Molybdenum complexes chemical shift ranges

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). 

Se chemical shifts span a wide range of ca. 3300 ppm the extremes are marked by selenoaldehydes, some molybdenum selenides, and cationic heterocycles (up to S = 2434) at the high-frequency end and bridging selenium (p) in tungsten complexes (3 = —900) at the low-frequency end. A detailed discussion of electronic influences on Se chemical shifts and its variation in different classes of compounds has been published. In the following, some general characteristic tendencies are collected. 

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