Sandwich compounds,

Sandwich Compounds Containing Linked Cyclopentadienyl or Aryl Groups 

A great deal of effort has been devoted to the study of linked metallocene units in order to probe the redox properties of compounds containing two or more identical electron-transfer sites much of the interest in these systems stems from the fact that one-electron changes lead formally to mixed-valence species. Iron compounds, including biferrocene 1 (M = Fe), bridged biferrocenes 2 [X = (CH2) , S, Hg, etc.], bis(ful-valene)diiron 3 (M = Fe), ferrocenophane 4 (M = M = Fe), and their derivatives, have attracted the most attention. 

Biferrocene 1 (M = Fe) oxidizes in two separate one-electron steps (6, 7) (Ei/2 = 0.31 and 0.64 V), of which only the first is chemically reversible (i.e., the monocation is stable but the dication is not) mild oxidation yields the monocation [l] (M = Fe) as a mixed-valence Fe(II)Fe(III) species (8-10). Bis(fulvalene)diiron 3 (M = Fe) similarly undergoes two successive one-electron oxidations (the dication is also stable), but in this case the Mdssbauer spectrum of the monocation shows the iron atoms to be equivalent, implying a delocalized electronic structure and an oxidation state of 2.5 for each metal (9,11). The separation of 

The electronic absorption spectrum of [BFcJ shows a broad band at 2200 nm, assigned to a charge-transfer transition and supporting the formulation of the complex as a zwitterion containing three neutral Fe(II) ferrocenyl groups, one cationic Fe(III) center, and, at least formally, a negatively charged boron atom (20). 

The cyclic voltammogram of [BFC4] shows four separate waves, implying that delocalization through the central boron is greater than through the central tin atom in the chromium compounds mentioned above (79). The reported electrochemical preparation (20) of the monocation [BFc4] is, however, hard to reconcile with the published cyclic voltammetric data, which show that several of the waves have shifted in potential after electrolysis. 

The cyclobutadiene derivatives [Co(i7 -C4Fc Ph4 )Cp] (n = 1-3) (Cp = 17 -cyclopentadienyl) undergo one-electron oxidation to stable cations (27) which show charge-transfer bands in the near infrared region. An analysis of the spectra led to the conclusion that the cations are Class II mixed-valence compounds and that the charge-transfer transition arises from a weak interaction between Co(I) and Fe(III) rather than between Fe(II) and Fe(III). Each compound showed further oxidations at more positive potentials, including an irreversible process ascribed to electron loss from the cobalt center. 

For the purposes of this article, a sandwich compound is regarded as one in which a metal is n bonded to two planar, delocalized rings. Such compounds almost invariably undergo electron-transfer reactions which are, however, rarely uncomplicated. The enhanced reactivity of the more reduced and oxidized sandwich complexes is finding increased synthetic application. 

Iodine and [TiCp(f/-cot)] yield the air-stable monocation as an iodide or triiodide salt (408). 

Vanadocene, [VCp2], is oxidized and reduced in one-electron steps to highly reactive ions. The reversible formation of [VCp2] at the very 

Although [V(tj-arene)2] has not been studied electrochemically, both the cation and the anion are well authenticated. Indeed, [V(t/-arene)2]+ is the precursor to the neutral compound, being formed in the reaction between VC14 and the arene under Friedel-Crafts conditions (409). 

The addition of lithium naphthalide to [V(r -arene)2] leads to the disappearance of the ESR spectrum typical of the neutral vanadium radical. The postulated (410, 411) one-electron reduction was confirmed by NMR spectroscopy which showed that the blue solution generated when [V( /-CeH6)2] reacts with potassium in 1,2-dimethoxyethane contains [V( j-C6H6)2r (412). 

---

C, b.p. 249 C. The prototype of metallocenes, a typical cyclopentadienylide. A sandwich compound. Oxidized to the blue ferricinium cation ((h —CsHjijFe)". ... 

