Tetrahydrofuran ligand structure

The molecular structure of [Yb2Cl4(Tp)2(THF)2]-C7H8 (Fig. 2.155) is reported by Rabe et al.639 This dimeric centrosym-metric complex exhibits a seven-coordinate Yb atom, bonded to three N atoms of the tridentate ligand, to the O atom of the neutral tetrahydrofuran ligand, and to one terminal as well as to two bridging Cl atoms. 

Fig. 2-23. Schematic view of the structure of Th2(BH3CH3)8(OC4H8)2 (only the oxygen atoms of the tetrahydrofuran ligands are shown dotted lines designate ri -bonds of boron via hydrogen and / means center of inversion) [2].

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

Again, weak coordination is expected for ether ligands to zinc, however, the larger number of structurally characterized examples is at least partially attributable to the frequent use of solvents such as diethyl ether or tetrahydrofuran which may provide additional ligands to the metal center. 

What specific properties of these complexes have allowed isolation of five-coordinate Pt(IV), in the form of the trimethyl complex and the dihy-dridosilyl complexes These two types of complexes are significantly different, and their stability is apparently due to different factors. Comparing the trimethyl complex in Scheme 21(A) with the related but six-coordinate complexes of a similarly bulky oc-diimine ligand (98), shown in Scheme 23, is instructive. In Scheme 23A, triflate is clearly coordinated, exhibiting an O-Pt distance of 2.276(3) A (98), which is typical for Pt-coordinated triflate (108). This triflate complex A in Scheme 23 was obtained from dry tetrahydrofuran. The aqua complex cation B, also structurally characterized, was obtained from acetone containing trace water. An equilibrium between coordinated triflate and coordinated water, very likely via a common five-coordinate intermediate, was indicated by NMR spectroscopy (98). 

Complex C (Scheme 21) seems to shows stable penta-coordination for apparently very different reasons. The compound is cationic, but the counterion is a non-coordinating tetraarylborate. Interestingly, neither the dichloromethane solvent nor the diethylether present in the reaction mixture seem to coordinate to the open site in solution. The compound was crystallized from a tetrahydrofuran/pentane mixture as the tetrahy-drofuran (THF) solvate, but in the crystal structure, the THF is remote from the open site at platinum. The open site is shielded somewhat by the methyl groups of the protonated TpMe2 ligand, but it does not appear completely inaccessible. A reasonable explanation for the... 

The sterically hindered dianionic bidentate phenoxide ligand (205) gives several tetrahedral iron(II) complexes, e.g., [Fe (205)(THF)2], [Fe (205)(py)2], [Fe (205)(bipy)], and [Fe (205)(2,6-xylylNC)2]. The first of these is prepared from FeCl2 and (205)H2 in tetrahydrofuran the others are prepared from the dimer [Fe2(205)2]. The 2,6-xylylNC complex is low-spin, the others high-spin. There is also a five-coordinate iron(III) complex, red-black [Fe(205)(bipy)Cl], whose structure is intermediate between trigonal bipyramidal and square pyramidal." ... 

Simple Mo xanthate complexes were obtained by the reaction between Mo(II) acetate and the xanthate ligands in inert atmosphere (550, 581a). The red crystalline compounds had the stoichiometry Mo2(EtXant)4 and reacted readily with donor molecules such as tetrahydrofuran (550) pyridine, 4-picoline, and Et3As (581a). A Mo(lI)-acetate type of dimeric structure was proposed (581a) for this molecule and was confirmed by a subsequent x-ray study (550). 

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