Bridged Dimers

Linearly bridged dimers are complexes having the M-X-M structural type. One of the more common ions that act as the bridging group X is the cyanide ion. 

The trimetallic cyanide-bridged complexes (bpy)(H20)Pt°-NC-M°(CN)4-CN-Pt°(H20)(bpy) (M = Fe, Ru, Os) have been prepared. These complexes, however, show a characteristic intense MLCT band that involves an electronic 

This complex then dissociates to give Fe(CN)6 and Pt(NH3)4 (Refs. 11,12)  

When iron is replaced by ruthenium or osmium the trinuclear anions are photo-chemically inert. For these compounds, once the photoexcited species M -Pt -M° is formed, the rate constant for the formation of products is much smaller than for Fe, and the thermal reaction to regenerate the trinuclear complex is favored by a lower activation barrier.  

it does not involve a preadjustment of the coordination spheres of the different metal centers in order for electron transfer to occur. 

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Halides. Gold(III) chloride [13453-07-1] can be prepared directiy from the elements at 200°C (167). It exists as the chlotine-bridged dimer, Au2Clg ia both the soHd and gas phases under an atmospheric pressure of chlorine at temperatures below 254°C. Above this temperature ia a chlorine atmosphere or at lower temperatures ia an iaert atmosphere, it decomposes first to AuCl [10294-29-8] and then to gold. The monochloride is only metastable at room temperature and slowly disproportionates to gold(0) and gold(III) chloride. The disproportionation is much more rapid ia water both for AuCl and the complex chloride, [AuCy, formed by iateraction with metal chlorides ia solution. 

Mono- and dialkylboranes usually exist as bridged dimers, yy -dialkyldiboranes and yy -tetraalkyldiboranes. Only very hindered alkylboranes, eg, bis(2,3-dimethyl-2-butyl) borane (39), are monomeric. However, for convenience of presentation monomers are shown in the equations. 

Attempts to prepare the mono(cyclopenta-dienyl) derivatives are sometimes frustrated by a Schlenk-type equilibrium (see p. 132), but judicious choice of ligands, solvent etc. occasionally permits the isolation of such compounds, e.g. the centrosymmetric halogen-bridged dimer (t - -C5Me5)Ca(/i-l)(lhf)2 2] which cry.stallizes from toluene solution. The complex is isostruc-tural with the dimeric organosamarium(ll) analogue. - ... 

InPhj mp 208°, TlPhj mp 170°. For Ga and In compounds the primary M-C bonds can be cleaved by HX, X2 or MX3 to give reactive halogen-bridged dimers (R2MX)2. This contrasts with the unreactive ionic compounds of T1 mentioned above, which can be prepared by suitable Grignard reactions ... 

Ph3TeCl is a chloride-bridged dimer with 5-coordinate square-pyramidal Te (Fig. 16.13c). 

The M(05H5)201 compounds, which are actually 01-bridged dimers, [(05H5)M(/l-01)2-M(05H5)], provide an extensive substitution chemistry in which /r-Ol can be replaced by a variety of ligands including H, ON, NH2, MeO and alkyl groups. 

In /r-bridged dimers, two phthalocyanine moieties are linked by one or more bridging atoms such as oxygen or halogens. Niobium(V) oxide tribromide (NbOBr,) when treated with phthalonitrile in 1-chloronaphthalene at 230CC for 3 hours produces /r-bromo-/t-dioxobis[nio-bium(V) phthalocyanine] tribromide in 52%.232... 

Reaction of Rh2(OCOR)4 with dimethylglyoxime leads to a non-bridged dimer [81]. 

Cp derivatives (Scheme 185) to form -oxo-bridged dimeric complexes. The stabilities of the Af,0-bound carbamate intermediates depend mainly on the cyclopentadienyl ring substituents, but there is also a small dependence on the nature of the length of the (dimethylamino)alkyl chain for the C5H4Me systems. ... 

Two chloro-bridged dimeric vanadyl(V) amidinates, [ PhC(NPr )2 VOCl2]2 and [ PhCH2C(NPr )2 VOCl2]2 have been reported to promote the polymerization... 

The Mossbauer spectrum of the rearranged compound corresponds with that of a Sn(IV) compound, and the most probable structure appears to be that of a chlorine-bridged dimer. 

Reactions of stannylenes Sn[N(SiMe3)2][(NCy)2CR] (R=Me, t-Bu) stabi-hzed by bulky trialkyl amidinates with elemental sulfur proceeded efficiently to afford the corresponding tetrathiastannolane Sn[N(SiMe3)2][(NCy)2CR]S4 [23]. This is in sharp contrast to the reaction of M[N(SiMe3)2]2 (M=Ge, Sn) with elemental sulfur giving the bridged dimer [M N(SiMe3)2 2S]2 [24, 25] (Scheme 2). 

Complexity in the manganese and rhenium pentarbonyl halides substitution arises from the fact that these decompose in inert solvents to form the halogen-bridged dimers [M(CO)4X]2. Both monomers and dimers react with phosphines, arsines, pyridine, aniline etc. to give the disubstituted compounds M(CO)3XL2. Thus three final products or any mixture of them may be obtained, depending upon the precise reaction conditions... 

A series of complexes of type [Ni(T] -allyl)Cl(NHC)] highlight the important influence of the NHC on the reactivity of the resulting complex towards O. It was shown that 0,-activation is disfavoured when the rotation around the Ni-C, bond is restricted. On the other hand, with complexes displaying free rotation around the Ni-Cj, bond, the complexes react cleanly with O. The overall reaction results in the oxidation of the allyl group and the formation of hydroxy-bridged dimers (Schane 10.4) [14,15]. 

In studies involving the reaction of a cA-[PdCl(Me)(NHC)]j chloro-bridged dimer with CO it was demonstrated that reductive elimination is also extremely facile for NHC-Pd-acyl complexes, yielding 2-acylimidazolium salts and Pd(0) (Scheme 13.4) [22]. The product distribution was shown to depend on the stracture of the complexes from which reductive coupling took place (pathways A and B, Scheme 13.4). 

The hydroxo complexes [Au(6-Rbipy)(OH)2], postulated as intermediates in the formation of 44c-i, are not isolated nevertheless, in all the mass spectra (FAB conditions) a weak peak is found corresponding to these species. Unchanged 44c-i are quantitatively recovered from the reaction with aqueous solutions of HX (X = BF4 or PFfi) neither dihydroxo complexes similar to those observed in the vapor phase, nor hydroxo bridged dimers are obtained [25]. 

High-valent iron also occurs in -nitrido bridged dimers with linear [Fe °-N=Fe" ]" and [Fe =N=Fe ] " cores [209, 210] (and references therein). Such compounds have been prepared first by thermolysis [247] or photolysis [248] of iron(III)-porphyrin complexes with an azide ligand, (N3). Mixed-valent iron-nitrido porphyrin dimers exhibit valence delocalization as can be inferred from the... 

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