TC-Cyclopentadienyl Nickel Complexes. Nickel bromide dimethoxyethane [29823-39-9] forms bis(cydopentadienyl)nickel [1271 -28-9] upon reaction with sodium cyclopentadienide (63). This complex, known as nickelocene, 7T-(C3H3)2Ni, is an emerald-green crystalline sandwich compound, mp 173°C, density 1.47 g/cm. It is paramagnetic and slowly oxidi2es in air. A number of derivatives of nickelocene are known, eg, methylnickelocene [1292-95-4], which is green and has mp 37°C, and bis( 7t-indenyl)nickel [52409-46-8], which is red, mp 150°C (87,88). 

Several compounds such as BaZrS [12026-44-7], SrZrS [12143-75-8], and CaZrS [59087-48-8], have been made by reacting carbon disulfide with the corresponding zirconate at high temperature (141), whereas PbZrS [12510-11-1] was produced from the elements zirconium and sulfur plus lead sulfide sealed in a platinum capsule which was then pressurized and heated (142). Lithium zirconium disulfide [55964-34-6], LiZrS2, was also synthesized. Zirconium disulfide forms organometaUic intercalations with a series of low ionization (<6.2 eV)-sandwich compounds with parallel rings (143). 

Many stmctures are possible for the smaller metaHacarboranes and various synthetic strategies are available. Especially noteworthy is the occurrence of triple- and tetradecker sandwich compounds (214). The polyhedral expansion synthetic strategy can also be used with small carboranes (212). Eor instance, the small metaHacarborane /(9j (9-l,l,l-(CO)3-l,2,3-EeC2B4H3 [32761-40-3] (212) is obtained from C2B4Hg upon treatment with Ee(CO)3. 

TThe x-ray stmcture of the novel bis-dicarboUide sandwich compound i (9y y (9-3,3 -Si(3,l,2-SiC2B2H22)2 contains a siUcon atom iu a highly unusual bonding mode (245). TThis compound, which is quite stable thermally, is prepared iu good yield. 

R)C2B4H ions and appropriate main group element haUdes, have stmctures containing central main group elements iu the 4+ oxidation states similar to the bis-dicarboUide sUicon sandwich compound. TThe stmcture of the sUicon sandwich compound commo- ](GH.])fi fZ, fri fii is shown iu Figure 27. 

Other types of bonding include donation by Ligand TT-orbitals, as in the classical Zeiss s salt ion [Pt( 7 -CH2=CH2)Cl3] [12275-00-2] and sandwich compounds such as ferrocene. Another type is the delta (5) bond, as in the Re2Clg ion, which consists of two ReCl squares with the Re—Re bonding and echpsed chlorides. The Re—Re 5 bond makes the system quadmply bonded and holds the chlorides in sterically crowded conditions. Numerous other coordination compounds contain two or more metal atoms having metal—metal bonds (11). 

The sandwich compounds, feiTocene and nickelocene, FefCg 115)2 and Ni(C5H5)2, so called because die metal atom is sandwiched between the organic radicals, C5H5, have also been used to prepare iron and nickel deposits. 

Although the chemistry of zirconium in its lower oxidation states is still relatively unexplored, it is developing. Examples which offer the possibility of further exploitation include the blue, paramagnetic zirconium(III) compound 32) [L2Zr(/r-Cl)2ZrL2] L = C5H3(SiMe3)2-l,3, and the sandwich and half-sandwich compounds derived from cycloheptatriene red... 

At about 159°C it turns pink and adopts the sandwich structure, expected for [M(C5H5)2] compounds, and this is retained in the gaseous phase and in hydrocarbon solutions. Using substituted cyclopentadienyls a variety of analagous sandwich compounds have been prepared and their magnetic properties indicate that the... 

Numerous half-sandwich compounds of the type [M()7 -C5R5)L2], M = Rh, Ir R = H, Me L = CO, phosphine etc.) are known and are useful reagents. [Ir()7 -C5Me5)(CO)2] for instance is an excellent nucleophile and is also used in the photochemical activation of C-H in alkanes. It is particularly effective in the latter role when supercritical CO2 is the solvent. ... 

