes which provide energy to the bacteria are centered around either a Fe-Fe or Fe-Ni active site
Active site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
. H2 metabolism is not used by humans or other complex life forms, but proteins in the mitochondria of mammalian life appear to have evolved from hydrogenase enzymes, indicating that hydrogenase is a crucial step in the evolutionary development of metabolism.
The active site of Fe-Ni containing hydrogenase enzymes often is composed of one or more bridging sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
ligands, carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
, cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
and terminal sulfur ligands. The non-bridging sulfur ligands are often cystine
Cystine
Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...
amino acid residues that attach the active site to the protein backbone. Metal-metal bonds between the Fe and Ni have not been observed. Several oxidation states of the Fe-Ni core have been observed in a variety of enzymes, though not all appear to be catalytically relevant.
The extreme oxygen and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
sensitivity of these enzymes presents a challenge when studying the enzymes, but many crystallographic studies have been performed. Crystal structures for enzymes isolated from D. gigas, Desulfovibrio vulgaris, Desulfovibrio fructosovorans, Desulfovibrio desulfuricans, and Desulfomicrobium baculatum have been obtained, among others. A few bacteria, such as R. eutropha, have adapted to survive under ambient oxygen levels.
These enzymes have inspired study of structural and functional model complexes in hopes of making synthetic catalysis for hydrogen production (see Fe-Ni and Hydrogen Production, below, for more detail).
Fe-Ni and Hydrogen Production
In the search for a clean, renewable energy source to replace fossil fuels, hydrogen has gained much attention as a possible fuel for the future. One of the challenges that must be overcome if this is to become a reality is an efficient way to produce and consume hydrogen. Currently, we have the technology to generate hydrogen from coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
, natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
, biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. The majority of hydrogen currently produced comes from natural gas reformation, and hence does not help remove fossil fuel as an energy source. A variety of sustainable methods for hydrogen production are currently being researched, including solar, geothermal and catalytic hydrogen production
Hydrogen production
Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...
.
Platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
is currently used to catalyze hydrogen production, but as Pt is expensive, found in limited supply, and easily poisoned by carbon monoxide during H2 production, it is not a practical for large-scale use. Catalysts inspired by the Fe-Ni active site of many hydrogen producing enzymes are particularly desirable due to the readily available and inexpensive metals.
The synthesis
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
of Fe-Ni biomimetic catalytic complexes has proved difficult, primarily due to the extreme oxygen-sensitivity of such complexes. To date, only one example of a Fe-Ni model complex that is stable enough to withstand the range of electronic potential required for catalysis has been published.
When designing model complexes, it is crucial to preserve the key features of the active site of the Fe-Ni hydrogenases: the iron organometallic moiety with CO or CN- ligands, nickel coordinated to terminal sulfur ligands, and the thiolate bridge between the metals. By preserving these traits of the enzyme active site, it is hoped that the synthetic complexes will operate at the electrochemical potential necessary for catalysis, have a high turnover frequency and be robust.