Vacant coordination sites

Figure 11.11 Schematic representation of the bonding in NO complexes. Note that bending would withdraw an electron-pair from the metal centre to the N atom thus creating a vacant coordination site this may be a significant factor in the catalytic activity of such complexes. ...

The active form of a Ziegler-Natta catalyst is an alkyltitanium intermediate with a vacant coordination site on the metal. Coordination of alkene monomer... 

The wide diversity of cocatalysts and transition metal complexes suggests that the oxidation state of the transition metal is not a critical parameter. More important seems the availability of vacant coordination sites. In agreement with this, in the case of heterogeneous systems also,... 

Some data allow the realization of the second possibility to be proposed (8, 140) at least in the case of catalysts formed with the use of 7r-allylic compounds of chromium. In the reaction of Cr(7r-C3H3)2 with the Si02 surface, complexes are formed that may have vacant coordination sites... 

The Formation of Ions with Vacant Coordination Sites... 

It should be noted that, similarly to olefin, the insertion of carbon monoxide in the active bond in the propagation centers of polymerization catalysts also follows the coordination mechanism 175). The insertion of carbon monoxide into the active bond was not feasible when a vacant coordination site of the metal ion had been occupied by phosphine. 

The components of the Ziegler-Natta catalyst unite in such a way that there is a vacant co-ordination site on the transition metal to which a molecule of monomer can bond, a second molecule of monomer then attaches itself to another vacant coordination site, from which it is able to react with the first molecule of coordinated monomer. This causes the second coordination site to become vacant once again, thereby allowing a further molecule... 

It is to be noted that solvent effect on the reaction is rather small e.g., a 12-fold increase in rate was observed on going from THF to dimethyl-formamide (Table V, reactions 2 and 4). Addition of pyridine or PPhj, which may take up the vacant coordination site in the acyl, slows down the reaction by less than a factor of 2 (Table V, reaction 5). Furthermore,... 

Ancillary Hgands, the role of which is to stabiHze complexes or nanoparticles and Hberate a vacant coordination site when necessary... 

It has been noted (84) that U-dioxygen (type 1) complexes formed in solution often contain a u-hydroxo bridge, in addition to the /i-dioxygen bridge, provided that each cobalt atom has a vacant coordination site. Fig. 4 illustrates the dibridged M-(02, OH)—M moitey... 

A trinuclear system, whose structure was published,555 was formed in high yield by the cyclometalation of a l,3-bis(alkylbenzimidazol-2-yl)benzene ligand, followed by the trimerization of the unit, which possesses a vacant coordination site on the palladium atom and a pendant noncoordinated benzimidazole moiety. The resulting product forms a cup-shaped trimer or tricorn, presented in Figure 46, that includes a molecule of acetonitrile in the crystal structure.556... 

A number of examples have been reported documenting the use of palladium phosphine complexes as catalysts. The dialkyl species [PtL2R2] (L2 = dmpe, dppe, (PMe3)2 R = Me, CH2SiMe3) catalyze the reaction of [PhNH3]+ with activated alkenes (acrylonitrile, methyl acrylate, acrolein).176 Unfunctionalized alkenes prove unreactive. The reaction mechanism is believed to proceed via protonation of Pt-R by the ammonium salt (generating PhNH2 in turn) and the subsequent release of alkane to afford a vacant coordination site on the metal. Coordination of alkene then allows access into route A of the mechanism shown in Scheme 34. Protonation is also... 

The reduction of C02 can be driven electrochemically at metallic cathodes, however, it requires large overpotentials (<—1.5 V) and electrode poisoning often occurs.65 Those problems can be addressed by adding catalysts. Metal complexes are a priori good candidates as electrocatalysts. It is expected that their reduction will be accompanied by the appearance of a vacant coordination site able to bind C02 and thus activate its reduction in the metal coordination sphere.1... 

BLM transport systems for ferrioxamine B were also devised based on first coordination shell recognition via ternary complex formation utilizing vacant coordination sites on the Fe(III) center (Fig. 29) (199). The tetra-coordinated substrate complex selectively transported was partially dechelated diaqua-ferrioxamine B and coordinately unsaturated di-hydroxamato iron(III) complexes, which utilized a hydrophobic membrane bound bidentate chelator as a carrier for selective transport. Active transport for these systems was accomplished using a pH gradient (199). 

We would like at this time to amplify the earlier brief comments on the catalytic decomposition of formate ion to H2 and C02 by the group 6b metal carbonyls. This process requires the presence of a vacant coordination site on the metal for formate binding, i.e., formation of m(C0)s02CH species. Consequently, the reaction of Cr(Co)0 with formate ion in 2-ethoxyetha-nol was found to take place under more rigorous conditions than those needed for the production of H2 in the Cr(C0)e/K0H system. That is, whereas Cr(C07e in aqueous K0H/2-ethoxyethanol commences to produce H2 at 50°C (where the rds appears to succeed the formation of Cr(C0)sH ), the analogous Cr(CO)e/formate ion reaction necessitates temperatures approaching 100°C. 

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