E. O. Fischer (Munich) and G. Wilkinson (Imperial College, London) pioneering work, performed independently, on the chemistry of the organometallic so-called sandwich compounds. 

Irradiation of tricarbonyl(r(4-cyclobutadienc)iron(0) with ethyl 1//-azepine-l-carboxylate results in the formation of the novel sandwich compound (t 4-cyclobutadiene)[l-(ethoxycar-bonyl)-1 //-azepine]iron(0) (27).223... 

Many other shapes are possible for complexes. The simplest are linear, with coordination number 2. An example is dimethylmercury(O), Hg(CI l,)2 (4), which is a toxic compound formed by bacterial action on aqueous solutions of I Ig ions. Coordination numbers as high as 12 are found for members of the / block, but they are rare in the d block. One interesting type of d-mctal compound in which there are 10 links between the ligands and the central metal ion is ferrocene, dicyciopentadi-enyliron(O), [Fe(C5H5)2] (5). Ferrocene is an aptly named sandwich compound, with the two planar cyclopentadienyl ligands the bread and the metal atom the filling. The formal name for a sandwich compound is a metallocene. 

Another type of five-membered aromatic compound is the metallocenes (also called sandwich compounds), in which two cyclopentadienide rings form a sandwich... 

Interactions between non-halogen-containing IIIB compounds and transition-metal complexes are found in 6.5.3. Most of these IIIB compounds are boron-containing heterocycles. A series of interesting sandwich compounds, including triple- and tetradecked complexes, are synthesized by methods in 6.5.2.1-6.5.3. 

Both T)5-CpCo(Me2C2B2Me2CH2) and r -CpRh(Et2C2B2Et2CHMe) have been used to synthesize special 2,3-dihydro-1,3-diborole sandwich compounds. Via the anions [i7 -CpM(R2C2B2R 2CR")] , available by reduction with K, the AuPPhj derivatives are obtained ... 

Mixed cyclopentadienyl-thiadiborolene tetradecked sandwich compounds are available using a novel zinc complex, e.g. ... 

Each of the sandwich compounds forms two isomers, described as clockwise and counterclockwise, respectively. Clockwise means that the atomic sequence in both rings is the same, counterclockwise that the atomic sequence is opposite. The syntheses occur best in THE at -78°C. After warming, the solvent is removed. Purification can be carried out by crystallization from petroleum, ether or better by sublimation at 60-70°C and 10 torr. The yields vary between 25 and 85%. The 17-and 18-electron complexes with V and Fe atoms show the metal atoms to be fixed above and below the ring centers. In contrast, the 19- and 20-electron complexes of Co and Ni possess slipped rings. 

Recent work on [CpFe(CO)2]2 was intended to test whether once again a complex molecule could be found to have a high yield and also to test a possible preferential formation of metal carbonyls over metal sandwich compounds. In this compound, thermal decomposition of the starting compound gives rise predominantly to ferrocene (28, 68). The data (50) given in Table VIII show that indeed the carbonyl is preferentially formed... 

The borole ring and various 1,3-diborolyl anions have been extensively employed as ligands to prepare a huge array of transition metal complexes and multidecker sandwich compounds.96 97 Inevitably, the electronic character of the borole is profoundly changed upon complexation, so a study of such complexes can reveal nothing certain about B—C multiple bonding in the isolated ligand. 

Goti, A. et al., J. Organomet. Chem., 1994, 470(1-2), C4 This half-sandwich compound is extremely pyrophoric. 

This sandwich compound, nominally the Ce(IV) derivative of cyclooctate-... 

E(d 2) ligand field interpretations of the magnetic properties of sandwich compounds. 

The considerable number of molecular orbital calculations which have recently been made for sandwich compounds are however considered in some detail in Section 6. This has been done in order to make clear the relationship between the ligand field and molecular orbital approaches, and also to indicate the need for the use of a more sophisticated molecular orbital scheme than that adopted in this Introduction, i.e. one in which the a-framework of the rings is specifically included in the basis set as well as the rr-type orbitals. 

---