Catenation

Adding benzyne to the disiLoLe PhC=CPhPhC=CPhSi(Re)CMe)SiPhC=CPhPhC=CPh 

Lene adduct at 350 C with anthracene transfers the ReSisSiMe residue. Si 

ArAr Si=SiArAr isomers, while HgCOCOCF,), oxidises Mes(R)Si=Si(R)Mes to the 

2-diacetate through a disilene-HgCll) adduct. Oxidation of 

The structures of C(t1e,Si) CH3,MT (M=tie,Sn) are supported by calculations on the trans-folded structure is the most stable. 

Stirring MejSiaH with Fe2(CO)9 produces the first stable mononuclear dimethylsilanediyl iron complex (56) as an air sensitive distillable oil, while the diferriosilane CpFe(CO)2 2SiMeH loses CO photochemically to give a silanediyl bridged mixture of isomers [equation (66)]. Palladium complexes catalyse the 

Dodecamethylcyclohexasilane can now be prepared in very good yield by the one step reduction of MeaSiCla using lithium in excess. With a deficiency of lithium, however, larger cyclopolysilanes resulted (MeaSi) (x = 7, 8, or 9). A series of silyl and phenyl substituted cyclopolysilanes have been synthesized, and while ethylene oxide inserts into Li(SiPha)5Li, dimethylsilylene, generated from (MeaSi)6, will additively cyclodimerize adamantanone and norbornone through a mechanism thought to involve the formation of an oxasilacyclopropane intermediate.  

The structure of (PhaSOg shows the ring to be between Cg and Ca symmetry, while (MeaSi)6 shows the largest temperature range for plastic crystalline behaviour for a monocyclic compound.  

The first optically active hexaorganoditin has been prepared by the stereoselective reaction of Ph(PhCMe2CH2)MeSnH in the presence of Pd. Other methods give a racemic mixture. The distannanes, formed by oxidizing RjSnH (R=2,4,fi-trimethylphenyl or 2,4,6-triethylphenyl) with AIBN [equation (68)], provide the first example of a reversible thermal dissociation in solution at 100 °C. The dissociation energies of 190 and 125 kJ mol are less than that observed for Me Sn2.  

The decomposition of an extensive range of aryldisilane derivatives has been studied under nucleophilic , photolytic2 2 and catalytic conditions2 3. A trisilapropellane has been made along with aminodi and trisiianes  

The disilene complex Cp2W(T -Me2Sia iMe2) reacts widi S, Sc. and Tc to give the symmetric Si-Si insertion product, while with phosphine chalcogenides, a small amount the 

R2CHCOCI to give HCl and ketene adducts Photolysing Mes2Si(SiMe3)2 with 

Pd catalyses the silylation of a-diketonees with MegSi2 to give Inssiloxyethenes, aryl bromides to give aryl and hetetoarylsilanes. and allylic silylations of allylic chlorides, the addition of disilanes to 1,3-dienes to give a,ci -disilyl octa-2,6-dienes, and disilanyl ethors frt n 

Reducing ls 2Si(H)Q and subsequent coupling gives mixed disilanes R2SiHSiHIs 2 which 

Terminally metallated methyloligosilanes result from the appropriate a,o -dichIcnrkfe, while photolysing Cp(CO)2FeSiMe7SiMe2X (X=H, Me or CgH4Y) gives the suspected disilene-Fe complex (X=H), the silylene complex (X=Me), while for X=Aiyl, the expelled 

The hindered ring Mesgde i thermolyses to the germylene and silagermene, irradiation of PhMe2MM Me3 eliminates Me2M (M=Ge) or gives R3M. radicals. Pd(0) catalyses the 

Bu 2Si and the disilene which adds Ph2CO to give the disilaoxetane 

Calculations show disilene and silylsilylene isomers of Me jSi2 to be almost isoenergetic, the former having a very flat potential energy surface for trans-bending or twisting about the Si-Si bond. 6 6 about half the aromaticity of benzene, but in contrast is 

Photolysing mesitoylSi(SiMo 3 gives the isomeric silene which 

Halogenation of R-f luorenylSi (SiFle ) with I2 gives cleavage of only 

In DME, (Me2fii)4Si and MeLi give [(He2Si)3Si Li ]2bME3 which 

A range of (disilylene)oligoarylylenes and thienylenes have been made and their photolytic and electrochemical properties explored and octabutyltetrasila[2.2]terthiophenophane prepared, with Si-Si bonds of 236.3 pm247. 

