Molybdenum complexes ligand structure

Norbornadiene (NBD) in (NBD)M(CO)4 (353) is readily displaced by CO (274) or (2-allylphenyl)(diphenyl)phosphine (50, 312). Although the latter reaction gives the compound of expected composition, (C2iHi9P)M(CO)4, both the chemical and spectral data indicate that it has the structure (26) in which the C21H19P ligand is the isomeric (2-propenyl)(diphenyl)phosphine. For the molybdenum complex this structure has been confirmed by X-ray diffraction (379). 

Comparison of the photoelectron spectra and electronic structures of M-NS and M-NO complexes, e.g., [CpCr(CO)2(NX)] (X = S, O), indicates that NS is a better a-donor and a stronger r-acceptor ligand than NO. This conclusion is supported by " N and Mo NMR data, and by the UV-visible spectra of molybdenum complexes. 

Similarly, metathetical exchange of the labile acetonitrile ligands in the octahedral molybdenum complex (CO)2(MeCN)2BrMo( 3-C3Ph3) with a variety of bidentate amines led to the corresponding amino complexes (equation 267)311. These form crystalline acetonitrile solvates suitable for X-ray determination, which confirm the octahedral structure of these complexes and the occurrence of the cyclopropenyl and bromide groups in a tram-relationship. 

While some molybdenum complexes such as Mo03(dien) were found to be inactive,236 the rates of molybdenum-catalyzed epoxidation of alkenes were found to be independent of the structure of the complex used, after an induction period representing the time for exchange of anionic ligands by the alkyl hydroperoxide. cis-Dioxomolybdenum(VI) diolates such as (78) were isolated... 

Tables I and II summarize the structural studies of mononuclear and binuclear vinylidene complexes, and Table III those of propadienylidene complexes which had been reported to mid-1982. As can be seen, the C=C bond lengths range from 1.29 to 1.38 A, and the M-C bond (1.7-2.0 A) is considerably shorter than those found in alkyl or simple carbene complexes. Both observations are consistent with the theoretical picture outlined above, and in particular, the short M-C bonds confirm the efficient transfer of electron density to the n orbitals. In mononuclear complexes, the M—C=C system ranges from strictly linear to appreciably bent, e.g., 167° in MoCl[C=C(CN)2][P(OMe3)2]2(fj-C5H5) these variations have been attributed to electronic rather than steric factors. In the molybdenum complex cited, the vinylidene ligand bends towards the cyclopentadienyl ring (111).

The crystal structure of 116 has been completed (471). The dimeric molybdenum complexes CpMo SC(NR)S)2MoCp (R = Me, Bu, C6H, CH2Ph), containing bridging dithiocarbonimidate ligands, were synthesized from [CpMoS(CH2)3S]2 and excess isonitrile... 

One approach to limit dimer formation in model complexes involves the use of bulky ligands and weakly coordinating solvents. Holm and coworkers [196,198-201] have studied oxygen atom transfer reactions of 2,6-bis(2,2-diphe-nyl-2-thioethyl)pyridinate [2] molybdenum oxo complexes. In contrast to the structurally similar dithiocarbamate molybdenum complexes, the tendency of the 2,6-bis(2,2-dipheny 1-2-thioethy l)pyridinate MoIV monoxo complex to undergo di-... 

The lanthanide metals, with their larger coordination numbers, invariably have the ligand coordinated in either a trismonodentate or a tetrakismonodentate fashion (42, 55, 57) (see Figs. 10 and 11). Thus, the ligand apparently tends toward the maximization of its coordinating capacity where circumstances allow, giving rise in many cases to the formation of sheet or cage structures. Limited chain formation was observed in the platinum, palladium, and rhodium complexes described, while the molybdenum complexes described by Hilbers et al. consist of dinuclear and tetranuclear units. 

---