Tetravalent complexes, transition metal

These heterocycles contain at least one transition metal as part of a strained three-membered ring. Their structures therefore are partially described as heterocycles and partially as transition metal n complexes of heteroalkenes (Equation (1)). Evidence for metallacyclic structures is therefore be highlighted. A wide variety of transition metal complexes could be covered but this chapter is restricted to compounds where the ring carbon is tetravalent, bonded to hydrogen or carbon, and bonded to not more that two metal atoms. Related areas which have been reviewed include CO bridged bimetallics <84AOC(23)2i9> and C02 metal complexes <83AOC(22)l29>. 

The diazo reactions in this chapter are characterized by processes run either in the gas phase, in relatively inert matrices, or in — typically, but not exclusively — aprotic and comparatively apolar solvents, either thermally or photolytically or with transition metal catalysis of various types. The metastable intermediates are carbenes (RR C ), i. e., neutral, apparently divalent, carbon compounds, or their transition metal complexes (coined carbenoids, see later in this section). It is interesting to recall that the synthesis of a compound that we now call a carbene, namely methylene (H2C ), was already attempted in the early 19th century, i.e., before the tetravalency of carbon was established. Dumas (1835) and Regnault (1839) thought then that it should be possible to obtain a compound consisting of one carbon and two hydrogen atoms by dehydration of methanol (a compound of which only the atomic ratio 1C 4H lO was then known). ... 

Using this approach, a dynamic combinatorial library (DCL) of rapidly equilibrating metal complexes was formed by incubating a series of salicylaldimines with the transition metal salt ZnCl in aqueous solution (Fig. 3.3). Divalent zinc was chosen as the transition metal for its known tetravalent coordination geometry with salicylaldimines [7] and its compatibility with DNA. (In retrospect, this was a somewhat naive view of both... 

Most zeolites have an intrinsic ability to exchange cations [1], This exchange ability is a result of isomorphous substitution of a cation of trivalent (mostly Al) or lower charges for Si as a tetravalent framework cation. As a consequence of this substitution, a net negative charge develops on the framework of the zeolite, which is to be neutralized by cations present within the channels or cages that constitute the microporous part of the crystalline zeolite. These cations may be any of the metals, metal complexes or alkylammonium cations. If these cations are transition metals with redox properties they can act as active sites for oxidation reactions. 

Organostannylene and plumbylene complexes of transition metals can be synthesized from both divalent and tetravalent starting materials. 

The nonaqueous chemistry of the tri- and tetravalent ions follows similar trends as the aqueous complexes. Coordination numbers are dictated by the steric bulk and electronic properties of the hgands two structures, U(MeBH3)4 (11) and [ U(N(CH2CH2NSiMe3)3) 2(/r - 2 JJ -Nz)] (12), are shown. Direct comparisons of the latter species can be made to the behavior of transition metals in both coordination and reactivity. 

The complex Li2[PtMc4] has been formed from Pt(COD)Cl2 in the presence of xs LiMe Platinum also has an extensive organic chemistry of the tetravalent state and Li2[PtMeg] has been isolated . Neutral compounds are more common, however, mainly via LiR or MgRX alkylations, including the historic prototype for alkyl transition metals, [PtMe4l]4 and a wide variety of PtMe4L2, formed from... 

Sn Mossbauer spectra of a number of the complexes were examined. Data are summarized in Table III together with relevant reports from the literature. There are essentially two types (a) the dialkylstannylene complexes such as R2SnCr(CO)s in which divalent tin is three-coordinate, and (b) dialkylstannylene insertion products into C-Hal, M-H, M-Cl, M-Me, or M-M bonds (M = a transition metal) in which tetravalent tin is four-coordinate. (SnR2)2 and Class (a) complexes are characterized by... 

Complexes of transition metals in which a Group IV metal moiety functions as a donor ligand arc quite profuse. Many routes for their synthesis are available, and many types of structure are adopted. The formally tetravalent organometallic R = organic group, X = halogen ... 

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