Trigonal planar coordination

Reaction of 2 equiv of H0Si(0 Bu)3 with B(0 Bu)3 as a neat mixture at 80 °C results in the formation of Bu0B[0Si(0 Bu)3]2, which can be readily isolated in crystalline form (79%) [64], X-ray crystallography revealed that BuOB[OSi(O Bu)3]2 is monomeric in the soUd state, with a trigonal planar coordination geometry about boron (Fig. 5). Similarly, B[0Si(0 Bu)3]3 can be formed by reaction of B(0 Bu)3 with 3 equiv of H0Si(0 Bu)3 in toluene [64]. The related triphenylsiloxide compound B(OSiPh3)3 had been previously reported [109]. 

The molecular structure of the f-butyl derivative [BpBut]ZnBul has been determined by an x-ray diffraction study, which confirms a distorted trigonal planar coordination environment for zinc, as shown by the two views in Figs. 26 and 27 (80). In comparison to tetrahedral coordination, three-coordinate zinc alkyl complexes are rare (84). 

Cobalt(II) alkoxides are known and monomeric forms are part of a wider review.413 The interest in these compounds pertains to a potential role in catalysis. For example, a discrete cobalt(II) alkoxide is believed to form in situ from a chloro precursor during reaction and performs the catalytic role in the decomposition of dialkyl pyrocarbonates to dialkyl carbonates and carbon dioxide.414 A number of mononuclear alkoxide complexes of cobalt(II) have been characterized by crystal structures, as exemplified by [CoCl(OC(t-Bu)3)2 Li(THF)].415 The Co ion in this structure and close relatives has a rare distorted trigonal-planar coordination geometry due to the extreme steric crowding around the metal. 

Herrmann and coworkers183 reported a series of Cp-manganese carbonyl complexes which bind Ge, Sn and Pb as central atoms linearly coordinated in clusters, to two Mn atoms in one series and trigonal-planar coordinated to three Mn atoms in another series 8 and 9. The group 14 atoms are double-bonded to two Mn atoms in these compounds, or carry one double bond and two single bonds to three Mn atoms. Potentiometric measurements of these compounds show irreversible reductions and oxidation by CV. No products could be isolated from either reduction or oxidation. The exceptionally high oxidation potential of (/i-Pb) r/ -CsHs )Mn(CO)2]2 as compared to the apparently similar Sn compound is noteworthy (Table 15). 

Recent results on the chemistry of persistent vinyl cations are summarized. / , / -Disilyl-substituted vinyl cations were synthesized by intramolecular addition of transient silylium ions to alkynes. The vinyl cations are stable at ambient temperature and were isolated in the form of their tetrakispentafluorophenylborate and hexabromocarboranate salts. The vinyl cations were characterized by IR and NMR spectroscopy and by X-ray crystallography. The experimental results for the a-alkyl- and a-aryl-substituted vinyl cations confirm their Y-shape structures, consisting of a linear dicoordinated, formally positively charged a-carbon atom and a trigonal planar coordinated /f-carbon atom. In addition, the spectroscopic data clearly indicate the consequences of, / -silyl hyperconjugation in these vinyl cations. Scope and limitations of the synthetic approach to vinyl cations via addition of silylium ions to C=C triple bonds are discussed. 

As expected, for the metalated carbon atom a trigonal planar coordination geometry is achieved. Both lateral carbene moieties which stabilize the carbon(O) atom are planar but are tilted relative to each other to relieve allylic strain [20, 21]. 

The structural features of the dialkylzinc units in these compounds are closely related, i.e. the zinc atoms are trigonal-planar coordinated, with C—Zn distances as expected. The Zn—N coordination bonds range from 2.078 to 2.184 A and the C—Zn—C bond angles range from 132 to 145 . 

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