1 / 5

2) Terms and absorption bands of octahedral d 1 metal ion. Selection rules

Lecture 25 Electronic Spectra of Coordination Compounds ML x (x = 4,6) 1) Octahedral complexes of d 1 configuration. Absorption of a TMC in the UV and visible regions results from transitions of electrons between the energy levels available in the metal complex.

channary-khin
Download Presentation

2) Terms and absorption bands of octahedral d 1 metal ion. Selection rules

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 25Electronic Spectra of Coordination Compounds MLx (x = 4,6)1) Octahedral complexes of d1 configuration Absorption of a TMC in the UV and visible regions results from transitions of electrons between the energy levels available in the metal complex. • We will be interested in those transitions which occur within the metal valence shell (d-subshell). Of our interest will be: 1) The number and the order of the d-energy levels (terms); 2) The number of absorption bands and their intensity; 3) Absorption bandwidth 1. Consider terms of an isolatedd1metal ion (Ti3+) The number of microstates possible for dXconfiguration is given by formula d1 case corresponds to X = 1 and N = 10 (maximum occupancy of d-level). The number of microstates is then 10 which means that any of the five degenerate d-orbitals may be occupied by an electron with a spin of ½ or - ½. The orbital angular momentum for Ti3+, L = 2, the spin S = 1/2 and the term is 2D.

  2. 2) Terms and absorption bands of octahedral d1 metal ion. Selection rules • The wavefunction for a D term is of the same symmetry as for a d-orbital. As a result, D terms behave as d-orbitals do. • In particular, when d1 metal ion is exposed to an octahedral ligandfield, the five-time degenerate 2D term will be split into 2Eg (doubly degenerate) and 2T2g (triply degenerate) terms (recall how d-orbital split in octahedral complexes in crystal field theory!). • Since we have two terms only, one can expect a single band in the spectrum of d1 metal complexes due the only possible T2g Eg transition within the metal d-shell. This band is weak [e of 1 – 103 L/(mol cm)]. Selection rules help rationalize the observed intensity of electronic transitions. • The Laporte rule states that symmetry allowed are the transitions with the change of parity, g  u or u  g. • It means that T2g Eg transitions are symmetry forbidden. Nevertheless, due to asymmetric vibrations there is always a small fraction of molecules of a reduced symmetry so that such a vibronic transition is allowed though with a low intensity. • Another selection rule states that spin allowed are the transitions with DS = 0 (both states should be of the same multiplicity). This holds true for the 2T2g  2Eg transition.

  3. 3) Estimating Do from electronic absorption spectra of d1 species • Values of Do can be readily obtained from absorption spectra of d1 transition metal complexes. • In the case of a d1 metal complex [Ti(H2O)6]3+ (spectrum is shown below) lmax = 500 nm what corresponds to the Do = n = 1/ lmax = 1/(5.00 10-5cm)= 20000 cm-1. • In the case of dn (n>1) species the relationship between the measured lmax and Do may be much more complex. lmax

  4. 4) Complexes of dn, d10-n, d5-n and d5+n configurations • In more complex cases of dn (n > 1) configurations the number of terms available is much more significant than for d1 species. Here are some rules to make analysis of the terms simpler: • For any given ligand field, term splitting pattern for a metal of d10-n configuration will be the same as for dn one, but the energy level sequence will be the opposite. • d10-n configuration can be considered as a combination of n positrons (“hole formalism”) and a d10 metal shell. Because of the opposite charge of positrons, their interaction with the ligands will be of the opposite sign resulting in the opposite energy level sequence . • Similarly, in the case of weak field ligands, dn, d5-n and d5+n configurations have the same term splitting pattern. The ordering of the energy levels is the same for dn and d5+n and the opposite for dn and d5-n.

  5. 5) Tetrahedral vs octahedral complexes of dn configuration • Octahedral and tetrahedral ligand fields cause the opposite ordering of split levels for each of the metal ion terms. • The term splitting pattern for octahedral and tetrahedral complexes is the same. • The term sequence is the opposite for octahedral and tetrahedral complexes of the same configuration. • The term sequence is of the same order for dn octahedral and d10-n tetrahedral complexes. Note: Tetrahedral terms have no center of inversion and thus no labels g or u.

More Related