Crystal Field Theory (CFT).

Crystal Field Theory (CFT).

Crystal Field Theory (CFT), An Introduction.

Lecture 3. CHEM1902 (C 10K) Coordination Chemistry

In the ionic CFT, it is assumed that the ions are simple point charges. When applied to alkali metal ions containing a symmetric sphere of charge, calculations of energies are generally quite successful. The approach taken uses classical potential energy equations that take into account the attractive and repulsive interactions between charged particles (that is, Coulomb's Law interactions).

Electrostatic Potential is proportional to q1 * q2/r

where q1 and q2 are the charges of the interacting ions and r is the distance separating them. This leads to the correct prediction that large cations of low charge, such as K+ and Na+, should form few coordination compounds.

For transition metal cations that contain varying numbers of d electrons in orbitals that are NOT spherically symmetric, however, the situation is quite different. The shape and occupation of these d-orbitals then becomes important in an accurate description of the bond energy and properties of the transition metal compound.

To be able to understand and use CFT then, it is essential to have a clear picture of the shapes (angular dependence functions) of the d-orbitals.


d-orbitals


Jmol is an Open Source application for the display of molecular graphics that is capable of displaying atomic orbitals so it is possible to see the relationship between their orientation and ligands in different stereochemistries. See the cases for octahedral, tetrahedral and square planar complexes.

A cube provides a convenient reference for displaying the coordination centre of complexes since octahedral compounds have cubic symmetry i.e. the six anions sit at the centres of the faces of the cube and for tetrahedral complexes the 4 ligands sit at opposite diagonal edges.

You should be able to see that two of the d-orbitals, the dz2 and dx2-y2 meet the faces of the cube, but the remaining three (dxy, dyz and dxz) point towards the edges of the cube and actually have a node meeting the centres of the faces.

What happens then to the energy of the d-electrons if negatively charged ligands are brought in towards the metal centre to give an octahedral complex - where the ligands sit at the centre of the faces of the cube i.e. on the X, Y and Z axes? Since the d electrons are themselves negatively charged they would experience repulsion and their energies would be raised. For 's' and 'p' electrons this is of little consequence but for 'd' electrons there will be an energy difference involved depending on the orbital distribution and occupation.

Consider the simple example of TiCl63- in which six chloride ions octahedrally surround the Ti3+ cation. There is only one d-electron to be allocated to one of the five d- orbitals. If it were to occupy the dz2 or dx2-y2 orbital, both of which meet the face of the cube and thus point directly towards the chloride ligands, it would be strongly repelled. The geometry of these orbitals and their nodes would require the electron to stay near the negatively charged ligands causing even more repulsion than a spherically distributed electron would experience. 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


d-orbs in sphere
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.
CFT splitting

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.e all four coordinate complexes are tetrahedral except for d8 which may be tetrahedral or square planar.

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. 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).

It has been established that the ability of ligands to cause a large splitting of the energy between the orbitals is essentially independent of the metal ion and theSPECTROCHEMICAL SERIES is a list of ligands ranked in order of their ability to cause large orbital separations.

A shortened list includes:

I- < Br- < SCN- ~Cl- < F- < OH- ~ ONO- < C2O42- < H2O
< NCS- < EDTA4- < NH3 ~ pyr ~ en < bipy < phen < CN- ~ CO

When metal ions that have between 4 and 7 electrons in the d orbitals form octahedral compounds, two possible electron distributions can occur. These are referred to as either weak field - strong field or high spin - low spin configurations.

Take for example, Fe2+- d6.
high spin low spin
The diagram on the left represents the case for the aqua ion (small Δ) and on the right that of the hexacyano ion (large Δ).

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. Conversely, CN- is a strong field ligand, since it causes large splittings of the energy of the d-orbitals.

A simple JAVA applet that shows the electronic configuration for octahedral and tetrahedral complexes and calculates the spin-only magnetic moment is available.

One method of determining the number of unpaired electrons is by looking at the magnetic properties of the compounds. A simple technique to determine a magnetic moment (the GOUY METHOD) involves weighing the sample in the presence and absence of a strong magnetic field. By careful calibration using a known standard, such as Hg[Co(SCN)4] the number of unpaired electrons can be determined.

To predict the magnetic moment, we can use the simple spin-only formula:

      μ     =   √[4S(S+l)]       Bohr Magneton    (BM) 
where S is the spin quantum number (1/2 for each unpaired electron).
An alternative representation is:
      μ     =   √[n(n+2)]        Bohr Magneton    (BM) 
where n is the number of unpaired electrons.
These simple formulae give good results for most first row transition metal compounds and can be refined to include orbital contributions (COVERED IN MORE DETAIL IN CHEM2101 (C21J)).

It is expected that all students will be able to calculate the spin-only magnetic moment of any coordination compound. To do this, you must know the number of d-electrons in the central metal ion, the stereochemistry and whether weak field-strong field considerations are necessary.

K3[Fe(oxalate)3] 3H2OK2[CuCl4]
metal ionFe3+Cu2+
number of d electrons59
stereochemistryoctahedraltetrahedral
High Spin/Low SpinHigh SpinNot relevant
# of unpaired electrons51
magnetic moment√(35) B.M√(3) B.M

Note that if there are 1-3 or 8-9 d electrons in an octahedral complex, the spin-only magnetic moment will have the same value irrespective of whether the ligands present are considered weak field or strong field. For octahedral Co(III) complexes, we will make the simplification that all are low-spin, so regardless of the types of ligands present the magnetic moment will be μ=0, that is, diamagnetic.