Tetramesityldisilene adds to either cis- or treais-stilbaie oxide to give an E/Z mixture disilyl enol ethers, a 3-oxa-l,2-disilacyclq entane and a 3-oxa-l,2-disilacyclo nDpane 7. 

4-Benzo-l,l,2,2-Et4-l,2-disilacyc]obut-3-ene undergoes Pd(0) catalysed dimerisation to the l,2,3,6-tetrasilacycloocta-4,7-dieiie, and not the 1,2,5,6-isomer as first suq ected. 

HG/AICI3 give Et2SiQ2 and Cl(SiEt2)3Q. PhCHO and alkynes insert the Si-Si bond under similar conditions. In the presence of M(CO)5 (M=Cr,Mo,W), l,lX2-mtrafluoto-3-t-Bu- 

2- diene with one less Si atom dirough Me2Si extiusion.  

Palladium(ii) catalyses cleavage of MegSia by PhCOCl, giving the benzoylsilane, and (PhaP)4Pd the addition of chlorodisilanes to acetylene. 9-Fluorenyltris- 

Yamamoto, S. Suzuki, and J. Tsuji, Tetrahedron Lett., 1980, 1653 H. Matsumoto, I. Matsubara, T. Kato, K. Shono, H. Watanabe, and Y. Nagai, J. Organomet. Chem., 1980, 199, 43. 

Although organotin deuterides result from reaction of the chloride and LiAFH or from RsSnNa and HgO, better yields are given by halide/deuteride exchange [equation (8)]. Bu ,Sn H is best prepared from the tin-Grignard reagent (17) using HaO [equation (9)].  

Functionally-substituted allylstannanes result from the corresponding allyl-sulphone and Bu sSnH, which forms adducts with conjugated dienes, and reduces the keto functional group in a-keto esters. Hydrostannation of 1-alkynols 

AIBN initiates the radical addition of RgSnH to vinyl phosphines, the reduction of monochloromethyl polycyclic benzenoid hydrocarbons by PhgSnH, and of the isonitrile intermediate formed in the deamination of amino-acid esters.  

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All Group IV elements form tetrachlorides, MX4, which are predominantly tetrahedral and covalent. Germanium, tin and lead also form dichlorides, these becoming increasingly ionic in character as the atomic weight of the Group IV element increases and the element becomes more metallic. Carbon and silicon form catenated halides which have properties similar to their tetrahalides. 

Compounds with Tin—Tin Bonds. The most important class of catenated tin compounds is the hexaorganoditins. The ditin compounds are usually prepared by reductive coupling of a triorganotin haUde with sodium in Hquid ammonia ... 

Intramolecular heteroatom coordination may also influence the stabilities or structures of catenated tellurium compounds. For example, a rare example of a tritelluride, bis[2-(2-pyridyl)phenyl]tritelluride, is stabilized by a Te N contact of 2.55 The ditelluride (2-MeOCelFtCOTe) has an unusual planar structure. Although the C=0 Te interaction is longer (3.11 A) than the Me 0 contact (2.76 A), ab initio molecular orbital calculations indicate that the planarity results predominantly from the former intramolecular connection. 

The hydrides of the later main-group elements present few problems of classification and are best discussed during the detailed treatment of the individual elements. Many of these hydrides are covalent, molecular species, though association via H bonding sometimes occurs, as already noted (p. 53). Catenation flourishes in Group 14 and the complexities of the boron hydrides merit special attention (p. 151). The hydrides of aluminium, gallium, zinc (and beryllium) tend to be more extensively associated via M-H-M bonds, but their characterization and detailed structural elucidation has proved extremely difficult. 

