Crystal Field Theory (CFT) In Detail. (H&S 3rd Ed. Chpt. ). CFT provides a simple model for d orbital splitting: • works for several oxidation states and. CRYSTAL FIELD THEORY. Hans Bethe () and Van Vleck (). Failures of VBT. 1. Tetrahedral or Square planar?? 2. Distortions in complexes – No. 3. We investigate a simple model for the prediction of the splitting of the $3d$ orbitals of a metal ion in the environment of ligands. The electrons are considered to be independent and their interaction with the ligands is represented by the Dirac delta function in three dimensions.
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Crystal Field Splitting in an Octahedral Field eg. Energy. 3/5 o o t2g. 2/5 o e g. - The higher energy set of orbitals (d z2 and d x2-y2.) t. 2g. - The lower energy set . Crystal Field Theory. (Text: JD Lee; pp). •This theory (CFT) largely replaced VB Theory for interpreting the chemistry of coordination compounds. Crystal Field Theory of Coordination Complexes. • historically developed for solid state crystal lattices. • adapted for molecular complexes (later versions: ligand.
Introduction to Inorganic Chemistry/Coordination Chemistry and Crystal Field Theory
Tetrahedral complexes are the second most common type; here four ligands form a tetrahedron around the metal ion. The lower energy orbitals will be dz2 and dx2-y2, and the higher energy orbitals will be dxy, dxz and dyz - opposite to the octahedral case. Furthermore, since the ligand electrons in tetrahedral symmetry are not oriented directly towards the d-orbitals, the energy splitting will be lower than in the octahedral case.
Square planar and other complex geometries can also be described by CFT. The reasons behind this can be explained by ligand field theory. It is useful to note that the ligands producing the most splitting are those that can engage in metal to ligand back-bonding. In complexes with these ligands, it is unfavourable to put electrons into the high energy orbitals.
Therefore, the lower energy orbitals are completely filled before population of the upper sets starts according to the Aufbau principle. Complexes such as this are called "low spin". This low spin state therefore does not follow Hund's rule.
In this case, it is easier to put electrons into the higher energy set of orbitals than it is to put two into the same low-energy orbital, because two electrons in the same orbital repel each other.
So, one electron is put into each of the five d-orbitals in accord with Hund's rule, and "high spin" complexes are formed before any pairing occurs. As noted above, eg refers to the dz2 and dx2-y2 which are higher in energy than the t2g in octahedral complexes. Therefore, the energy required to pair two electrons is typically higher than the energy required for placing electrons in the higher energy orbitals.
Thus, tetrahedral complexes are usually high-spin.
The use of these splitting diagrams can aid in the prediction of magnetic properties of coordination compounds. A compound that has unpaired electrons in its splitting diagram will be paramagnetic and will be attracted by magnetic fields, while a compound that lacks unpaired electrons in its splitting diagram will be diamagnetic and will be weakly repelled by a magnetic field.
Crystal field stabilization energy[ edit ] The crystal field stabilization energy CFSE is the stability that results from placing a transition metal ion in the crystal field generated by a set of ligands.
On the other hand, if the electron were to occupy the dxy, dyz or dxz orbital, it would spend less time near the ligands than would a spherically distributed electron and would be repelled less.
This difference between for example the dx2-y2 orbital, the dxy orbital and a spherical distribution can be graphically represented by The result of these differences for the d-orbitals is an energy difference between the dz2 and dx2-y2 orbitals compared to the dxy, dyz and dxz orbitals.
The CFT approach can be easily extended to other geometries and the next most important case is the tetrahedron. To predict the splitting pattern of the energy of the d-orbitals under a tetrahedal crystal field you may once again find it convenient to consider how the ligands can fit into a cube to give a tetrahedron.
The next step is to consider how the d-orbitals interact with these incoming ligands. For tetrahedral complexes, the energy of those orbitals which point towards the edges should now be raised higher than those which point towards the faces. That is, the exact opposite of the situation we just dealt with for the octahedral crystal field.
The end result is a splitting pattern which is represented in the splitting diagram above. The square planar case can be considered as an extension of the octahedral, where we remove the two ligands from the Z axis. Consequently, repulsion of an electron in the dz2 orbital will no longer be equivalent to that experienced by an electron in the dx2-y2 orbital, and the end result is shown above. For the first year course, the only square planar complexes will be for d8 complexes, i.
Once we accept that the energy of the 5 'd' orbitals are no longer degenerate in a coordination compound we can begin to explore some of the implications. Note that a different CFT energy splitting diagram has to be applied for each stereochemistry.
In this course we will only be concerned with diagrams for octahedral, tetrahedral and square planar complexes. One of the important aspects of CFT is that not all ligands are identical when it comes to causing a separation of the energy of the d-orbitals.
Crystal field theory
For transition metal compounds, there is clear evidence for this from the multitude of colours available for a given metal ion when the ligands or stereochemistry are varied. For octahedral complexes this is a reflection of the energy difference between the higher dz2, dx2-y2 eg subset and the dxy, dyz, dxz t2g subset.
These are referred to as either weak field - strong field or high spin - low spin configurations. What this means is that if we use a technique that can detect the presence of unpaired electrons in each compound, then for the first we should find 4, while in the latter none.
This accounts for the names high spin compared to low spin. The terms weak field, strong field give an indication of the splitting abilities of the ligand. Water always give rise to small splittings of the enregy of the d orbitals for first row transition metal ions and hence is referred to as a weak field ligand.It had mysterious properties, in that it dissolved in water like an ionic solid, but it retained its six ammonia molecules when recrystallized.
These electrons can come from lone pairs, pi or sigma donors. Z-type ligands are those that accept two electrons from the metal center as opposed to the donation occurring with the other two types of ligands.
Crystal field theory of transition metal complexes
This situation allows for the most number of unpaired electrons, and is known as high spin. Spatial arrangement of ligands in the an octahedral ligand field with respect to the five d-orbitals. Thus, tetrahedral complexes are usually high-spin.
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