Metal carbonyl complexes,

Metal carbonyl complexes are an interesting series of coordination compounds in which the ligands are CO molecules, and in many cases the metals are present in a zero oxidation state. In these complexes, both the metal and ligand are soft according to the Lewis acid-base definitions. Although the discussion at first will be limited to the binary compounds containing only metal and CO, many mixed complexes are known that contain both CO and other ligands. 

Metal carbonyls form several complexes but those of major interest in MOCVD are the carbonyl halides and the carbonyl-nitric-oxide complexes. 

The carbonyl halides have the general formula Mx(C0)yX2 with X being fluorine, chlorine, bromine, or iodine. With the notable exception of nickel, most metals form carbonyl halides including all the metals listed in Table 3.4. 

A particularly interesting case is that of the platinum metal group which, in addition to platinum (Pt), comprises ruthenium (Ru), osmium (Os), rhodium (Rh), iridium (Ir), and palladium (Pd). These carbonyl halides are usually the most practical precursors for metal deposition because of their high volatility at low temperature. Indeed two of them, palladium and platinum, do not form carbonyls but only carbonyl halides. So does gold. 

The hydrogen reduction of the metal halides, described in Sec. 1.2, is generally the favored reaction for metal deposition but is not suitable for the platinum-group metals since the volatilization and decomposition temperatures of their halides are too close to provide efficient vapor transport. 1 1 For that reason, the decomposition of the carbonyl halide is preferred. The exception is palladium which is much more readily deposited by hydrogen reduction than by the carbonyl-halide decomposition. 

Preparation of Carbonyl Halides. The carbonyl halides are generally produced by the direct reaction of the metal halide and CO, usually at high pressure and at the temperature range of 140-290°C. A typical reaction is 1 1 

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Conjugated dienes, upon complexation with metal carbonyl complexes, are activated for Friedel-Crafts acylation reaction at the akyhc position. Such reactions are increasingly being used in the stereoselective synthesis of acylated dienes. Friedel-Crafts acetylation of... 

The nature of the bonding, particularly in CO, has excited much attention because of the unusual coordination number (1) and oxidation state (-f2) of carbon it is discussed on p. 926 in connection with the formation of metal-carbonyl complexes. 

It has been observed, however, that the enantioselectivity of reactions of tartrate ester modified allylboronates with metal carbonyl complexes of unsaturated aldehydes are significantly improved compared with the results with the metal-free, uncomplexed aldehydes72. Two such examples involve the (benzaldehyde)tricarbonylchromium complex and the hexacarbonyl(2-... 

Reagent and catalyst induced substitution reactions of metal carbonyl complexes. M. O. Albers and N. J. Coville, Coord. Chem. Rev., 1984, 53, 227-259 (153). 

Pseudo-allyl metal carbonyl complexes. T. Inglis, Inorg. Chim. Acta, Rev., 1973, 7, 35-42 (43). 

Metal carbonyl complexes with ligands of biological origin. A. A. Ioganson, Russ. Chem. Rev. (Engl. Transl), 1985,54, 277 (124). 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Carbonyl Nitric Oxides. Another group of metal-carbonyl complexes, worthy of investigation as CVD precursors, consists of the carbonyl nitric oxides. In these complexes, one (or more) CO group is replaced by NO. An example is cobalt nitrosyl tricarbonyl, CoNO(CO)3, which is a preferred precursor for the CVD of cobalt. It is a liquid with a boiling point of 78.6°C which decomposes at 66°C. It is prepared by passing NO through an aqueous solution of cobalt nitrate and potassium cyanide and potassium hydroxide. ... 

The reactions of nucleophilic reagents with cationic and uncharged metal carbonyl complexes have received much attention in the past, and it is not surprising that these studies have now been extended to isocyanide metal complexes. Different products in these reactions can arise by three general routes these include ligand substitution, reactions involving attack at a ligand, and reduction of the metal complex. All have been observed in reactions with metal isocyanide complexes. 

N.m.r. studies are reported on the triethylphosphine and trisdi-methylaminophosphine complexes with boron halides, and triethylphos-phine complexes with aluminium chloride. A correlation of Sp with the number of phosphorus ligands in metal carbonyl complexes has led to a qualitative rationalization of 8p in terms of a- and 7r-bonding. ... 

Uson, R., Laguna, A., Abad, J.A. and Dejesus, E. (1983) Preparation Of Monomeric Neutral Or Anionic Tris (Polyfluorophenyl)-Thalbum(III) And Of Anionic Heteronuclear Tris (Polyfluorophenyl)-Thallium Metal-Carbonyl-Complexes. Journal of the Chemical Society, Dalton Transactions, (6), 1127-1130. 

Figure 17 Some examples of metal carbonyl complexes with M-Ag bonds.

Mathur, Pradeep, Chalcogen-Bridged Metal-Carbonyl Complexes. 41 243... 

Therefore bonding in PdSiO may be compared to that in typical transition metal carbonyl complexes. 

While the chemistry of metal carbonyl complexes has enjoyed a rather long and colorful history, being extensively studied and widely reviewed (7-3), the synthesis and reactivity of the group 4B (Ti, Zr, Hf) metal carbonyls have developed relatively slowly. Although the first well-characterized group 4B metal carbonyl complex, bis(i7-cyclopentadienyl)-dicarbonyltitanium (1), was reported by Murray of Monsanto Co. in... 

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