The structures of metal-rich borides can be systematized by the schematic arrangements shown in Fig. 6.6, which illustrates the increasing tendency of B atoms to catenate as their concentration in the boride phase increases the B atoms are often at the centres of trigonal prisms of metal atoms (Fig. 6.7) and the various stoichiometries are accommodated as follows ... 

Figure 6.6 Idealized patterns of boron catenation in metal-rich borides. Examples of the structures (a)-(f) are given in the text. Boron atoms are often surrounded by trigonal prisms of M atoms as shown in Fig. 6.7.

The extent to which B3O3 rings catenate into more complex structures or hydrolyse into smaller units such as [B(OH)4] clearly depends sensitively on the activity (concentration) of water in the system, on the stoichiometric ratio of metal ions to boron and on the temperature (7-A5). 

The ability of C to catenate (i.e. to form bonds to itself in compounds) is nowhere better illustrated than in the compounds it forms with H. Hydrocarbons occur in great variety in petroleum deposits and elsewhere, and form various homologous series in which the C atoms are linked into chains, branched chains and rings. The study of these compounds and their derivatives forms the subject of organic chemistry and is fully discussed in the many textbooks and treatises on that subject. The matter is further considered on p. 374 in relation to the much smaller ability of other Group 14 elements to form such catenated compounds. Methane, CH4, is the archetype of tetrahedral coordination in molecular compounds some of its properties are listed in Table 8.4 where they are compared with those of the... 

The pseudohalogen concept (p. 319) might lead one to expect the existence of a cyanate analogue of cyanogen but there is little evidence for NCO-OCN, consistent with the known reluctance of oxygen to catenate. By contrast. 

Some metahrich silicides have isolated Si atoms and these occur either in typical metallike structures or in more polar structures. With increasing Si content, there is an increasing tendency to catenate into i.solated Si2 or SU, or into chains, layers or 3D networks of Si atoms. Examples are in Table 9.3 and further structural details are in refs. 24, 26 and 27. 

Catenation is well established in organotin chemistry and distannane derivatives can be prepared by standard methods (see Ge, p. 396). The compounds are more reactive than organodiger-manes e.g. Sn2Meg (mp 23°) inflames in air at its bp (182°) and absorbs oxygen slowly at room temperature to give (Me2Sn)20. Typical routes to higher polystannanes are ... 

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

Some of the alkali metal-group 15 element systems give compounds of stoichiometry ME. Of these, LiBi and NaBi have typical alloy stmc-tures and are superconductors below 2.47 K and 2.22 K respectively. Others, like LiAs, NaSb and KSb, have parallel infinite spirals of As or Sb atoms, and it is tempting to formulate them as M+ (E )" in which the (E )" spirals are iso-electronic with those of covalently catenated Se and Te (p. 752) however, their metallic lustre and electrical conductivity indicate at least some metallic bonding. Within the spiral chains As-As is 246 pm (cf. 252 pm in the element) and Sb-Sb is 285 pm (cf. 291 pm in the element). 

NaAs03 has an infinite polymeric chain anion similar to that in diopside (pp. 349, 529) but with a trimeric repeat unit LiAs03 is similar but with a dimeric repeat unit whereas /6-KASO3 appears to have a cyclic trimeric anion As309 which resembles the cyc/o-trimetaphosphates (p. 530). There is thus a certain structural similarity between arsenates and phosphates, though arsenic acid and the arsenates show less tendency to catenation (p. 526). The tetrahedral As 04) group also resembles PO4) in forming the central unit in several heteropoly acid anions (p. 1014). 

More extensive catenation occurs in the cyc/o-polyarsanes (RAs) which can readily be prepared from organoarsenic dihalides or from arsonic acids as follows ... 

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