Name each complex. Do your best to sketch a 3D representation of each Are isomers possible? If so, what are they? Record the observed color of each solution. What does this suggest about the wavelength of light absorbed for each? Use your observations to rank ethylenediamine and EDTA in the spectrochemical series. Which is a stronger field ligand?
- Spectrochemical Series Demonstration. Below are many of the experimental steps you will perform in this lab. Be sure to consult the procedure for the detailed instructions. Click on an image to open an enlarged view.
- Coordination compounds. Use water as a blank sample for zeroing the spectrophotometer. Set the wavelengthrange from 425 to 700 nm.Part B: Ti3+ion complexesPlace 0.5 mL of 0.1 M Ti2 (SO4)3 into 3 wells of a well-plate. Then add the designated amounts ofreagent or water to the corresponding well.
Terms:
Complex Ion: An ion containing a central cation bonded to one or more molecules or ions.
Ligand: A molecule or an ion that is bonded to the metal ion in a complex ion.
Ex. A ligand is a oxygen atom in [Fe(H2O)6]3+ or a nitrogen atom in [Zn(NH3)4]2+.
D Orbitals: a type of atomic orbital (regions of space around the nucleus of an atom where an electron is likely to be found) with five distinct shapes; they give transition metals the ability to easily give and take electrons.
Crystal Field Splitting: The energy difference between two sets of d orbitals in a metal atom when ligands are present.
Ex. (Crystal field splitting only applies to octahedral and tetrahedral complexes since they have no more than two energy levels.)
Ex. (CO and CN- are strong-field ligands while the Br- and I- are weak-field ligands.)
Since there is no single theory for bonding in coordinate compounds that takes into account properties such as color, magnetism, stereochemistry, and bond strength, several different approaches have to be applied. However, in this case, we will only consider crystal field theory and transition metal complexes.
Crystal field theory explains the bonding in complex ions in terms of electrostatic forces. In a complex ion, there are two types of electrostatic interaction: the attraction between the positive metal ion and the negatively charged ligand, and the repulsion between the lone pairs on the ligands and the electrons in the d orbitals of the metals. The crystal field theory is very helpful due to the fact that it accounts for both the color and magnetic properties of many coordinate compounds.Rank Dmg In The Spectrochemical Series. Free
In an octahedral complex ion, a central metal atom is surrounded by six lone pairs of electrons (on the six ligands). As a result, all five d orbitals experience electrostatic repulsion.
However, the magnitude of this repulsion depends on the orientation of thedorbital. For example, an electron in the experiences a greater repulsion from the ligands than an electron does in thedxy orbital.
As a result of the metal-ligand interactions, the five d orbitals in an octahedral complex are split into two sets of energy levels: a higher level with two orbitals (dx2-y2 and dz2) and a lower level with three equal-energy orbitals (dxy, dyz, and dxz) as shown above.
In this case, the crystal field splitting is the energy difference between the two levels.The magnitude of Δ depends on the metal and the nature of the ligands; it has a direct effect on the color and magnetic properties of complex ions.
Color
All the rules that apply to reflected light are the same for transmitted light. When the energy of a photon is equal to the difference between the ground state and an excited state, absorption occurs as the photon strikes the atom (or ion or compound), and an electron is promoted to a higher level.
To calculate the energy change involved in the electron transition and the energy of a photon, use the equation E = hv, where h represents Planck's constant (6.63 × 10−34 J · s) and v is the frequency of the radiation.
The best way to measure crystal field splitting is to use spectroscopy to determine the wavelength at which light is absorbed. Then, using the equations
where c is the speed of light and λ is the wavelength,we find the energy to excite one ion of a particular molecule.
With spectroscopic data for a number of complexes, all having the same metal ion but different ligands, chemists have been able to calculate the crystal splitting for each ligand and establish a spectrochemical series,which is a list of ligands arranged in increasing order of their abilities to split the d orbital energy levels:
With spectroscopic data for a number of complexes, all having the same metal ion but different ligands, chemists have been able to calculate the crystal splitting for each ligand and establish a spectrochemical series,which is a list of ligands arranged in increasing order of their abilities to split the d orbital energy levels:
The order in the spectrochemical series is the same no matter which metal atom or ion is present.
