Stable homoleptic metal alkyls

TJ ecently there has been much interest in the synthesis, structure, char-acterization, and thermal decomposition of unusual stable homoleptic metal alkyls MR (I). (The term homoleptic is used to describe a metal complex in which all the ligands are identical (2), i.e. TiR4 and Sn(NR 2)2 are examples of homoleptic metal alkyls and dialkylamides whereas Ti(Cl)R3 and Sn(NR 2)R are heteroleptic compounds. ) There are three classes (2) of these complexes (a) transition metal compounds, (b) diamagnetic lower valent main group element derivatives such as the... 

Table I. Stable Homoleptic Metal Alkyls, MRh of Neopentyl Type...

Iron, as found in the porphyrin derivative hemoglobin, complexes CO to form a stable metal carbonyl. Iron also forms a variety of metal carbon monoxide derivatives such as the homoleptic Fe(CO)5, Fe2(CO)9 and Fe3(CO)i2, the anionic [Fe(CO)4] and its covalent derivative Fe(CO)4Br2, [CpFe(CO)2] and its alkylated covalent derivatives CpFe(CO)2-R with its readily distinguished n (and and a (and / ) iron carbon bonds. By contrast. Mg in its chlorin derivative chlorophyll, which very much resembles porphyrin, forms no such bonds with CO nor is there a rich magnesium carbonyl chemistry (if indeed, there is any at all). 

Osmium forms a wide variety of alkyl and aryl complexes including homoleptic alkyl and aryl complexes and many complexes with ancillary carbonyl (see Carbonyl Complexes of the Transition Metals), cyclopentadienyl (see Cyclopenta-dienyl), arene (see Arene Complexes), and alkene ligands (see Alkene Complexes). It forms stronger bonds to carbon and other ligands than do the lighter elements of the triad. Because of this, most reactions of alkyl and aryl osmium complexes are slower than the reactions of the corresponding ruthenium complexes. However, because osmium is more stable in higher oxidation states, the oxidative addition (see Oxidative Addition) of C-H bonds is favored for osmium complexes. The rate of oxidative addition reactions decreases in the order Os > Ru Fe. 

The transition metal homoleptic complexes containing alkyl ligands are kinetically and thermally less stable than analogous compounds of the main group metals. For some time, it was believed that this is a result of the diminished strength of the carbon-transition metal bond. However, thermodynamic studies showed that the M —C(sp ) bond energy for the transition metals and the main group metals is the same (Table 4.2). 

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