Molybdenum complexes four-electron alkyne ligands

Four-Electron Alkyne Ligands in Molybdenum(ll) and Tungsten(ll) Complexes... 

As stated in the Introduction, Templeton7 published a comprehensive review entitled Four Electron Alkyne Ligands in Molybdenum(II) and Tungsten(II) Complexes, including halocarbonyl complexes, which dealt with the literature up to 1987. Hence, this section of the review deals with the literature on halocarbonyl alkyne and alkene complexes published from 1987 to mid-1995. The vast majority of papers that have appeared during this period are concerned with the reactions of [MI2(CO)3(NCMe)2] (M = Mo, W) and their derivatives with alkynes, which begin this section of the review. 

J.L. Templeton - Four-Electron Alkyne Ligands in Molybdenum (II) and Tungsten (II) Complexes, Adv. Organomet. Chem. 29, 1,1989. 

Davidson and Sence (904) investigated the chemistry of a number of molybdenum(II) alkyne complexes. For example, addition of NaS2CNMe2 to [CpMoCl(CO)(PhC2Ph)] yields [CpMo(PhC2Ph)(S2CNMe2)J, which contains a four-electron alkyne ligand (Eq. 79). 

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). 

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