Bridging alkylidene and alkylidyne

COMPOUNDS WITH BRIDGING ALKYLIDENE AND ALKYLIDYNE LIGANDS. 

The isolobal connections of carbynes W(C0)2(CR)Cp with alkynes and carbene Cr[C(OMe)Ph](CO)3 with alkenes are reviewed.They react with late transition metal centres forming heterobimetallic bridging alkylidene and alkylidyne derivatives, there are many such examples and their chemistry towards species such as nucleophiles protons, alkynes, and metal fragment precursors is prolific. 

Finally, an alkyl group can also occasionally bridge two or three metals in various modes. Metal-alkyl complexes are examined in this chapter, terminal and bridging alkylidene and alkylidyne complexes being studied in the following chapter. 

Just as alkylidenes and alkylidynes may bridge M-M bonds, vinyli-denes may also be employed to support metal-metal bonds. This approach has included the addition of preformed terminal vinylidene complexes to unsaturated metal complexes or, occasionally, the direct assembly from terminal alkynes with unsaturated metal complex precursors (Figure 5.50). 

In some cases, hydrogenation of the alkylidenes and alkylidynes reduces the metal-carbon multiple bonds to single bonds. The alkyhdene hgand in (29) is converted to an alkyl gronp when exposed to H2, leading to the formation of an interesting tantalum hthium bridging hydride complex. 

The general synthetic approaches for expanded porphycenes rely upon the insertion of alkylidene or alkylidyne moieties either as the meso-bridge or between the 2 and 2 positions of the bipyrrole subunits. Thus, the vinylogous dialdehyde 72 (Scheme 35) obtained from 4,4 -dipropyl-2,2 -bipyrrole with 3-(dimethyl-l-amino)acrolein has been converted to (1993PAC143) to 73 using a standard McMurry-type coupling. Expanded porphycene 73 with TFA precipitates as dication 74. 

The generation and interconversion of hydrocarbon fragments on metal surfaces is an important aspect of transition metal catalysis. In an effort to model and understand these transformations, much attention has been focused on the synthesis and reactivity of organic species coordinated at polynuclear transition metal centers. Organodiruthenium complexes have provided a particularly rich area of study. The availability of a variety of organometallic derivatives of the bis(T) -cyclopentadienyl)diruthenium carbonyl system has allowed extensive examination of the reactivity of bridging alkylidene, alkylidyne, and ethenylidene ligands. 

The bond between tetrahedral carbon and transition metals mainly exists in the form of terminal metal-alkyl complexes such as [WCCHs) ]. - Metal-alkyl complexes are now numerous. It has been understood that they are stable if the p-elimination path is blocked either because the complexes have 18 valence electrons, - or have less valence electrons, but no p hydrogen. - Complexes of other tetrahedral carbon are also known in which two, three or four metal substituents are metal fragments. They are respectively bridging alkylidene, alkylidyne and carbide complexes, below ... 

We referred earlier to the significance of reactions at the alkylidyne carbon atoms of the dimetal species. Our studies in this area are in a preliminary stage, but Schemes 1 and 2 summarise some chemistry at the bridged carbon centres for the compounds (1 ) and (3,)(12). It will be noted that protonation of the neutral bridged al ylidyne compounds yields cationic alkylidene species in which one C—C bond of the tolyl group is n2 co-ordinated to tungsten, a feature revealed by both n.m.r. and X-ray diffraction studies. 

Bridging alkylidyne (see Alkylidyne) complexes (28a-b) (Section 4.1) react with TMEDA, leading to C-H activation of the methyl group in -NMe2 and the elimination of ZnCb, giving the alkylidene (92a-b)." hii ... 

Addition of 2 eq HC1 to (22) in ether produced a thermally unstable bis(alkylidene) complex, which upon addition of excess PMe3 generated binuclear (41). The bridging alkylidyne ligands of (41) exhibited inequivalent Ta—C bond lengths of 2.073(6)-2.099(6) A and 1.922(6)-1.942(7) A, consistent with the different nature of the Ta centers.73,74... 

Scheme 67. General formation of alkylidyne- and alkylidene-bridged M-Pt bimetallic complexes (L = PMe3, R = Me, C6H4Me-4, Tp = Tp, pzTp).

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