Ligand Carbonyls

Not all ligands use just two electrons to bond to transition metals Chromium has the electron configuration [Ar]4s 3rf (6 valence electrons) and needs 12 more to satisfy the 18 electron rule In the compound (benzene)tricarbonylchromium 6 of these 12 are the tt elec Irons of the benzene ring the remammg 6 are from the three carbonyl ligands... 

Imidazole with [Re(CO)3(phen)Cl] or [Re(CO)3(phen)(CF3S03)] in the presence of sulfuric acid gives [Re(CO)3(phen)(im)]2SO (95ICA(240)169). Imidazole with [Mri2(CO) g] gives [Mn2(CO)g(imH)] (84P707). This path involves the nucleophilic substitution of the carbonyl ligand. However, it is complicated by some redox... 

Bis(l-methylimidazol-2-yl)methane and -ketone with the dimer [Rh(CO)2Cl]2 in the presence of sodium tetraphenylborate give the dicarbonyl complexes 68 (X CHj, CO L = CO) where the carbonyl ligands may easily be substituted by the triphenyl phosphine ligands to yield 68 (X = CH, CO L = PPh ) (99JOM(588)69). The bis(l-methylbenzimidazol-2-yl)methane analogs of 68 (X=CH2 L=C0, PPhj) can be prepared similarly. 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

The formal [3+2+1]-cycloaddition involves an a,ft-unsaturated carbene ligand (C3-synthon),an alkyne (C2-synthon) and a carbonyl ligand (Cl-synthon) and takes place within the coordination sphere of the chromium(O), which acts as a metal template (Scheme 2). 

Scheme 54 Complementary benzannulation via photo activation of carbonyl ligands...

In a similar vein, we observe nickel(O), possessing ten electrons in its valence shell, to require four carbonyl ligands to satisfy the eighteen electron rule and form [Ni(CO)4l, whilst chromium(O), with six electrons in its valence shell forms [Cr(CO)6]. These latter compounds are tetrahedral and octahedral respectively. 

In—W bond. Use of Ph3Al leads to a complex in which the oxygen atom of a carbonyl ligand is the site of electron pair basicity in a WC=OAl link. Solutions of [n-Bu4N][Ph3GaCpW(CO)3] in CH2CI2 contain, in addition to free [CpW(CO)3], two isomeric complexes a metal-metal-bonded species and a C- and O-bonded adduct of the type found in the Ph3Al case. 

Kiindig EP, Dupre C, Bourdin B, Cunningham A Jr, Pons D (1994) New C2-chiral bidentate ligands bridging the gap between donor phosphine and acceptor carbonyl ligands. Helv Chim Acta 77 421 28... 

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