These ligands are arranged in the order of increasing value of Δ. CO and CN− are called strong-field ligands, because they cause a large splitting of the d orbital energy levels. The halide ions and hydroxide ion are weak-field ligands, because they split the d orbitals to a lesser extent.
Magnetic Properties
Ions with only one d electron are always paramagnetic. However, for an ion with several d electrons, the situation is less than simple. According to Hund's rule, maximum stability is reached when the electrons are placed in five separate orbitals with parallel spins. But this arrangement can be achieved only at a cost; two of the five electrons must be promoted to the higher-energy orbitals, the dx2-y2 and dz2 orbitals. No such energy investment is needed if all five electrons enter the dxy, dyz, and dxz orbitals.
In the image below, the distribution of electrons among d orbitals that results in low- and high-spin complexes is shown.
The actual arrangement of the electrons is determined by the amount of stability gained by having maximum parallel spins versus the investment in energy required to promote electrons to higher d orbitals.
With a weak-field ligand, the five d electrons enter five separate d orbitals with parallel spins to create a high-spin complex. On the other hand, with a strong-field ligand, all five electrons are in the lower orbitals because it is energetically preferred; as a resul, a low-spin complex is formed.
Thus, high-spin complexes are more paramagnetic than low-spin complexes.
Tetrahedral and Square-Planar Complexes
![Rank Rank](/uploads/1/3/4/3/134383952/100517460.jpg)
The splitting pattern for a tetrahedral ion is just the reverse of that for octahedral complexes. In this case, the dxy, dyz, and dxz orbitals are more closely attached to the ligands and therefore have more energy than the
![Series. Series.](/uploads/1/3/4/3/134383952/292652938.webp)
However, in the case of square-planar complexes, crystal field splitting cannot be applied because there are more than two energy levels. Clearly, the dx2-y2 orbital possesses the highest energy and the dxy orbital the next highest. However, the position of the dxy, dyz, and dxz orbitals have to be determined through certain calculations.
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< Advanced Inorganic Chemistry
MO Diagrams of Pi Donor Ligands and Pi Acceptor Ligands
The nature of ligands coordinated to the center metal is an important feature of a complex compound along with other properties such as metal identify and its oxidation state. More specifically, it is the identity and consequently the ability of the ligand to donate or accept electrons to the center atom that will determine the molecular orbitals.
The spectrochemical series shows the trend of compounds as weak field to strong field ligands. Furthermore, ligands can be characterized by their π-bonding interactions. This interaction reveals the amount of split between eg and t2g energy levels of the molecular orbitals that ultimately dictates the strength of field of the ligands.
Examples of Weak Field LigandsX-, OH-, H2O ;Examples of Strong Field LigandsH-, NH3, CO, PR3
Electron configuration of high and low spin.
In a π-donor ligand, the SALCs of the ligands are occupied, hence it donates the electrons to the molecular σ σ* and π π* orbitals. The orbitals associated to eg are not involved in π interactions therefore it stays in the same energy level (figure 1). On the other hand, the occupied ligand SALC t2g orbitals that would form molecular orbitals with the metal t2g orbitals (ie. dxy, dxz, dyz) are lower in energy than its metal counterparts. The resulting MO has π* orbitals that are energetically lower than the σ* orbitals that are formed from the non bonding orbitals (eg). The difference between the t2g π* and eg σ orbitals is denoted as Δ, split. In the π-donor case, the Δ is small due to the low π* level.
Conversely, the t2g SALCs of a pi accepting orbitals are higher in energy than the metal t2g orbitals because they are unoccupied. The resulting t2g π* orbitals are higher than the σ* orbitals. This creates a larger Δ between the eg and t2g π orbitals, making these π-accepting orbitals high split ligands.
Finally, the magnitude of Δ as influenced by the identify of the ligand will dictate how electrons are distributed in the metal d orbitals (figure 2). Weak field ligands produce a small Δ hence a high spin configuration. Strong field ligands produce a large Δ hence a low spin configuration on the d electrons.
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