Tetracovalent

The puzzling discovery of Collie and Tickle in 1899 that 2,6-dimethyl-4-pyrone (8) affords crystalline salts (9) with acids, which were the first monocyclic pj rylinm salts to be isolated, was interpreted by a formula (21) with tetracovalent oxygen.An active period of research followed. The methosulfate or methiodide of 2,6-dimethylpyrone was converted by ammonia into 4-methoxy-2,6-lutidine, therefore, the exocyclic oxygen of the pyrone must be involved in the salt formation. Thus, formula 21 was disproved and formula 22 was demonstrated for these salts. 

Potts and co-workers have used phosphorus pentasulfide and pyridine as a standard reagent for constructing annelated diphenylthiophene rings from o-dibenzoyl compounds. Many of these annelated thiophenes can be written only in the tetracovalent sulfur or ylid dipolar form (96) and are thus termed nonclassical 123 other examples, particularly of the synthesis of non-classical thienoisothiazoles, have been reported by Gotthardt.124... 

Mercaptophenylacetic acid, acetic anhydride, and sodium nitrite yielded the first example (169, R = Ph) of a meso-ionic 1,3,2-oxathiazol-5-one this compound has a dipole moment of 4.5 D. An X-ray crystallographic investigation of the 4-phenyl-l,3,2-oxathiazol-5-one (169, R = Ph) has been reported. This compound is incorrectly described as a thiosydnone. The bond distances support the structure with tetracovalent sulfur, and on this evidence alone the compound 169, R = Ph, could be judged not to be meso-ionic because it can, on the basis of the X-ray evidence, be more satisfactorily represented by a covalent formulation. 

In 1967 Cava and Pollack obtained derivatives of the fourth, so-called nonclassical , thienothiophene— thieno[3,4-c]thiophene (4), a condensed heterocycle with formdly tetracovalent sulfur (42)j. The reaction of 3,4-bischloromethyl-2,5-dimethylthiophene (141) with sodium sulfide afforded 4,6-dimethyl-lif,3ff-thieno[3,4-c]thiophene (142) periodate oxidation of 142 gave die corresponding sulfoxide (143) in 91% yield. Attempts to convert the sulfoxide (143) into the thieno-[3,4-c]thiophene by the method used for S3mthesizing benzo[c]-thiophene led only to polymer. However, 24% of adduct 144 and 10% of 145 were obtained by refluxing sulfoxide (143) with N-phenylmaleimide in acetic anhydride, indicating that the thieno[3,4-c]-thiophene was formed as an intermediate. 

Conversely, doping Ge with As introduces an extra electron that cannot be accommodated in the tetracovalent network (valence band), and this creates a narrow band of occupied donor levels, just below the conduction band in energy. The Fermi level is now located between the donor band and the conduction band, and electrons in the donor band can be readily excited thermally into the conduction band (Fig. 5.5). Thus, a negative or n-type semiconductor is created. Semiconductors can exhibit electrical conductivities in the range 10-3 to 104 S m 1, as compared to 103 to 107 S m 1 for metals. 

Structures involving tetracovalent sulfur (279) have been used to represent... 

Stereochemical evidence for the mechanism of nucleophilic substitution at a tetracovalent phosphorus atom has been mainly forthcoming from the experiments of Michalski et a/.48-50 with esters containing thiophosphoryl centers. Until recently, the inaccessibility of suitable optically active phosphorus compounds precluded stereochemical investigations on the mechanisms of their hydrolysis. 

A remarkable reaction in which a monomer reacts to form a polymer has been observed by Foxman and his students.37 They prepared [Ni P(CH2CH2CH2NH2)3 2Br2], in which the bromine and phosphorus atoms are coordinated to the tetracovalent nickel. Upon heating, the nickel... 

Divalent nickel forms two main types of complexes. The first consists of complexes of the spin-free ( ionic or outer orbital) octahedral type (see also Ligand for their discussion) in which the ligands are principally H2O, NH3, and various amines such as ethylenediamine and its derivates, e.g., Ni(H20>62+. Ni(NH3)e2+, Ni(en)62+. These complexes usually have colors toward the high-frequency side of the spectrum, i.e., violet, blue, and green. The other class consists of tetracovalent square complexes with ligands such as CN, the dioximes and their derivatives, and other chelates, which usually have colors on the low frequency side of the spectrum, i.e., red. orange, and yellow. The structure of the nickel-climethylglyoxime complex is... 

The overall reaction is quite complex but involves a rearrangement similar to that described for the hydroboration-oxidation of alkenes (Section 11-6E). The first step is hydroboration of the alkene to a trialkylborane. When the trialkylborane is exposed to carbon monoxide, it reacts (carbonylates) to form a tetracovalent boron, 9 ... 

