Dicoordinated

In this compound, synthesized in the low temperature reaction between diborane and excess ammonia, the cationic boron is coordinatively saturated in a tetrahedral environment. More recendy, cations having boron in tricoordinate or dicoordinate environments have been observed. These cationic species, called borenium and borinum ions, respectively, have been reviewed (19,20). 

The univalent halogen atoms are included because of the need to name rings containing such atoms in dicoordinate form, as halonium cations. 

Many reactions are known which involve the sulfur atom in sulfoxides and other tricoordinate S(IV) species. Three situations are common in these reactions, i.e. the sulfur atom may remain tricoordinate, its coordination number may be reduced to two, or it may be increased to four. If the sulfur atom is rendered dicoordinate, it can no longer be stereogenic so such transformations will not be considered here. Reactions which leave the coordination number at three usually take place with inversion of configuration or... 

Carbenes - molecules with a neutral dicoordinate carbon atom - play an important role in all fields of chemistry today. They were introduced to organic chemists by Doering and Hoffmann in the 1950s [1] and to organometallic chemists by Fischer and Maasbol about 10 years later [2,3]. But it took another 25 years until the first carbenes could be isolated [4-8] examples are given in Scheme 1. 

The iminophosphenium cation was also of interest for more physical investigations. It was noted to be a stable entity in gas phase experiments [52], the parameters were investigated in detail [53] and a systematic study of the nucleophilic addition of CH, NH and OH bonds was performed [54] with a concomitant interpretation of the chemical shifts (at the dicoordinate phosphorus centres). The latter authors also confirmed the loose interaction of a triphenylphosphine with the iminophosphenium cation (PP = 2.625 A). 

The first product of dicoordination, 18, was synthesized via treatment of phos-phinoiminophosphanes with selenium. The latter then inserted into the PP-bond and formed finally a bis-coordinated donor-acceptor complex [55]. The overall synthetic route is shown in Scheme 10. 

With increasing electronegativity of one ligand in the diphosphene the ZPPR angle shrunk and decreased finally below 90° for the diphosphene 27. The experimental findings were accompanied by the quantum chemical calculations. There are no other reports regarding further experimental investigations or possible dicoordination in these compounds. 

As mentioned in the discussion of the reaction mechanism for this transformation, the active species is a dicoordinate Pd(0) complex, and it is unclear whether an associative or a dissociative process is operative for oxidative addition. In this context, different NHC complexes containing only one carbene ligand have been tested in the Mizoroki-Heck reaction. The most successful are those prepared by Beller, which were able to perform the Mizoroki-Heck reaction of non-activated aryl chlorides with moderate to good yields in ionic liquids (Scheme 6.13). The same compounds have also been applied to the Mizoroki-Heck reaction of aryldiazonium... 

The stannaketenimine (entry 3, Table V) shown below was found to have a bent structure with an angle at the dicoordinate carbon atom of I540 S7 The coordination geometry of the tin atom is distorted owing to the short contact with one of the fluorine atoms of the trifluoromethyl substituents. The bond between the tin atom and the doubly bonded carbon... 

The "unfolded-drum" or "ladder" compound 2 has crystallographic symmetry. This corresponds to the idealized molecular symmetry and, therefore, there are three chemically inequivalent types of Sn atoms in the molecule, although all are hexacoordinated. The oxygen atoms in the open form can be subdivided into two types, as in the case of the drum molecule tricoordinate framework oxygen atoms and the dicoordinate oxygen atoms of the bridging carboxylate ligands. 

Mononuclear silver(I) complexes with nitrogen ligands can have several coordination numbers, the simplest is dicoordinated and compounds of the type [AgL2]+ occur for a variety of ligands... 

The strong bonding with these compounds is due to dicoordination with two carbonyl oxygens. The stronger bonding in succinamide relative to glycylglycine should be due to the better electron pair donor ability of the NH2 group relative to OH. 

The following serves as one more example of the synthesis of a P,B-containing heterocycle (127) with a dicoordinated phosphorus atom [Eq. 

Prasad and Joshi121 presented a conceptually different catalyst system—zinc amides of oxazolidine. Because the addition of dialkylzinc to aldehyde is known to involve a chiral zinc alkoxide with a coordinately unsaturated tricoordinated center, they anticipated that a zinc amide with dicoordinate zinc should be a better Lewis acid. Examining three different zinc species 128-130, zinc amide derived from the corresponding oxazolidine 130 was found to lead to a very fast reaction (4 hours, 0°C) and 100% ee (Scheme 2-50). The reaction proceeds even faster at room temperature (completed within 1 hour) without significant loss of stereoselectivity. This reaction can provide excellent ee for aromatic aldehydes,... 

Double-bonded species (i.e., dicoordinate ) , cr2 metal species) were reported for As, Sb, and Bi.2 The stability of these species toward the formation of oligomers with metal-carbon single bonds is increased by steric protection with... 

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. 

For a recent monograph on vinyl cations Dicoordinated Carbocations Rappoport, Z., Stang, P. J., Eds. Wiley New York, NY, 1997. 

A recent review on vinyl cations Rappoport, Z. Stang, P. J. Eds. Dicoordinated Carbocations, John Wiley Sons Chichester, 1997. 

---