18-electron count, alkyne complexes

In addition, complexes like 11 are also capable of catalyzing [2 - - 2 - - 2] cycloadditions of alkyne moieties resulting in the formation of substituted benzenes. Furthermore, Fe(I) catalysts like 22 with an odd electron count (17-electron species) have been studied in this context (Fig. 12) and the initial results demonstrate that they are catalytically relevant, uncovering a previously largely unrecognized aspect. 

Nearby elements also display chemistry reflecting alkyne 7rx donation to a vacant metal dir orbital. In Group V, Lippard s coupled carbonyl product Ta(Me3SiOC=COSiMe3)(dmpe)2Cl is a d4 Ta(I) alkyne monomer (116), similar in electron count to CpV(CO)2(RC=CR) complexes (231). The preparative route and physical properties of a series of Ta(CO)2-(RC=CR)(I)L2 d4 monomers are compatible with a four-electron donor description for the alkyne ligands (231a). The d2 configuration has also... 

Carbyne ligands may bridge two (p) or three (p3) metal centres, providing a total of 3VE to the overall electron count. Earlier synthetic routes to such complexes involved the reactions of carbonyl metallates with 1,1,1-trihaloalkanes, or the cleavage of alkyne ligands coordinated to clusters (Figure 5.46). 

Tp NbI(CO)(PhC=CMe)(RC=N) (Scheme 47).621 The assigned formal electron counts on the alkyne and nitrile ligands are compatible with detailed 13C NMR data. Electrochemical studies indicated that an equilibrium existed in solution between the f72(3e)-alkyne/ 72(3e)-nitrile and 72(4e)-alkyne/771(2e)-nitrile complexes.50 PhCN is displaced by PMe2Ph or PhC=CMe to provide Tp Nb(CO)(PhC=CMe)(PMe2Ph) and Tp Nb(CO)(PhC=CMe)2, respectively. Protonation of these species can induce intramolecular redox coupling reactions to produce Nbv metallocycles (e.g., Scheme 47). 

The NLO properties of organometallic and coordination complexes are also rich (21, 184, 279-296). Metal-alkyne complexes were first reported 1960 (297) and have recently attracted significant interest because of their potential in materials applications (2, 298). Studies of these types (299) have resulted in the development of structure-NLO response relationships for quadratic optical nonlinearities (p-value), which increase with valence electron count and ease of oxidation of metal. The amplitude is also tunable by ancillary ligand modification and substitution. Select small alkynyl complexes have been shown to exhibit p values at 1064 nm > 2600 x 10 ° esu (299). 

For the metal—alkyne fragmait, nonlinearity also increases upon increasing valence electron count [14 valence electron (triphenylphosphine) gold alkynyl compounds <18 valence electron (cyclopentadienyl) (triphenylphosphine)nickel, and (cyclopentadienyl) bis(triphenylphosphine) ruthenium alkynyl compounds] and increasing ease of oxidation (less easily oxidizable (cyclopentadienyl)(tri-phenylphosphine)nickel alkynyl complexes < more easily oxidizable (cyclopen-tadienyl)bis(triphenylphosphine)ruthenium alkynyl complexes). 

Scheme 4-42) [134]. Werner s group has shown that alkynylsilanes also undergo 1,2-silyl migration in the same system via an intermediate 7i-complex [135]. Alternatively, the rearrangement may be catalyzed by base [136] or induced by sequential deprotonation/protona-tion [137]. The relative stability of the alkyne and vinylidene complexes is dependent on the electron density and the d-electron count of the metal, as illustrated by the behavior of the d -Mo complexes 84 in which the alkyne is a four-electron donor addition of CO causes the 84 85 conversion whereas tautomer 84 is favored with the phosphite ligand (Scheme 4-43)... 

A subtle example of variable electron donor properties is provided by alkynes which may be either 2- or 4-electron donors, with negligible change in coordination properties (Figure lO)." " Thus, in addition to the G-G 7r-bond in the [MG2] plane (tth) serving as a 2-electron cr-donor in a similar manner to that in metal-olefin complexes, the perpendicular 7r-bond of the alkyne (tt ) may also serve as donor, but in a tt rather than a sense. Thus, alkynes may be classified as 2-electron (a) or 4-electron (cr-f tt) donors. To facilitate electron counting, these interactions may be represented with single and double arrows, respectively. 

The [2+2] reactions of the zirconium-imido compounds with alkynes and alkenes occurs by a mechanism similar to that for the [2+2] reactions of carbenes with alkynes and alkenes. The alkene or alkyne first binds to an open coordination site at the metal, and this coordination is followed by conversion of the alkyne or alkene complex to the metallacyhc product (e.g. Equation 13.76). Thus, the [2+2] reaction requires a 16-electron intermediate to bind the olefin or alkyne, even though the metallacyHc product and the imido complex have the same overall electron count. In support of the coordination of alkyne or alkene, albeit weakly, to the d° metal center, the rate of the reaction of alkynes with the 18-electron zirconocene-imido compound containing bound pyridine-N-oxide was inhibited by added pyridine-N-oxide (Equation 13.76). ... 

When R = 2,6-Me2C H3—, tungstenacyclobutadiene complexes in which the metal atom adopts an approximately trigonal bipyramidal geometry, have been isolated from the reaction with 3-heptyne. X-ray crystallography shows that the four membered WC3 ring is essentially planar. On account of the low coordination number (five) and low electron count (12), vacant sites are available at the metal centre for attachment of alkyne molecules. These properties are consistent with the high activity of these catalysts. 

To Fischer carbenes correspond Fischer carbynes that are 18-electron complexes of electrophilic carbyne, and to Schrock carbenes correspond Schrock carbynes that are d° high-oxidation-state early transition-metal complexes of nucleophilic carbyne, often with a less-than-18-electron count. This latter category is important in alkyne metathesis, and all carbyne complexes show a rich chemistry. 

The cluster valence electron (c.v.e.) count usually corresponds to 12 + 22 electrons. Bonding of the C2 unit involves stabilization of a, a, and ji orbitals by interaction with radial metal MOs of the same symmetry, together with overlap of ji orbitals with filled metal MOs, i.e., a similar synergic interaction to the familiar bonding mode found in alkyne-metal complexes. For the model [Co8(C2)(n-L)(L)8]4 based on two trigonal... 

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