Lower oxidation states organometallic compounds

Organometallic Compounds. Ruthenium, predominately in the oxidation states 0 and +2, forms numerous mononuclear and polynuclear organometaUic compounds. A few examples of compounds in both higher and lower oxidation states also exist. The chemistry of polynuclear mthenium complexes is extensive and has been reviewed (53—59). 

Organometallic Compounds. Osmium forms numerous mononuclear and polynuclear organometaUic complexes, primarily iu lower oxidation states. There are many complexes of carbon monoxide, such as [Os(CO)3] [16406-49-8], [Os(CO) H2] [22372-70-9], [Os3(CO)2 H2] [56398-24-4],... 

Complexes in which the metal exhibits still lower oxidation states (such as I, 0, —I, —II) occur amongst the organometallic compounds (pp. 1006 and 1037). 

The organometallic chemistry of aluminum is dominated by the chemistry of aluminum(lll), but lower oxidation state compounds are now accessible. The first examples of this class of compounds are carbonyl complexes such as Al(CO), A1(C0)2, and Al3(CO), which were generated upon exposure of aluminum atoms to CO in matrix-isolation experiments near 20 K. The number, relative intensities, and frequency of the carbonyl stretches in the IR spectra, along with isotopic labeling and EPR studies were used to verify these compositions. These complexes exhibit vco values of 1868, 1985 and 1904, and 1715 cm , respectively, indicative of Al- CO 7t backbonding. The carbonyl species are unstable at higher temperatures and no stable carbonyl complex of aluminum, in any oxidation state, has been reported. The monomeric aluminum-alkene adducts A1( -C2H4) and k rf-CeHe) were similarly identified in inert matrices at low temperature. No room-stable alkene complexes of aluminum have been reported. 

Much new information about compounds of the lower oxidation states of uranium and thorium has accumulated during the last decade, and the rapid development of the organometallic chemistry of these elements has enhanced interest in this topic. 

Therefore 4d and 5d electron metals interact with ligands in a more effective manner and thus form more covalent compounds. Because of valence orbital energy and orbital sizes, compounds of these elements in their lower oxidation states, particularly organometallic ones, are more stable than analogous complexes of M electron metals. The increased stability of olefin and acetylene compounds with increasing atomic number in a given group may serve as an example. Olefin complexes of cobalt are few and very unstable, while rhodium and iridium olefin compounds are quite common and usually air-stable. 

Transition metal ions can bind ligands (L) to give a coordination compound, or complex ML , as in the familiar aqua ions [M(OH2)6] (M = V, Cr, Mn, Fe, Co, or Ni). Organometallic chemistry is a subfield of coordination chemistry in which the complex contains an M—C or M—H bond [e.g., Mo(CO)6]. Organometallic species tend to be more covalent, and the metal is often more reduced, than in other coordination compounds. Typical ligands that usually bind to metals in their lower oxidation states are CO, alkenes, and arenes, for example, Mo(CO)g, (C6H6)Cr(CO)3, or Pt(C2H4)3-... 

The number of organometallic compounds with a double metal-metal bond is notoriously higher than those with higher bond orders. This feature agrees with the relatively lower oxidation states of the metal in these compounds and it also points out to the idea of a transition toward the behavior observed in cluster compounds. 

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