Metal alkynyls, bridged

The number of X-ray crystal structures of terminal metal-alkynyl complexes has increased ten-fold since the last review 13 years ago (7). Metrical data for the M—C C—R fragments of terminal metal-alkynyl complexes are set out in Table I 34-151). Bridging alkynyl complexes of the type L M—C=C—ML (and higher L M——ML analogs) are not included in this survey these complexes have been the subject of recent reviews (4b, 10). 

Alkynyl-bridged dinuclear zirconocene(m) compounds such as 22-24 (and 30 below) exhibit dynamic behavior and rapidly exchange alkynyl ligands between the two metal centers, in a process that can be monitored by variable-temperature NMR.27,32... 

For the time being, the best model system to exemplify the particular type of alkynyl bridging may be the aluminium compound (CH3)2AIC2CH3 which is dimeric both in the crystalline and in the gaseous state (39). The Av(CEC)-values known for this main group metal alkynyl system match nicely those listed in Table VII. 

Heterobimetallic and polymetallic structures featuring bridging acet-ylides can also be prepared from the respective Cu(I), [or Ag(I)], and Re(I) substmctures described above. Excitation of both the penta- (224, 230) and decanuclear (203) mixed-metal alkynyl complexes in the solid state and in solution at X > 400 nm results in strong red luminescence (680-700 nm) characteristic of predominantly [i Ti(Re) ti (NN)] MLCT excitation mixed with ti(C=C) —> 7t (NN) LLCT perturbed by the Mg or Mg cores of the components. The MLCT state energy in the absorption spectrum for M = Ag(I) is shghtly blue-shifted relative to M = Cu(I) due to the poorer donating ability of the Agg versus Cug core, which causes the Re(I) center of the [Re(CO)g(NN)]" fragment to be less electron rich. 

The efficiencies for these metal alkynyl compounds are dependent on the nature of the bridging alkynyl ligand where CeUj < CeUjCeUi < CCeH4 < (E)-... 

The aryl- and alkyl-transition metal bonds are thermodynamically less stable than the alkynyl-metal bond [223]. The number of intermolecular transmetala-tion reactions of alkynyl transition metal complexes, however, is much larger than those of alkyl and aryl complexes as shown above. The stable bridging coordination of alkynyl ligands of the intermediate dinuclear complexes with a 0,7t-coordination probably makes the transmetalation, which involves activation of the thermodynamically stable metal-alkynyl bond, kinetically favorable. 

Two analogous dimeric palladium(l) complexes have been reported where an alkynyl moiety binds both palladium atoms, interacting <7-bonded to one of the metals and in an 77 -fashion with the other. Equation (25) shows the preparation by a comproportionation reaction of the allyl, alkynyl-bridged complex.A butadiene alkynyl complex has been prepared from a bis-butadiene Pd(i) dimer and a terminal alkyne. 

B. Fluorine- and Alkynyl-Bridged Macrocyclic Metal Complexes... 

If the metals M and M are competitive in their affinities towards the alkynyl ligands RC=C, or if steric factors prevent the formation of type B tweezer complexes, then structural type C or D species are formed. In these molecules, the ligands RC C are more evenly shared, either with both metals, p-bridged by both alkynyl ligands (type C) or with each metal a-bonded by one alkynyl unit and rr-coordinated to the other (type D) [14]. 

Fluorine- and alkynyl-bridged macrocydic metal complexes... 

Alkynyl-bridged macrocycUc metal complexes, 704 a-phase, 110 Alternate layers, 731. 751 Alternating Heisenberg model, 786 Amphiphilic molecule, 728 Analogues of TCNQ, 229. 245 Aniline linked fullerenes, 444 Anisotropic conductivity, 758 Anisotropy, 100. 113. 667 Anodic intercalation, 651 of graphite, 653... 

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