Complexes, alkyne-metal electron donor-acceptor

The isomerization, itself, originates from the a complex (B in Figure 3). However the total activation energy depends critically on the relative energy of A and B (Figure 3). An alkyne C=C triple bond binds more efficiently to a transition metal complex than a o C-H bond since the % C-C orbital is a better electron-donor and the 71 C-C orbital a better electron acceptor than the a and a C-H orbitals, respectively. However, the difference in energy between the two isomers is relatively low for a d6 metal center because four-electron repulsion between an occupied metal d orbital and the other n C-C orbital destabilizes the alkyne complex. This contributes to facilitate the transformation for the Ru11 system studied by Wakatsuki et al. 

The extent of alkyne ir donation in Mo(RC=CR)(SBu )2(CNBu% (SO) is not easily quantified as discussed in the structural and molecular orbital sections. Proton shifts for HC=CH and PhC=CH ligands in these complexes are near 10.4 ppm, above the N = 4 median value and approaching an N = 3 chemical shift. The 13C chemical shifts range from 170 to 185 ppm, also above classic four-electron donor alkyne values, presumably reflecting competition with the two adjacent equatorial thiolates for donation to the two vacant metal dir acceptor orbitals. Single bond VCH values of 215 and 211 Hz are typical of terminal alkynes bound to molybdenum(II) (133). 

Some simpler ligands can also serve as ir-donors. We saw earlier that acetylenes can act as CT-donors and ir-acceptors in the same way as olefins. However, the rr-bonding orbitals that are perpendicular to those oriented toward the metal can also serve as ir-donors, as shown in Figure 1.29. Acetylene ligands in complexes that possess less than 18 electrons without such TT-donation are sometimes considered to be "four-electron donors." This is the origin of the listing of alkynes as 2-electron or 4-electrori ligands in Table 1.1. 

ABSTRACT. The redox properties of a variety of isocyanide, rutrile arvj alkyne-derived vinylidene and phenylatlene complexes, commonly with an electron-rich Re(l) or Fe(ll)-phosphinic centre, as well as of the derived protonated species (with aminocarbyne, methyteneamido, carbyne, i vinyl or hydride ligands), as studied by < dic voltammetry and controlled potential electrolysis in aprotic media, are discussed in terms of ligand and metal site effects on the redox potential, and the values of the electrochemical parameter which measures the ligand net etectron-donor/acceptor character are also presented for these ligands. Expressions are indicated which allow to predict the oxidation potential for the octahedral-type 18-electron complexes or 3) or [MsU-1-... 

Table XI contains data for other organometallic complexes. The chloro complexes are of interest because they were reacted with terminal alkynes to afford alkynyl complexes, and the greater nonlinearity of the alkynyl complex eompared to the sum of the precursor alkyne and chloro complex suggests the importance of electronic communication between ligated metal and alkynyl ligand. The aryl complexes have a donor-bridge-acceptor composition, but nonlinearities are...

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