The role of orbital orientation in organic and enzymatic reactions has been of considerable current interest. Deslongchamps and coworkers(1) in studying tetracovalent carbon species have recently demonstrated selective cleavage of bonds which are trans, antiperiplanar (app) to lone pairs on directly bonded oxygen and nitrogen atoms. 

Molecular orbital calculations have also provided theoretical justification for these stereoelectronic effects in tetracovalent and pentacovalent phosphorus species (2-7). As has been shown in molecular orbital calculations on the X -P-X2 (X = 0,N) structural fragments, the X.-P bond is strengthened (as indicated by an increase in the Mulliken overlap population) while the P-X3 bond is weakened when the X atom lone pair is app to the P-X3 bond. Thus, in the g,t conformation of dimethyl phosphate (Structure ll the overlap population for the trans P-0 bond is. 017 electron lower than the overlap population for the gauche P-0 bond. As shown for g,t dimethyl phosphate one lone pair (shaded in 1) on the gauche bond oxygen is app to the trans bond, while no lone pairs on the trans bond oxygen are app to the gauche bond. Thus, the weakest X.-P bond has one app lone pair and no lone pairs on X. app to the P=X2 bond. 1... 

Methyl triflate is a powerful alkylating agent, which methylates tetracovalent P=0, P=S, and P=Se systems at the oxygen, sulfur, and selenium atoms, respectively (1,2, 3). Its reaction with analogs containing a hydrogen substituent (e .j>., phosphinate, phosphonate or secondary phosphine oxide species), however, appears not to have been reported. 

Black spheres representing Carbon this tetracovalent element has four holes ... 

Formula II presents a pentacovalent nitrogen atom surrounded by 10 electrons. All five valences are the ordinary, nonpolar ones. Compounds of pentacovalent nitrogen such as the type N(CH3)5 are not known. On the other hand formulae III present nitrogen as a tetracovalent atom, surrounded by an electron octet. Here, one of the nitrogen atoms is bonded with an oxygen atom by a coordinate (semi-polar) link. 

This chapter describes heterocyclic systems which are isoconjugate with the pentalenyl dianion (13) and which cannot be represented by classical Kekule structures. Molecules of this type (e.g. 9-12) can only be represented by dipolar structures, or possibly by structures involving tetracovalent sulfur atoms, and are described as conjugated mesomeric betaines or non-classical heteropentalenes. The structure and chemistry of this type of heterocycle have been the subject of two earlier reviews (77T3203, 77HC(30)317) and their classification as conjugated mesomeric betaines and relationship to other dipolar heterocycles has also been discussed (83UP43700). 

The question of the extent of sulfur 3ef-orbital participation in the bonding of some type A and C heteropentalenes is an interesting one. In valence bond terms ef-orbital participation is introduced by assuming that canonical forms involving tetracovalent sulfur e.g. 23 + 24) make a significant contribution to the structure. In terms of MO theory, rZ-orbitals are introduced by mixing the HOMO i.e. pseudo-NBMO) with a sulfur dyz-orbital (25). The... 

There are cases when one of the four different groups associated with a tetracovalent center of asymmetry is merely an unshared electron pair. 

Divalent cobalt may assume a coordination number of either 4 or 6 in its complexes. The large majority of the tetracovalent complexes, including such simple ones as C0F72, CoBr72, and Co(SCX)72, are tetrahedral, but a few of the more complicated Co(II) chelates are known to be planar. The majority of the hexacovalent complexes, such as Co(NH )t2, Co(H20)f, and Co(C204)74, are of the spin-free, rather than the spin-paired, type. As indicated below, a spin-paired octahedral complex of Co(II) has one electron beyond a sextet of low-energy 3d orbitals ... 

Tetracovalent. Given to combination with four atoms or radicals, the bonds being of a covalent or nonionized character. 

Structures involving tetracovalent sulfur (279) have been used to represent this class of compound but the role of d-orbitals in their bonding is not clear (see Section IV,A). 

Selenium is more easily oxidized to Se(IV),but the oxidation stage Se(VT) is obtained with more difficulty than with the corresponding sulfur compounds. For example, selenuranes (tetracovalent selenium compounds) are more easily obtained than the sulfur analogs. The synthesis as well as the chemistry with selenium at higher oxidation stages is reviewed in Chap. 6 by M. Mikolajczyk and J. Drabowicz. 

A steric factor, reflecting that for each metal ion there is a particular preferred orientation of coordinated groups, is occasionally to be considered. This factor seems to be of secondary importance. For example, a square planar configuration is preferred over the tetrahedral configuration for tetracovalent complexes of Cu(II). Nevertheless, a reagent such as trimethylenetetramine, which could not form a planar complex, for steric reasons forces the tetrahedral configuration upon Cu(II) with only a small loss of expected stability. ... 

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