Sandwich species

Finally, brief mention should be made of various mixed sandwich species for which magnetic data are available. Thus the (Cp)7l/(Bz) species are known for M = Mn, and Cr, the former being diamagnetic with a 12+ ground state, and the latter giving a moment of 1.70 B.M., virtually the spin-only value, thus indicating a 2Z+(a S4) ground level (113). 

As expected the band corresponding to the ionisation of the el Cot level lies at higher energies than for the e1 Ch level, which was in turn found at higher energy than for the e, Bz level, in other mixed sandwich species. 

The graph-theoretical 4N + 2 Hiickel rule analogy with the aromaticity of two-dimensional polygons requires that N = 0 in all the three-dimensional deltahedra. The Jemmis-Schleyer interstitial electron rule [55], originally introduced for nido half-sandwich species, also relates the 4N + 2 Hiickel rule to the delocalized deltahedra directly In this treatment, N is typically 1. 

Table II. Electronic Absorption Spectra of A) Cerium Bisporphyrinate Salts (1), B) Praseodymium Bis(tetra-p-tolylporphyrinate) Sandwich Species (14) (2 in nm, log in brackets, Hewlett-Packard spectrophotometer 8451 A NIR Zeiss photometer DMR 21)...

An alternative approach to aromaticity in some deltahedral boranes is based on the Jemmis and Schleyer [41] interstitial electron rule, originally introduced for nido half-sandwich species. This also relates aromaticity in deltahedral boranes to the 4A + 2 Hiickel mle, but for deltahedral boranes k is typically 0. 

By the description magnesium sandwich species are meant compounds with the general structural formula [(CH)m]nMgp (and their substituted and/or ionized derivatives) where m 3, n 2 and p 1. In addition, it is tacitly assumed that the (CH) rings are attached to the metal by at least three carbons, i.e., that they are species with k > 3. 

The classic (if not classical) metal sandwich species bis(cyclopentadienyl)iron has the formula [(CH)5]2Fe (CpaFe) and the semisystematic name ferrocene, and so the related magnesium-containing species [(CHfsJaMg (CpaMg) is often accompanied by the name magnesocene. We commence our discussion with this species. 

The species (CH)8Mg, or we should say its THF 2.5-solvate, is readily formed from cyclooctatetraene and Mg. The NMR spectrum shows eight equivalent ring atoms and so suggests either the cyclooctatetraene dianion and Mg salt or a putative (and highly fluxional) solvated magnesacyclopentene (or more properly magnesabicyclonon-atriene). However, there is no structural data for the rf open sandwich species and the enthalpy of formation of this simple and sensible half-sandwich, or tight ion pair, cannot even be estimated. 

This is not surprising—there are many textbook discussions of the difference between transition and main group elements. Consonant with this is the finding that (Cp )2Mg (the sandwich species alternatively called bis(pentamethylcyclopentadienyl)magnesium and decamethyfinagnesocene) does not react with CO, unlike the corresponding species with Ca and some other metals and metalloids . Indeed, in (Cp )2Mg, there is little room for another ligand around the central metal and Mg seems electronically satisfied. 

In the gas phase, the co-condensation of chromium vapors with azines and subsequent cooling (77 K) led to the first 7r-complexes of pyridine (44, in the presence of PF3) [75AG(E)273] and (45) (76JA1044). The gas-phase synthesis was utilized to prepare the parent bis(pyridine) sandwich species (46) (88CB1983). An attempt to prepare the -complex of pyridine was... 

The 15-crown-5 ring binds a larger range of. s-block ions than does 12-crown-4, and simple [M(15-crown-5)]" " or [LnM(15-crown-5)]+ (L = H20, halide, ether, acetonitrile, etc.) complexes are common. Sandwich species of the form [(15-crown-5)2M] (M = Cs, Ba ", ) are... 

The 18-crown-6 ether is widely represented among the i-block elements, and is found in a large range of compounds, either as the simple [(18-crown-6)M]" ion, coordinated with various anions ((18-crown-6)ML L = F[20, ethers, alcohols, HMPA, NH3, etc.) or as the sandwich species [(18-crown-6)2M]+. It is often thought to fit best with K or Sr, but Rb+ can sit in the center... 

The divalent metal derivatives of the A-type sandwich complexes (Table 14) are readily converted, e.g., by boiling aqueous solutions, to new sandwich species with the general formula [(XW9034)2M4(H20)2] that incorporate B-type lacunary anions. The first example of this type of anion, with X = P and M Co", had been reported in 1973212, but the range of heteroatoms... 

Although the elements of the lanthanide and actinide series have long been known to exhibit a quite extensive organometallic chemistry, it is only within the last decade that typical sandwich species have been prepared and studied. These systems however, although resembling the familiar metallocene and bis-arene compounds of the d-block elements, are not strictly their analogues since in both f-orbital series the known sandwich complexes are derived only from the cyclooctatetraenyl dianion. 

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