Transition metals dimers

Two elasses of systems illustrate eases for whieh heterolytie bond dissoeiation lies lower than the homolytie produets. The first involves transition metal dimer eations, M2. Espeeially for metals to the right side of the periodie table, sueh eations ean be eonsidered to have ground-state eleetron eonfigurations with a d d +i eharaeter, where the d eleetrons are not heavily involved in the bonding and the a bond is formed primarily from the metal atom s orbitals. If the a bond is homolytieally broken, one forms X + Y = M (s d +i)... 

In an earlier work, we have proposed a theoretical procedure for the spectroscopy of antiferromagnetically (AF) coupled transition-metal dimers and have successfully applied this approach to the electronic absorption spectrum of model 2-Fe ferredoxin. In this work we apply this same procedure to the [Fe2in - 82) P o - CeH48)2)2 complex in order to better understand the electronic structure of this compound. As in our previous work" we base our analysis on the Intermediate Neglect of the Differential Overlap model parameterized for spectroscopy (INDO/S), utilizing a procedure outlined in detail in Reference 4. 

Ortiz, G. and Ballone, P. (1991) Metastability of doubly charged transition-metal dimers in density-functional theory. Physical Review B -Condensed Matter, 44, 5881-5884. 

Only a single band maximum was observed for Cr2(CH3)g in solution from 230 to 2000 nm. This 22,000 cm 1 band was assigned to the 5- 8 transition, with the relatively large gap evident here resulting from metal-metal overlap at a metal-metal separation of 1.980(5) A in this first-row transition metal dimer (180). 

Hodgson, D. J., The Structural and Magnetic Properties of First-Row Transition Metal Dimers Containing Hydroxo, Substituted Hydroxo, and Halogen... 

Fig. 10. Structures of [Cp Mn(CO)2]2( /r-Te) (28) and [(MeCp)Mn(CO)2]2( m-CH2) (29) from X-ray structures, showing a series of transition metal dimers linked by isoelectronic main group element fragments (63b).

TABLE I. Selected Properties of the First-Row Transition Metal Dimers... 

A summary of the bond dissociation energies of first-row transition metal dimers is given by Cooper et al. in ref. 17, based mainly on the work of A. Kant ( 7) A more up-to-date summary has been prepared by K.A. Gingerich and is referred to in his contribution to this volume.)... 

This proposal ignited a worldwide effort by transition metal dimer and cluster chemists to show that polymetallic systems could be effective homogeneous catalysts. Pittman and... 

Particular areas of interest are the chemistry early-late metal-metal bonded complexes, main group metal-transition metal dimers, transition metal carborane derivatives, and lanthanide and actinide transition metal complexes. ... 

Noodleman, L. (1981) Valence bond description of antiferromagnetic coupling in transition metal dimers. Journal of Chemical Physics, 74, 5737. 

The LCGTO-Xa approach described so far has been successfully applied to a large variety of systems, including main group molecules (50,52,53), transition metal compounds, e.g. carbonyl complexes (27,28,55,56) and ferrocene (57), and a number of transition metal dimers (47). Besides these investigations on ground state properties useful information has also been obtained for selected problems involving excited states (52), such as the photolysis of Ni(CO)4 (58,59) and localized excitons in alkali halides (60) and in other ionic crystals ( ). 

Cuenoud B, Schepartz A. Altered specificity of DNA-binding 190. proteins with transition metal dimerization domains. Science 1993 259 510-513. 

A number of chemiluminescent studies of chemiluminescent multicenter reactions of transition metal dimers and trimers have been reported [117], namely... 

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. 

Calculations and experimental studies of the transition-metal dimers have previously been reviewed by Weltner and Van Zee and by Shim . The present section may be considered as a commented update of those reviews. Two very recent reviews should also be mentioned. While I have tried to make the reference list complete, I will limit my comments for the most part to the most accurate available conventional ab initio or density functional calculations. 

Experimental spectroscopic constants of transition-metal dimers. Adapted from Refs 189 and 190. 

See Refs 118, 273, 283, 293 and 294.) The earliest ab initio calculations for Mn2, or for that matter for any transition-metal dimer, were performed by Nesbet in 1964. He calculated a state at the RHF... 

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