Electron-density distributions in complexes

Much greater information may be forthcoming from diffraction analysis when it is possible to apply it readily to the determination of accurate electron density distributions in complex materials. How soon that time will come is another unknown that is difficult to predict. 

Contents Introduction. - X-Ray Difraction. -Conformational Analysis. - Structure and Isomerism of Optically Active Complexes. - Electron-Density Distribution in Transition Metal Complexes. - Circular Dichroism. - References. 

It is well known that metal carbenes can be classified as Fisher and Schrock carbenes. The classification is mainly based on the n electron density distribution on the M = C moiety (Scheme 4.2). On the basis of the n electron density distribution, carbene complexes of the Fisher-type (E) are normally electrophilic at the carbene carbon while carbene complexes of the Schrock-type (F) are nucleophilic at the carbene carbon. Similarly, metal vinylidenes could also be classified into the two types Fisher-type (G) and Schrock-type (H). The majority of isolated metal vinylidenes belong to the Fisher-type. On the basis of the 7t electron density distribution shown in... 

An X-ray atomic orbital (XAO) [77] method has also been adopted to refine electronic states directly. The method is applicable mainly to analyse the electron-density distribution in ionic solids of transition or rare earth metals, given that it is based on an atomic orbital assumption, neglecting molecular orbitals. The expansion coefficients of each atomic orbital are calculated with a perturbation theory and the coefficients of each orbital are refined to fit the observed structure factors keeping the orthonormal relationships among them. This model is somewhat similar to the valence orbital model (VOM), earlier introduced by Figgis et al. [78] to study transition metal complexes, within the Ligand field theory approach. The VOM could be applied in such complexes, within the assumption that the metal and the... 

In 1973, Iwata and Saito determined the electron-density distribution in crystals of [Co(NH3)6]fCo(CN)6l (37). This was the first determination of electron density in transition metal complexes. In the past decade, electron-density distributions in crystals of more than 20 transition metal complexes have been examined. Some selected references are tabulated in Table I. In most of the observed electron densities, aspherical distributions of 3d electron densities have been clearly detected in the vicinities of the metal nuclei. First we shall discuss the distributions of 3d electron density in the transition metal complexes. Other features, such as effective charge on transition metal atoms and charge redistribution on chemical bond formation, will be discussed in the following sections. 

Some Selected Measurements of Electron-Density Distributions in Crystals of Transition Metal Complexes... 

Crystallographic studies of transition metal hydride complexes Stereochemistry of six-coordination Five-coordinate structures Stereochemistry of five-coordinate Co complexes Absolute stereochemistry of chelate complexes Stereochemistry of optically-active transition metal complexes Electron density distributions in inorganic compounds... 

With this aim in mind, calculations of the electronic structure of the hydrated, M(H20) +, and hydrolysed, M(OH)jT, complexes of Nb, Ta, Db and Pa, have been performed using the DFT method (Pershina 1998a,b). The calculations have shown Ec to be the predominant type of the metal-ligand interaction, so that by calculating only AEC, correct trends in the complex formation can be predicted for all the elements under discussion (see Table 6.4). This electrostatic interaction must, however, be defined on the basis of the real (relativistic) electronic density distribution in the considered systems. 

Before showing more complex DOS, we will first concentrate on how to characterize the electron density distribution in molecules and solids because that is where we started this is also needed to further decompose the density-of-states and characterize the electronic structures in terms of bonding properties. Recalling the LCAO ansatz for the molecular orbital (MO) of the H2 molecule as given in Equation (2.11), we may square-integrate the MO over all space and yield... 

Takazawa H, Ohba S, Saito Y (1988) Electron-density distribution in crystals of dipotassium tetrachloropalladate(ll) and dipotassium hexachloropaUadate(lV). Acta Cryst B44 580-585 Guryanova EN, Goldstein IP, Romm IP (1975) The donor-acceptor bond. WUey, New York Timoshkin AY, Suvorov AV, Bettinger HE, Schaefer HF 111 (1999) Role of the terminal atoms in the donor-acceptor complexes MX3-D (M=A1, Ga, In X=F, Q, Br, I D=YH3, YX3, X Y=N, P, As). JAm Chem Soc 121 5687-5699... 

Electrophilic attack of the tri-t-butylcyclopropenium cation on [M(CO)3( j-CsHs)] (M = Mo or W) takes place at the C5H5 ring to give complexes (17) and, since the displaced hydrogen is transferred to the metal, exo attack is implied. With [Fe(CO)2(7 -C6H5)] attack at co-ordinated CO is observed preferentially since the oxocyclobutenyl complex (18) is obtained. This stereoselectivity is difficult to rationalize on steric grounds and differences in electron density distribution in these complexes are consequently thought to control the site of attack. 

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