Osmium alkylidyne complexes

Sullivan reported the photophysical properties of the luminescent osmium alkylidyne complex... 

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,... 

Osmium carbyne (see Carbyne Complexes) or aUcylidyne complexes have a triple bond between the metal and the carbon atom of the ligand. Carbyne complexes are related to singlet carbenes. They are analogous to linear nitrosyl (see Nitrosyl Complexes) complexes and the osmium is usually in a lower oxidation state. Alkylidyne complexes are related to triplet carbenes and the bonding between the osmium and the carbon atom is similar to the C-C bond in an alkyne. 

Given the isoelectronic relationship between [CR] and [NO] and the ubiquity of this latter ligand in the coordination chemistry of later transition metals, the scarcity of mononuclear alkylidyne complexes of metals from groups 8-10 is surprising [1-4]. Isolated examples have been reported for iron [5], cobalt [6], ruthenium [4,7], osmium [4,8-9] and iridium [10]. Most of the examples known employ routes with extensive precedent in early transition metal systems, i.e., either electrophilic attack at the p-atom of a hetero carbonyl (CS [5], CTe [4], or C=CH2 [10]) or the Lewis-acid assisted abstraction of an alkoxide group from a carbene precursor [5] (Scheme 1). The one approach which is, too date, peculiar to group 8 metals involves reduction of a divalent dichlorocarbene complex by lithium aryls [4]. The limitation of this procedure to ruthenium and osmium is presumably not a feature of these metals but rather a result of the present lack of synthetic routes to suitable dihalocarbene precursor complexes of earlier metals. 

More recentiy, Ruiz et al. have developed the synthesis of the ruthenium(II) complex 28 [154]. This makes use of an appropriate combination of previous strategies. The ruthenium atom is stereogenic and this leads to a mixture of dia-stereoisomers, which are eight times more active than cisplatin on T47D breast cancer cell line. This approach can be extended to include complexes of rhodium and iridium [155], and other variants of the method have been used with ruthenium [156]. Alkylidyne and alkylidene derivatives of osmium, exemplified by molecule 29, have been reported [157], but as yet they have not been evaluated biologically. 

There will be circumstances other than those I have described here in which "high oxidation state" organometallic chemistry of rhenium in a catalytic reaction will be viable, although it is becoming clear that the balance necessary to achieve this feat is more difficult to maintain as one moves to the right in the transition metal series, and that some of the d rhenium chemistry in fact may look like chemistry of dP osmium species. On this basis it would seem unlikely that the principles that have been used to prepare Re(VII) alkylidyne and alkylidene complexes (a hydrogen migration reactions) can be extended further (to technetium, or especially osmium or ruthenium), at least in a routine fashion. 

---