Bond complexes having

The dinuclear compounds of molybdenum(II) have strong, quadmple, molybdenum—molybdenum bonds and ecHpsed stmctures, such as those of Mo2CF g (Fig- 6 ) Mo2(02CCH3)4 (Fig. 6b). These quadmply bonded complexes have been intensely studied from the point of view of their... 

Similar hydrogen-bonded complexes have been detected kinetically in ester hydrolyses65. 

Many electrophilic agents have been employed for ring closure and the nature of the electrophile plays an important role. The mechanism of the electrophilic attack, however, has not been completely clarified13 15. Thus, in halogen attack, both the formation of an onium ion and a concerted mechanism with attack of the nucleophile on a halogen-double bond complex have been proposed for substrates containing an internal nucleophile. 

The rates of formation of some hydrogen-bonded complexes have been measured by the ultrasonic and high-field dielectric relaxation techniques,... 

Third, the AH" and AS" valnes of many hydrogen-bonded complexes have been obtained from van t Hoff plots where the temperature range AT was nsnally too small. Enthalpies and entropies calcnlated with AT = 10° for the complexes of 4-nitrophenol with amines are inevitably less reliable than those calcnlated with AT = 78° for snbstitnted phenols hydrogen-bonded to dhnethylacetamide or with AT = 57° for snb-stimted phenols complexed with diphenyl snlfoxide , simply becanse the error in AH" is inversely related to AT. 

The requirement for coordination at an equatorial position (endo attack) is a likely consequence of the fact that the electron density necessary to cleave the H-H bond accumulates at the intermetallic region of these complexes. In agreement with this, other unsaturated metal-metal bonded complexes have been shown to undergo facile oxidative addition of dihydrogen, suggesting that metal-metal bonds are convenient sources of electron density for such additions. The reported examples involve dinuclear complexes of electron-rich metal centers such as Ir(II) [30-32] and Re(I), [33] but also compounds of early transition metals such as W(III), [34] Ta(III) [35] and Zr(III).[36]... 

Fully N-bonded complexes have been isolated for most transition metal ions forming stable M—N bonds Co", " RuUi 770.77, Rh", - Ir", Cu", Ni", Pd", Pt" and... 

Various metal-metal single o-bonded complexes have been obtained by the reaction of metal carbonyls with metal-carbon o-bonded porphyrins or by the reaction of metal carbonyl anions and chlorometalloporphyrins (Scheme 14). For example, the reaction of dimanganese carbonyl and methyl indium(III) porphyrin gives manganese pen-tacarbonyl indium porphyrin In(Por)Mn(CO)5. The same compound is isolated when chloroindium porphyrin is allowed to react with the manganese pentacarbonyl monoanion. Various iron, cobalt, tungsten, and molybdenum complexes have been prepared by these two methods. 

To date the most extensive NMR studies of transition metal-carbon er-bonded complexes have dealt with platinum compounds. Here an extensive... 

ESR spectra of many a metal-carbon bonded complexes have been investigated (Table 4.8). Numerous four-coordinate MR4-type complexes have symmetry lower than tetrahedral, for example, [Cr(CH2SiMe3)4] , [V(CH2SiMe3)4], [Cr(CH2SiMe3)4], etc. The compound ReMe has a structure of a distorted octahedron because of the Jahn-Teller effect. In contrast to ReF, the distortion of ReMeg is not dynamic. This situation may be due to a considerably larger lODq value for the methyl complex compared to the fluoro derivative. Based on ESR spectra, a square-antiprism structure was proposed for [ReMcg] . 

It is important to note that these data can be retrieved from measurements of IR spectra at room temperature in solution or even in the soUd state and need just a comparative analysis of the spectra of proton donor with and without base (Fig. 10). Application of Eqs. 12 does not require the knowledge of concentrations, but one should keep in mind that the base excess makes it easier to observe. Measurements at different temperatures provide not only the enthalpy (AHhb) but entropy (AShb) of hydrogen bond formation as the parameters of the temperature dependenee of hydrogen bond formation eonstant AThr. The AThr values (Eq. 4) can be obtained from the intensity drop of vah band (Fig. 10). In this case the knowledge of proton donor and proton acceptor concentrations is vital. In the absence of proton donor self-association (relatively low HA concentrations) the hydrogen bonded complexes have 1 1 composition (one HA molecule interacts with one molecule of proton acceptor) and calculations are straightforward. 

A range of mercury-transition metal bonded complexes have been subjected to X-ray structural analysis complexes with M-Hg-M links include CHg cis-RuMe(PMe )4 [Hg Pt(2,4,6-CgH2Cl3)(PPh-)2 2 ... 

Isomer selectivity At a finer level of discrimination, vdW complexes or clusters of a given size may occur as a distribution of structural isomers. Isomers of van der Waals and H-bonded complexes have been detected by rotationally resolved UV or IR spectroscopy [29-34], as have isomers of vdW solvent clusters with aromatic molecules [34-38]. The ionization potentials of vdW isomers can differ substantially [17,18,39], allowing mass- and isomer-selective electronic spectroscopy to be performed in a mixture of clusters by selective-ionization (SI) two-color R2PI combined with mass spectrometry in a mixture of clusters [17]. This increase in selectivity is quite general and may also be applied in IR-UV and microwave-UV excitation-ionization schemes. 

The development of the molecular homobimetaUic chemistry of G12 had a slow start. The first molecular Cd-Cd- and Hg-Hg-bonded compounds were reported in the 1990s and the first molecular Zn-Zn bond was not discovered until 2004. This breakthrough, however, stimulated intensive investigations on this topic and several research groups have successfully synthesized further Zn-Zn-bonded complexes, investigated their reactivities, and even found applications in catalysis. Some new Cd-Cd- and Hg-Hg-bonded complexes have also been prepared, although the number of these compounds is stm relatively small. 

Even G12-Tc-bonded complexes have been prepared. The tris(arylimido)technitium anion (90) reacted readily with 0.5 or 1 equiv. of HgBr2 to afford [Hg Tc(NAr)3 2] (91) or [BrHgTc(NAr)3] (Ar = 2,6-diisopropylphenyl) (92), respectively (Scheme 12.29) [70]. The crystal structure of Hg[Tc(NAr)3]2 shows a linear geometry around the mercury atom and the bond length of Tc-Hg is 2.615(1) A. Also the rhenium analog [Hg Re(NAr)3 2] (93) was obtained through reduction of tris(arylimido)rhenium chloride with sodium amalgam (Scheme 12.29) [70]. 

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