Bridging methyl groups

Mixtures of hexamethyldisilazane, /z-butyllithium, and dimethylzinc reacted, Scheme 55, with the formation of a lithium zincate 70. In the solid state, Figure 37, 70 consists of four-membered, -symmetric LiN2Zn rings, which form a polymeric structure through bridging methyl groups from zinc to lithium.126... 

The complex (CII ePt emF (en = ethylencdiamine) contains the centro-symmetric cation [en(CH3)3Pt-en-Pt(CfI,-,)3en]2+, in which there is only one ethylencdiamine bridge methyl groups are arranged in cfs-posi-tions, and the platinum atoms are 7.79 A apart 249). 

Several reactions, such as that of Complex XXVII with H+, have led to formation of an orange complex ion [Pt2Me3(/Lt-dppm)2]+. This complex does not contain a bridging methyl group, but probably contains a coordinate Pt —> Pt bond (20, 21). The structure is Complex XXXII, Figure 2. It is assumed that the electron-rich dimeth-ylplatinum center donates a pair of electrons to the vacant orbital... 

While this suggests that there should be two resonance signals observed in the PMR spectrum, one corresponding to the bridge methyl groups and the other to the terminal methyl groups, the initial investigation of the PMR... 

Protolysis of the bridging methyl groups in 48b with t-butylacetylene results in stepwise formation of the /t-alkynyl derivatives 49b and 50b (Scheme 6) [27]. 

Complexes with C—H—Cr bridges involving methyl groups, for example, 37n, 38n, 39j, 39k, 39k, 391,391, 39m, and 39n, exhibit a hindered rotation of this substituent. In [Fe P(OCH3)3 3( /3 CH-butenyl)]+, a hindered rotation of the bridging methyl group was also observed (44). 

The CpjYb unit can be coordinated by saturated hydrocarbons, as in MeBeCp. The positions of the hydrogens on the bridging methyl group show that this is not a methyl bridge having a 3c-2e bond between Yb-C-Be, as shown in Fig. 18.4.3(b). 

Organoaluminium compounds are dimeric but the bonding is different from that of halides as the bridging methyl groups in A12(CH3)6 (1) must be held by three-centre two-electron bonds similar to those in... 

A bridging methyl group may also show a large shift to high field. In the case of the ( 3-l,3-dimethylallyl)nickel methyl dimer (14), the structure of which has been confirmed by X-ray crystallography (26), the bridging methyl groups resonate at —20.3 ppm [ J(CH) = 118 Hz]. 

Al2(CH3)g is isostructural with diborane, B2Hg. On the basis of the orbitals involved, describe the A1 — C—A1 bonding for the bridging methyl groups in Al2(CH3)g. 

Roberts seems to have been the first to propose the existence of a carbonium ion intermediate with a bridging methyl group (a structure that we would now call a corner-protonated cyclopropane), in part as a result of a study on the deamination of propylamine-1-in an aqueous medium, in which it was believed that a small amount of label migration to both C(2) and C(3) of the 1-propanol product was observed (Figure 50). In fact, the analysis of the label position was later shown to be in error but, by then, other experiments had suggested the existence of some kind of protonated cyclopropane. Among these were studies showing the formation of cyclopropanes from deamination of alkylamines in non-aqueous media. This process was subjected to further scrutiny... 

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