Iron cluster unit

FIGURE 22 The structure of PrgFeuPb. Praseodymium, iron, and lead atoms are drawn as medium gray, black filled, and open circles, respectively. The layers of edge-sharing PbPrio polyhedra are separated by the iron cluster units. For details see text. 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

Spin-forbidden nonadiabaticity, 32 25 transitions, 32 25-26 Spin Hamiltonian, 38 194 four-iron clusters, 38 459-460 matrix, 38 453 parameters, 38 447, 449 Spin interactions, heterobinuclear units, 43 186... 

The three-iron cluster [Fe3S(S2-o-xyl)3]2-, where S2-o-xyl = o-xylene-a,a-dithiolate, has a pyramidal [Fe3(ju3-S)]4+ central unit.809... 

The structural features of P FeisPb are similar to the stannide Th4Fei3Sri5 (Manfrinetti et al., 1997 Moze et al., 2000 Principi et al., 2001), where a two-dimensional Fei3 cluster unit (243-268 pm Fe-Fe) is separated by layers of condensed SnTIv, octahedra. In both iron-rich compounds, the iron sublattices give a magnetic contribution. 

A stable trinuclear iron cluster 188 in which an allenyl unit is bonded in a /x3—T73 fashion has been derived from compound la by transmetallation... 

There is a large body of knowledge on the coordination chemistry of iron-sulfur complexes and iron clusters particularly directed at the modeling and nnderstanding of the action of nonheme proteins whose active sites contain iron-sulfur units. These topics are discussed in detail elsewhere in this Encyclopedia here, the emphasis is on the basic coordination chemistry. 

Figure 1. Orbital energies (in Rydberg atomic units) for the high spin form of the cubane three-iron cluster. Solid lines show spin-up orbital energies, dashed lines show spin-down. Numbers beside each line give the percent iron d-character in that orbital. The two orbitals marked with an asterisk (of a and e symmetry) are the potential locations for the final electron. The plot corresponds to the state with the final electron in the a orbital marked with an asterisk.

Astruc and co-workers have also isolated dendrimers using a tetrairon cluster [ CpFe(p,3-CO) 4], 302.355 Dendrimer 303, with nine organoiron clusters at the peripheries, was prepared via the reaction of the tetrairon cluster with dendrimer 301 (Scheme 2.78). These types of dendrimers were air-stable, and electrochemistry studies showed that the iron clusters were separated enough from each other to prevent any electrostatic interaction. These dendrimers are potential exoreceptors, which are generally confined to dendrimers containing metallocene units.355... 

VU Fig. 28.18 The structures of the two types of cluster unit present in the nitrogenase molybdenum-iron protein isolated from Azotobacter vinelandii (a) the P-cluster in its reduced state and (b) the FeMo cofactor. Colour code Fe, green Mo, pale grey S, yellow C, grey N, blue O, red. Each non-terminated stick represents the connection of a coordinated amino acid to the protein backbone. 

Fe3(CO)i2] was expected, but whereas reactions of the iron cluster normally lead to cleavage of the trimeric unit, the ruthenium analog seems to give stable trinuclear species. This was correlated with an increase in the stability of metal-metal bonds on descending the transition metal triad. For instance, when R = Et, Bu" or -in the case of osmium, Ph, the reaction in benzene or toluene under reflux affords [M3(CO)io( -H)(//-SR)] (M = Ru, Os) complexes. The structure of [Os3(CO)io(//-H)(/i-SEt)], synthesized in octane under reflux, has been reported. ... 

Scheme 6 Some of the important iron-sulfur cluster units found in metalloenzymes. [2Fe-2S] rhombus cluster is characteristic of [2Fe-2S] feiredoxins and Rieskie proteins, [4Fe-4S] cubane -e.g., in [4Fe-4S] feiredoxins, aconitase [3Fe-4S] clusters are present in the inactive form of aconitase, [3Fe-4S] feiredoxins. The iron vertices, designated as [Fe], have high-spin tetrahedral FeS4 coordination. Reprinted from [140], with the permission of Elsevier, copyright 2000...

The thermal reaction of cis or trans 1,4-poly(butadiene) with iron carbonyls results in geometrical isomerization of the alkenyl moieties and formation of polymers containing conjugated diene) iron tricarbonyl units. In the course of our continuing studies of the formation of stable colloidal iron dispersions by thermal decomposition of metal carbonyls in the presence of functional polymers, some aspects of this work have been repeated. Our objective was to isolate the soluble organometallic polymer which was intermediate to particle nucleation and independently examine the intramolecular condensation of metal atoms to yield metal clusters and metal particles. In this paper, the structure of the intermediate obtained on thermolysis of an excess of FeCCO) in a dilute xylene solution of c/5-poly(butadiene) has been described. 

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