D orbital transitions

Figure 9.15 Some possible transitions involving defects in a solid. Transitions involving transition-metal (TM) ions are between d orbitals, transitions involving lanthanide (LnM) ions are between/orbitals.

Scott and Wrixon have developed a quadrant rule for the c.d. of platinum(ii)-olefin complexes that depends on d-d orbital transitions. Application of the rule to monoterpenes was considered, and generally conformed to expectations based on known absolute configurations, but in some cases (notably 8-pinene) the results were not satisfactory. The complex measured may be that of a-pinene, for which a Cotton curve of the opposite sign is predicted. Further work on the use... 

Figure 3. The Cu2+ surface complex on h-Al203 (center), showing the corresponding d-orbital transitions (lower right), ESR (derivative) spectrum (upper right), proton ENDOR spectrum (upper left), and ESEEM spectrum (lower left).

The First d-Orbital Transition Series Building Up Period 4... 

In the past few years, the hydrogenation of a variety of diene polymers and copolymers made with anionic initiators and d-orbital transition metal catalysts has been studied extensively. It is of interest to investigate the hydrogenated... 

The discovery of rare earth coordination catalysts in stereospecific polymerization not only contributes to the development of Ziegler-Natta catalysts and stereospecific polymerization from the usual d-orbital transition elements to... 

The number of active species of the Nd(naph)3 and NdCl3 systems in Bd polymerization, determined by tritiated methanol quenching, kinetic and retarding agent methods, amounts to 0.6-10 mol% of Nd, while the Ti catalyst systems are generally only about 0.5%. Hu et al. (1982), and Hu and Ouyang (1983) proposed that the polymerization of conjugated diene with rare earth catalysts was the same as that of d-orbital transition metal catalysts such as Ti and Co and could be described as follows ... 

We consider first some experimental observations. In general, the initial heats of adsorption on metals tend to follow a common pattern, similar for such common adsorbates as hydrogen, nitrogen, ammonia, carbon monoxide, and ethylene. The usual order of decreasing Q values is Ta > W > Cr > Fe > Ni > Rh > Cu > Au a traditional illustration may be found in Refs. 81, 84, and 165. It appears, first, that transition metals are the most active ones in chemisorption and, second, that the activity correlates with the percent of d character in the metallic bond. What appears to be involved is the ability of a metal to use d orbitals in forming an adsorption bond. An old but still illustrative example is shown in Fig. XVIII-17, for the case of ethylene hydrogenation. 

The detailed theory of bonding in transition metal complexes is beyond the scope of this book, but further references will be made to the effects of the energy splitting in the d orbitals in Chapter 13. 

The splitting of the d orbital energy levels when ligands are bonded to a central transition atom or ion has already been mentioned (p. 60). Consider the two ions [Co(NH3)g] and [Co(NH3)g] we have just discussed. The splitting of the d orbital energy levels for these two ions is shown in Figure 13.2. 

Copper differs in its chemistry from the earlier members of the first transition series. The outer electronic configuration contains a completely-filled set of d-orbitals and. as expected, copper forms compounds where it has the oxidation state -)-l. losing the outer (4s) electron and retaining all the 3d electrons. However, like the transition metals preceding it, it also shows the oxidation state +2 oxidation states other than -l-l and - -2 are unimportant. 

A variation on MNDO is MNDO/d. This is an equivalent formulation including d orbitals. This improves predicted geometry of hypervalent molecules. This method is sometimes used for modeling transition metal systems, but its accuracy is highly dependent on the individual system being studied. There is also a MNDOC method that includes electron correlation. 

PM3/TM is an extension of the PM3 method to include d orbitals for use with transition metals. Unlike the case with many other semiempirical methods, PM3/TM s parameterization is based solely on reproducing geometries from X-ray diffraction results. Results with PM3/TM can be either reasonable or not depending on the coordination of the metal center. Certain transition metals tend to prefer a specific hybridization for which it works well. 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

Eor transition metals the splitting of the d orbitals in a ligand field is most readily done using EHT. In all other semi-empirical methods, the orbital energies depend on the electron occupation. HyperChem s molecular orbital calculations give orbital energy spacings that differ from simple crystal field theory predictions. The total molecular wavefunction is an antisymmetrized product of the occupied molecular orbitals. The virtual set of orbitals are the residue of SCE calculations, in that they are deemed least suitable to describe the molecular wavefunction. 

Normally, you would expects all 2p orbitals in a given first row atom to be identical, regardless of their occupancy. This is only true when you perform calculations using Extended Hiickel. The orbitals derived from SCE calculations depend sensitively on their occupation. Eor example, the 2px, 2py, and 2pz orbitals are not degenerate for a CNDO calculation of atomic oxygen. This is especially important when you look at d orbital splittings in transition metals. To see a clear delineation between t2u and eg levels you must use EHT, rather than other semiempirical methods. 

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