The Peptide Bond - Peptide Guide

The Peptide Bond

A peptide bond (sometimes mistakenly called amino bond) is a covalent bond that is formed between two molecules when the carboxyl group of one molecule reacts with the amino group of the another molecule, releasing a molecule of water. This is a a condensation reaction and usually occurs between amino acids. The resulting CO-NH bond is called a peptide bond, and the resulting molecule is an amide.

Formation of the peptide bond

The molecules must be orientated so that the carboxylic acid group of one can react with the amine group of the other. For example, two amino acids combining through the formation of a peptide bond to form a dipeptide.

Any number of amino acids can be joined together in chains of 50 amino acids called peptides, 50-100 amino acids called polypeptides, and over 100 amino acids called proteins. A number of hormones, antibiotics, antitumor agents and neurotransmitters are peptides (proteins).

A peptide bond can be broken down by hydrolysis (the adding of water). The peptide bonds that are formed within proteins have a tendency to break spontaneously when subjected to the presence of water (metastable bonds) releasing about 10 kJ/mol of free energy. This process, however, is very slow. Living organisms use enzymes to broken down or to form peptide bonds. The wavelength of absorbance for a peptide bond is 190-230 nm.

Structure of the Peptide Bond

X-ray diffraction studies of crystals of small peptides by Linus Pauling and R. B. Corey indicated that the peptide bond is rigid, and planer. Pauling pointed out that this is largely a consequence of the resonance interaction of the amide, or the ability of the amide nitrogen to delocalize its lone pair of electrons onto the carbonyl oxygen.

Because of this resonance, the C=O bond is actually longer than normal carbonyl bonds, and the N–C bond of the peptide bond is shorter than the N–Cα bond. Notice that the carbonyl oxygen and amide hydrogen are in a trans configuration, as opposed to a cis configuration. This configuration is energetically more favorable because of possible steric interactions in the other.

The Polarity of the Peptide Bond

The peptide bond is usually portrayed as a single bond between the carbonyl carbon and the amide nitrogen. Normally, this should allow free rotation about than bond. However, notice that the nitrogen has a lone pair of electrons, which are adjacent to a carbon-oxygen bond. Therefore, a reasonable resonance structure can be draw with a double bond linking the carbon and nitrogen, and which result in a negative charge on the oxygen and a positive charge on the nitrogen.

The resonance structure prevents rotation around the peptide bond. The real structure of course is a weighted hybrid of these two structures. Therefore, the question is how significant is the resonance structure in depicting the true electron distribution. It is know that the peptide bond has approximately 40% double-bond character and therefore it is rigid.

Charges give the peptide bond a permanent dipole. Because of the resonance, the oxygen has a -0.28 charge, while the nitrogen bears a +0.28 charge.



The Peptide Bond - Peptide Guide

The Peptide Bond - Peptide Guide

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IANS | Oct. 05, 2011

PATNA: Anand Kumar, who founded Super 30, Bihar's widely acclaimed free coaching centre for Indian Institute of Technology (IIT) aspirants, has blamed the institutes' entrance exam panel for the poor quality of students making the cut, a concern voiced by Infosys chairman emeritus NR Narayana Murthy.

Anand Kumar said if poor quality students, as felt by Narayan Murthy, were able to get into the IITs, it was the responsibility of"

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EklavyaJEE06: "Times of India :: 22 July 2011

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EklavyaJEE06: "tatesman Editorial :: 12 August 2011

The illegality within IIT

THE formality of the transfer of a case from a state government to the CBI is of relatively lesser moment than the astonishing fact of a campus within a campus. A fake institute styled the Institution of Electrical Engineering (IEE) was being run for quite a while within the holy of holies ~ the Indian Institute of Technology, Kharagpur. It was ostensibly engaged in the award of diplomas; neither the institution, not to mention the diplomas, were recognised by the All India Council for Technical Education. It is difficult to believe that the ITT authorities were not privy to this illegality. Yet it remained for the duped students to spill the beans and complain to the Central Vigilance Commission that it was being run by certain members of the faculty and IIT officials."

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EklavyaJEE06: "Telegraph :: 11 August 2011

CBI waits for Bengal nod on IIT case

BASANT KUMAR MOHANTY

New Delhi, Aug. 10: The CBI has refused to take up the probe into a fake institute that was being run on the IIT Kharagpur campus on the ground that the Bengal government has to first transfer the case that state police are now investigating.

“The state police will have to agree to transfer the case to the CBI. We have asked the state government to expedite the process of investigation by the state police or agree to transfer the c"

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EklavyaJEE06: "Telegraph :: 22 August 2011

Graft glare on IIT Bbsr director scam

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New Delhi, Aug. 21: The HRD ministry is likely to frame charges against IIT Bhubaneswar director Madhusudan Chakraborty for alleged involvement in irregularities related to equipment purchases worth Rs 2.5 crore last year.

The charges could be framed this month by the ministry’s vigilance wing on the basis of the lapses detected by the CBI in the purchases."

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EklavyaJEE06: "Rot starts at top: Indicted IIT chiefs go scot-free

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