Osmium allenylidene

In contrast to ruthenium and osmium, the reactivity of iron allenylidenes remains almost unexplored. Only the behavior of the cationic diphenylallenylidene-Fe(II) derivative frans-[FeBr(=C=C=CPh2)(depe)2]" has been studied in detail. Thus, it has been found that this complex reacts exclusively at Cy with both neutral (amines, phosphines) and anionic (H , MeO , CN ) nucleophiles [105-107]. This behavior contrasts with that of the neutral Fe(0) derivative [Fe =C=C=C(f-Bu)2 (CO)5] which undergoes PPhs-attack at Co- to afford the zwitterionic phosphonio-allenyl species [Fe C(PPh3)=C=C(f-Bu)2 (CO)5] [104]. 

Scheme 23 Coupling of an allenylidene ligand with a vinyl group at osmium...

Other synthetic methodologies have been used to prepare the unusual octahedral osmium(II) allenylidenes [OsH(=C=C=CPh2)(NCMe)2(PiPr3)2][BF4] and [Os k (C,0)-C(C02Me)=CH2 (=C=C=CPh2)C0)(PiPr3)2][BF4] from l,l-diphenyl-2-propyn-l-ol [22]. 

Cydoaddition reactions of allenylidene ligands with alkynes have also been described. Thus, heating a toluene solution of the neutral osmium complex 43 in the presence of dimethylacetylenedicarboxylate leads selectively to the allenyl-vinyli-dene 4S (Scheme 2.18) [31c[. The formal insertion of the alkyne into the Co =Cp... 

Mononuclear osmium half-sandwiches, with rf-cyclopentadienyls and 7]5-indenyls alkenyls and allenyls with t/-M-C bonds, 6, 558 alkenyl vinylidenes, 6, 593 alkyl, aryl, acyl complexes, 6, 552 with alkylidyne complexes, 6, 599 alkynyl and enynyl complexes, 6, 567 allenylidene and cumulenylidene complexes,... 

Some general reviews relating to the chemistry of Ru/Os-r hydrocarbon complexes appear in the literature the reactivity of Ru-H bonds with alkenes and alkynes/ aspects of ruthenium/osmium vinylidene/allenylidene/cumul-enylidene complexes,equilibria of M-R/M=CR2/M=CR complexes, the organometallic chemistry of metal porphyrin complexes, and the reactions of [Os(P Pr3)2(CO)HGl], ruthenium pyrazoly I borate complexes,and metallabenzynes. Other reviews relate more to applications of some of the complexes outlined in this chapter. See, for example, metal vinylidenes in catalysis,the development of Grubbs-type alkene metathesis catalysts, applications of ruthenium/osmium carbene complexes in metathesis polymerization, and the role of Ru /V-hetero-cyclic carbene complexes in metathesis polymerization. ... 

Not considered within this review are the wide range of porphyrin and other nitrogen macrocycle-stabilized ruthenium and osmium hydrocarbyl complexes. Readers should see the following as a starting point for information on this type of alkyl/aryl, alkynyl, vinylidene, allenylidene, and carbene complexes. 

Only a few examples have been obtained through the classical methodologies followed in group 6 metal chemistry. Most rf -Cs Fischer-type ruthenium and osmium carbenes arise from the nucleophilic additions of alcohol and amino groups at the electrophilic carbenic Ca-atom of both allenylidene and vinylidene complexes. The fate of the reaction depends on the electrophilicity as well as the steric hindrance around the Ca-atom, which can control its accessibility, especially for bulky nucleophiles. These features have been thoroughly discussed in a recent review. ... 

Fischer-type osmium alkoxycarbenes are scarce when compared to those of ruthenium. The addition of alcohols to the electrophilic Go -atom of vinylidene or allenylidene groups has also proved to be, as for the ruthenium complexes. 

All these 7 -Gp osmium carbynes have been obtained in almost quantitative yields and isolated as air-stable solids. These efficient synthetic routes prove the utility of vinylidene and allenylidene complexes as valuable starting materials, which are prone to undergo protonations even in cationic allenylidene precursors. 

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