Low oxidation state transition metals

In low oxidation states, transition metals possess filled or partly filled d shells. The Dewar-Chatt-Duncanson model envisages some of that electron density in (local) d (e.g. d., d y) orbitals being donated into the empty n orbitals of the carbon monoxide ... 

Bonnesen PV, Puckett CL, Honeychuck RV, Hersh WH (1989) Catalysis of Diels-Alder reactions by low oxidation state transition-metal Lewis acids fact and fiction. J Am Chem Soc 111 6070-6081... 

Allardyce, C. S. Dyson, P. J. The interactions of low oxidation state transition metal clusters with DNA potential applications in cancer therapy. /. Cluster Sci. 12, 563-569 (2001). 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

V Oxidative addition reactions of coordinatively unsaturated, low oxidation state, transition metal complexes, including established radical reactions especially of Co (II) 89... 

Unlike conventional homogeneous catalysis involving relatively low oxidation state transition metals, the proposed mechanism involves successive... 

Like the related diphosphenes, diarsenes are potential donors towards low oxidation state transition metals via either the arsenic lone pairs of electrons or through the n bond itself. In two chromium pentacarbonyl complexes investigated, one of the arsenic atoms donates a lone pair of electrons and the double bond remains essentially intact, increasing marginally from 2.224 A in the uncomplexed ligand to 2.246 A after coordination - . 

Organoarsines, as mentioned previously, often fragment in reactions with low-oxidation-state transition metals and the fragments can be stabilized by coordination to the metal. Many examples are known but the following are representative. Complete cleavage of the As—As bonds in (PhAs) and (MeAs)j can occur to give arsinidine... 

The historical move to early transition metals in rather high oxidation states and the use of a- and 7r-donor ligands, contrary to what was observed in the late and low-oxidation-state transition metals, has been the preferred approach to achieve a real electron-transfer from the metal to the N2 ligand. In this context, two major strategies have been particularly successful, namely the use of the ancillary ligands mentioned above and the macrocyclic effect. 

We have already pointed out that under these conditions, the true catalytic species is Ru(CO)3(To1-B1AN).R In this complex, the metal atom should be much more basic than in Ru(CO)s or Ru3(CO)i2, thus facilitating electron transfer from the metal to the nitro group. Such a process is now emerging as a common feature of the interaction of organic nitro compounds with low oxidation-state transition metal complexes. ... 

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