Transition metal molecules

There are a number of apparent exceptions to the VSEPR model, most of which can be classified into two main groups those due to ligand-ligand interactions and those due to the non-spherical cores found in transition metal molecules. 

Although the ligand field theory can be used to rationalize the geometry of some transition metal molecules and complex ions, the study of the shapes of transition metal molecules in terms of the electron density distribution is still the subject of research and it has not reached a sufficient stage of development to enable us to discuss it in this book. 

These papers discuss the affect of core distortion on the geometry of transition metal molecules. 

Table 2. Dissociation energies of diatomic transition metal, molecules, Dq(M-M) kj mol-1...

M. A. Buijse and E. J. Baerends, Theor. Chim. Acta, 79, 389 (1991). Orbital Localization in Transition Metal Molecules. 

In this simple case there is no advantage to the pseudopotential calculation (the 3-21G( ) geometry is actually better ), but more challenging calculations on very-heavy-atom molecules, particularly transition metal molecules, rely heavily on ab initio or DFT (Chapter 7) calculations with pseudopotentials. Nevertheless, ordinary nonrelativistic all-electron basis sets sometimes give good results with quite heavy atoms [64]. A concise description of pseudopotential theory and specific relativistic effects on molecules, with several references, is given by Levine [65]. Reviews oriented toward transition metal molecules [66a,b,c] and the lanthanides [66d] have appeared, as well as detailed reviews of the more technical aspects of the theory [67]. See too Section 8.3. 

A recent EH-type calculation by Hare et al. (34) and Cooper et al. 35) has been applied to diatomic transition metal molecules. Input data were chosen from previously explained procedures to determine which input data sets give the best fit to experimental data. The off-diagonal Hamiltonian elements were calculated using Eq. (8). A comparison of calculated and experimental data for transition element diatomics is shown in Table II. Although some discrepancies are apparent, the procedure seems qualitatively correct for these molecules. 

A comparison of EH and CNDO with experimental data has been made by Baetzold (30) for other metal homonuclear diatomic molecules. This work has employed the orbital exponents of Clementi et al. (10,11) and experimental atomic data for ionization potentials. Table III lists representative data for transition metal molecules calculated by CNDO and EH. No one procedure is universally superior to another. 

Radiofrequency or microwave/optical double resonance of transition metal molecules... 

As we have seen elsewhere in this book, the techniques of high-resolution rotational spectroscopy are beginning to be applied to transition metal molecules, and this is particularly true of double resonance methods. We now review some of the work which has been described, again in a mainly chronological order. 

Radiofrequency or microwave/optical double resonanceof transition metal molecules H 919... 

Dissociation Energies, D°/kJ mol- of Homonuclear Diatomic d-Transition Metal Molecules... 

The value of the maximum dissociation energy in a homonuclear diatomic transition metal molecule has been predicted (2) to be (600 40) kJ mol-1 for ditantalum. The dissociation energies of homonuclear transition metal molecules have been predicted by various methods of which the most recent is the cell model of Miedema (7). This proposes a relation between the enthalpy of vaporisation of the solid metal, AH ap, the dissociation energy, D°, of the diatomic molecule and the surface energy of the metal, Y° as follows,... 

Two models have been used to predict dissociation energies for heteronuclear diatomic transition metal molecules, the valence bond model (9), which proposes a polar single bond, and the atomic cell model (7). Their success when compared with experiment is indicated by the following examples ... 

The maximum bond energy of a heteronuclear diatomic transition metal molecule has been suggested (2) as (670 84) kJ mol l. 

In a diatomic transition metal molecule there is no ambiguity concerning the existence of a bond between the metal atoms. 

The use of empirical models of bonding has been Invaluable for the interpretation of the experimental dissociation energies of diatomrLc Intermetallic molecules as well as for the prediction of the bond energies of new molecules. In the course of our work, conducted for over a decade, we have extended the applicability of the Pauling model of a polar single bond (31) and have developed new models such as the empirical valence bond model for certain multiple bonded transition metal molecules (32,33) and the atomic cell model (34). 

Quite large downward corrections are also expected for other transition metal molecules due to the now expected metal like band electronic structure of other such molecules. Diatomic chromium is a recent example (53). The dissociation energies of Mo2, Nb2 and Pt2 (2 ) that have been recently measured in our laboratory and for which we had considered no (M02, Nb2) or only a... 

This chapter is concerned primarily, though not exclusively, with the study of valence levels in transition metal molecules. There are a number of recent reviews that address various aspects of this topic. 

To measure a gas-phase spectrum of a transition metal molecule, the compound must possess sufficient volatility to generate a pressure of 10 — 10 Torr at a temperature below its decomposition point. Such conditions clearly restrict the number of compounds that can be measured in this state. Many of the more covalent transition metal compounds will survive such treatment, and gas-phase PES has been... 

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