Molybdenum complexes alkynes, structures

In the same way that terminal oxo or nitrene units create a pyramidal distortion in M(0)L5 131) and M(NR)L5 132) complexes, alkynes also tend to pull the metal out of the plane of the four adjacent ligands. In W(CO)(HC=COAlCl3)L3Cl the two PMe3 ligands cis to the alkyne form an L—W—L angle of 161° 130), and in the molybdenum structure the... 

A hydrocarboxylation-type reaction of alkenes and alkynes with CO, using a homogeneous catalyst, is another avenue for the potential utilization of CO. Stoichiometric reactions between metal-coordinated alkenes and alkynes with CO are known. As shown by reaction 4.7.3.1, the molybdenum complex 4.56 reacts with CO to give 4.57 where acrylic acid units act as rf-, pMigands and bridge two molybdenum atoms. Although the structure of 4.57 may appear to be complex, its... 

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). 

The chemistry which has developed for molybdenum(II) and tung-sten(II) alkyne monomers encompasses syntheses, structures, spectra, molecular orbital descriptions, and reactions. The sheer volume of literature reports germane to this seemingly narrow topic is surprising. The Mo(II) and W(II) complexes addressed here are not unique in terms of alkyne 7rx donation. Related alkyne chemistry is appearing for d4 metals other than molybdenum and tungsten, as well as for d2 complexes in general. 

X-Ray Crystal Structure Determinations of Molybdenum(II) and Tungsten(II) Halocarbonyl Alkyne Complexes Reported since 1987... 

Nitrido complexes have also been formed by metathesis and atom transfer processes. The reaction of dinitrogen with a molybdenum(III) species forms a molybdenum-nitrido complex, as shown in Equation 13.103. and described in more detail in the section of Chapter 5 on dinitrogen complexes. In a metathesis process involving related complexes, the reaction of a metal-alkylid)me complex with a nitrile extrudes an alkyne to form a metal nitride fliat adopts a dimeric structure (Equation 13.104). - Related nitrido complexes have been formed from an azabicydic compoimd that eliminates anthracene after forming the M-N bond (Equation 13.105). ... 

The NMR characterisation of fBA adducts formed in the reaction of tungsten and molybdenum seven-coordinate compounds leaves little doubt concerning their formulation. However, we were unable to separate such compounds and to prepare a crystalline sample. To obtain a more stable terminal alkyne compound, we introduced two phosphine ligands into the coordination sphere of tungsten(II). In this way a cationic complex of tungsten(II), [WCl(CO)( BuC=CH)(NCMe)(PPh3)2]2[SnCl6], was prepared [45]. The crystal structure of this compound consists of two independent cations and a... 

It is clear that [3 + 2] cyeloaddition chemistiy and Sonagashira eross-coupling reactions are powerful tools for the development of more complex CO-RMs and indeed other pharmaceutieals agents. Onee a suitable CO-RM has been developed there are thousands of possible interesting structures that ean then be attached on to the CO-releasing moeity. The cyclopentadienyl molybdenum tricarbonyl complexes shown in Scheme 3 could also be easily modified to give a eomplex with a terminal alkyne funetionality which would allow the same sort of chemistry to be carried out on those too. 

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