Metal homonuclear diatomic //-transition

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. 

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,... 

TABLE 8.2 Properties of Some Homonuclear Diatomic Transition Metals... 

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

Fig. 15. The orbitals of a homonuclear diatomic responsible for bonding to transition metal fragments.

FIGURE 8.6 Orbital correlation diagram for homonuclear diatomic molecules of the transition metals... 

The electron affinities of the main group homonuclear diatomic anions have been measured by PES. A few experimental values for the transition metal dimers are also available. The electron affinities of all the 3d homonuclear diatomic molecules have been calculated using density functional methods [1-4], Only the AEa of I2, 2.524 eV C2, 3.27 Si2, 2.2o S2, 1.67 F2, 3.0g Cl2, 2.4s Br2, 2.5, and 02, 1.07 have been measured by more than one method [1-3]. CURES-EC calculations confirm these to within 0.1 eV. Positive excited states Ea have been measured for 02, C2, and I2 and are inferred for other X2 [5-8]. Just as in the case of the atomic Ea, the trends in the Periodic Table can support the assignments of AEa for the other elements. 

Observed interatomic distances for diatomic transition-element interactions are estimates of the fraction, d = 0.783 of nearest-neighbor approaches in the metals [5] and may be considerably in error in the present context, especially for the second transition series. Apart from first-order La2 and Ce2, with = 245 30kJmol homonuclear diatomics have weak interactions with an average Dx = 70 40kJmor in agreement with our estimates. Multiple bond orders, in general, are characterized by stepwise reduction of the first-order golden exponent, such that... 

Barden, C. J., Rienstra-Kiracofe, J. C., Schaefer, H. R (2000). Homonuclear 3d transition-metal diatomics A systematic density functional theory study. Journal of Chemical Physics, 113,... 

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