Carbonyl group, metal-bonded

When a carbonyl group is bonded to a substituent group that can potentially depart as a Lewis base, addition of a nucleophile to the carbonyl carbon leads to elimination and the regeneration of a carbon-oxygen double bond. Esters undergo hydrolysis with alkali hydroxides to form alkali metal salts of carboxylic acids and alcohols. Amides undergo hydrolysis with mineral acids to form carboxylic acids and amine salts. Carbamates undergo alkaline hydrolysis to form amines, carbon dioxide, and alcohols. 

The infrared spectrum of Rh6(CO)i6 has sharp bands at 2075 (s), 2025 (m), and 1800 (s) cm i in Nujol due to v (CO). The crystal-structure determination has established that there are twelve terminal carbonyl groups and four bridging carbonyl groups, each of which bridges three metal atoms. These four bridging carbonyl groups are bonded to alternate faces of the octahedron of rhodium atoms and give rise to v(CO) at 1800 cm-. ... 

Lewis bases such as R3N, RC=N, H2O, and R3P are also two-electron donors, as are carbon monoxide ( C=0) and the isoelectronic isocyanides ( C=NR). (When CO is attached to a metal it is called a carbonyl group.) The bond between a Lewis basic ligand and the Lewis acidic metal is sometimes called a dative bond. 

C. The structure, which involves two bridging carbonyl groups as shown in Fig. 26.8a, can perhaps be most easily rationalized on the basis of a bent Co-Co bond arising from overlap of angled metal orbitals (d sp hybrids). However, in solution this structure is in equilibrium with a second form (Fig. 26.8b) which has no bridging carbonyls and is held together solely by a Co-Co bond. 

Formation of C-C Bonds by Addition of Metalated Sulfoxides or Sulfoximides to Carbonyl Groups... 

The transition metal catalysed addition of a hydridosilane to a multiply-bonded system is known as hydrosilylation (1). Under such conditions, alkynes undergo clear cis-addition, so providing one of the most direct routes to vinylsilanes (Chapter 3). Hydridosilanes also add to the carbonyl group of saturated aldehydes and ketones, to produce alkyl silyl ethers. Fot example, under suitable conditions, 4-t-butylcyclohexanone (2) can be reduced with a high degree of stereoselectivity. 

The lobes of electron density outside the C-O vector thus offer cr-donor lone-pair character. Surprisingly, carbon monoxide does not form particularly stable complexes with BF3 or with main group metals such as potassium or magnesium. Yet transition-metal complexes with carbon monoxide are known by the thousand. In all cases, the CO ligands are bound to the metal through the carbon atom and the complexes are called carbonyls. Furthermore, the metals occur most usually in low formal oxidation states. Dewar, Chatt and Duncanson have described a bonding scheme for the metal - CO interaction that successfully accounts for the formation and properties of these transition-metal carbonyls. 

Among the compounds that form complexes with silver and other metals are benzene (represented as in 9) and cyclooctatetraene. When the metal involved has a coordination number >1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these eomplexes are called carbonyl groups. Thus, benzene-chromium tricarbonyl (10) is a stable compound. Three arrows are shown, since all three aromatic bonding orbitals contribute some electron density to the metal. Metallocenes (p. 53) may be considered a special case of this type of complex, although the bonding in metallocenes is much stronger. 

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