Donor structural investigation

A structural investigation of [Ta(=CH Bu)(CH2 Bu)3] grafted on siUca partially dehydroxylated at 700 °C using EXAFS [18] has revealed a short-range interaction of 2.64 between the O-atom from a siloxane bridge and tantalum. This O-atom acts as a two-electron donor ligand to stabiUze the formally ten-electron surface complex [(=SiO)Ta(=CH Bu)(CH2 Bu)2j, yielding the more stabiUzed twelve-electron species [(=Si0)Ta(=CH Bu)(CH2 Bu)2(=Si-0-Si j (Scheme 11.9). 

Complexes of thiourea (tu) with chromium(III) have not been studied in detail and the lack of structural investigations in particular has led to uncertainties regarding the ligand donor sites and geometrical configurations of some of the reported species. 

Clark and Schleyer20 were the first to report theoretical evidence for the complexation of H2Si by halogen donors when investigating the structural isomers of the silylenoid H2SiLiF. A silylene-lithium fluoride complex 14 was found to be only 5.4 kJ moL1 less stable at the MP2/6-21G//3-21G... 

That cyclic compounds 17 and 19 are in equilibrium with their corresponding dimers has been deduced from NMR spectra (40,48). If sulfur is the donor atom (18 and 21), the stabilization at the metal seems to be more efficient than in the case of an oxygen donor (19). Two remarkable isomeric forms, 23 and 24, have been shown to exist even in the solid state for the galliym derivative of N, AT-dimethyloxamide [X-ray structure investigations (54) see also compounds 22 and 24 (52,55)]. 

Based on the analysis of the electronic structure of vinylidene intermediates, we wanted to test whether allenylidene complexes of gold(I) could be prepared. First, the gold acetylides 49 were prepared then an alkylation delivered 50 (Scheme 4.20). A detailed structure investigation, based on experimental and computational methods, followed [43]. The strong donors, which allow 50 to be isolated, stabilize the carbenium ion with the lone pair of the heteroatom (O or N). Thus no significant Au-C double bond character could be detected. 

The structure of the complex of (S)-tryptophan-derived oxazaborolidine 4 and methacrolein has been investigated in detail by use of H, B and NMR [6b. The proximity of the coordinated aldehyde and indole subunit in the complex is suggested by the appearance of a bright orange color at 210 K, caused by formation of a charge-transfer complex between the 7t-donor indole ring and the acceptor aldehyde. The intermediate is thought to be as shown in Fig. 1.2, in which the s-cis conformer is the reactive one. 

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