Transition-metal-coordinated carbonyls complex hydrides

An attack of a nucleophilic transition metal hydride on coordinated CO has also been observed by Labinger and Wong, who treated carbonyls of Ct, Mo, W, Mn, Fe, Ku and Co with Cpi Nbllj. which is of a lower hydridic nature than the main group hydrides or group IV hydridic complexes (54]. Small amounts of C C3 hydrocarbons are formed. However, when the reaction of CpjNbH with Cr(CO)(( is carried out under H], only ethane t formed. Carbene intermediates are proposed to account for this selectivity (Scheme 14). 

Certain classical coordination complexes (see Coordination Complexes) of iron (e.g. Prussian blue) will be dealt with in other articles (see Iron Inorganic Coordination Chemistry and Cyanide Complexes of the Transition Metals), as will much of the chemistries of iron carbonyls (see Metal Carbonyls) and iron hydrides (see Hydrides) (see Carbonyl Complexes of the Transition Metals Transition Metal Carbonyls Infrared Spectra, and Hydride Complexes of the Transition Metals). The use of organoiron complexes as catalysts (see Catalysis) in organic transformations will be mentioned but will primarily be covered elsewhere (see Asymmetric Synthesis by Homogeneous Catalysis, and Organic Synthesis using Transition Metal Carbonyl Complexes). 

Asymmetric Synthesis by Homogeneous Catalysis Carbonylation Processes by Homogeneous Catalysis Coordination Organometalhc Chemistry Principles Electronic Stmcture of Clusters Hydride Complexes of the Transition Metals Hydrides Sohd State Transition Metal Complexes Organic Synthesis using Transition Metal Complexes Containing 7t-Bonded Ligands Oxidation... 

Deviating from the route via nucleophilic attack of the carbanion at the carbon atom of a CO ligand and then reaction of the acylmetallate with an electrophile are those methods which involve (a) addition of the carbanion to the carbon atom of a carbyne ligand, (b) displacement of halides from transition-metal carbonyl halides by cyclohepta-trienyllithium, or derivatives thereof, followed by hydride abstraction or (c) substitution of a coordinated solvent from a metal-carbonyl complex (see also reaction of LiR with carbene complexes). 

Trialkylborohydride reducing agents differ from borohydride in their ability to transfer hydride directly to a carbonyl ligand without prior substitution in the coordination sphere. They are used to synthesize formyl complexes " . When formyl complexes lose CO and undergo hydride migration from the formyl ligand to the metal, a transition-metal hydride results. The process is formally similar to nucleophilic attack by [OH] on a carbonyl ligand, followed by loss of CO and formation of a transition-metal hydride. Examples of hydride syntheses via formyl complexes are ... 

It has been shown that in a number of transition-metal hydrido complexes H atoms are directly bonded to M and occupy definite coordination positions (as in K2ReH9, HPtBr[P(C2Hs)3] 2, etc., for which see Chapter 27). This would also appear to be true in certain carbonyl hydrides, though the H atoms have been directly located by n.d. in only one form of Mn(CO)5H. In Mn(CO)5H five CO are situated at five of the vertices of an octahedron (Mn-C, 1 -84 A) and the H atom is situated at the sixth vertex, with Mn-H, 1-60 (C-0, M34 A). 

Silanes are widely recognized as efficient reagents for reduction of carbonyl and heterocarbonyl functionality. In the case of alkyl and arylsilanes, the reaction requires catalysis by Lewis acids or transition metal complexes 1, 3] however, with more Lewis acidic trichloro or trialkoxysilanes, an altemative metal free activation can be accomplished. Thus, it has been demonstrated that extracoordinate silicon hydrides, formed by the coordination of silanes to Lewis bases, such as tertiary amines 7a], DMF [7b] or MeCN, and so on [7], can serve as mild reagents for the reduction of imines to amines [8]. In the case of trichlorosilane, an inexpensive and relatively easy to handle reducing reagent, and DMF as a Lewis basic promoter, the intermediacy of hexacoordinate species has been confirmed by Si NMR spectro scopy [7b]. 

---