Halo bridges

Halides are ubiquitous co-ligands for cobalt(III), and are met throughout this review. Anation of (solvent)cobalt(III) complexes by halide has been examined from time to time. An example is substitution of coordinated acetonitrile in [Co(L)(MeCN)2]3+ (L = tetraaza-macrocycle) by Cl-and Br-.1096 A mechanism involving interchange from within tight ion pairs was proposed. Halo-bridged polymeric complexes are well known with both classical and organometallic complexes. 

Dinuclear Ru and Os complexes containirm halo bridges include a number of fluoro carbonyl complexes, described in the last section. The reactions between anhydrous HF and [MH2(CO)2(PPh3)2] (M = Ru, Os) produce cix,cA,tra i-[MF2(CO)2(PPh3)2], but when the precursor is [MH2(CO)(PPh3)3], the dinuclear complexes [M2(CO)2(PPh3)4(/u-F)3]" result. Initially, these form as the [Hp2] salts, but are better isolated as the air-stable [BPh4] salts. " ... 

The insolubility of the Cu(I) halide salts, as well as the possibility of /x-halo-bridge formation between two copper atoms (as discussed later) or between the copper complex and the electrode surface, suggests that the presence of hahdes may alter the electrochemical properties observed for copper-containing solutions. 

These complexes can be prepared from phosphorus ligands that are themselves substituted by electron-withdrawing groups. It is often beneficial, however, to employ a low ratio of phosphorus ligand to rhodium since in several instances excess ligand cleaves the halo bridges and forms four-coordinate monomeric complexes. Table 2 lists those complexes of this structure that have been isolated. 

The kinetics of the formation of [RhCl(PPh3)2]2 from [RhCl(PPh3)3] have been studied.91,92 The dimeric complex has the planar structure (11) this is similar to that found for [RhCl(cod)]2, but differs from the folded structure adopted by [RhCl(CO)2]2.93 Details of the far IR spectra, which indicate similar halo-bridged structures are collected in Table 3. 

Complexes such as [Ni(ROH)6] + and [NiX2(ROH)4] can be generated under anhydrous conditions, although they tend to be water sensitive and can be readily hydrolyzed. O-Donor ligands such as alcohols, ethers, and ketones are in general very poor donors to NF and so their complexes are labile and often unstable. The structure of [NiBr(EtOH)4]+ (75) shows it to be a halo-bridged dimer. 

Several complexes have beeai found to be dimeric. Infrared spectroscopy shows that the CuNCS(L) complexes (L = py, 2- or 4-picoline, 3,5-lutidine, or quinoline) contain bridging thiocyanato groups (332). The mass spectra of CuX P(cyclohexyl)3 complexes (X = Cl, Br, I) show no peaks of higher mass than those corresponding to dimeric molecules, and the far-infrared spectrum of the chloro complex indicates that they are dimeric with halo bridges (248). Triphenylphosphine reacts with copper(I) trifluoroacetate in dichloromethane to give [Cu(02C CF3)(PPh3)]2 which is dimeric in chloroform but partially dissociated in dichlorobenzene (107). 

The cod complexes react with allyl halides to give yellow, crystalline, halo-bridged complexes, Ru2X2(allyl)2(cod)2 (X = Cl, Br), and in boiling methanol a suspension of Ru(2-methylallyl)2(cod) reacts with triphenylphosphine with displacement of the diene ligand to give poorly soluble, pale yellow Ru(2-methylallyl)2(PPh3)2 [10],... 

In 1965, Murdoch described the synthesis of a series of halo-bridged allyl carbonyl complexes of the type [NEt4][M2(/a-X)3(CO)4(17 -03115)2] (5), which are prepared by reaction of [NEt4][MX(CO)5] with C3H5X. A year later, Murdoch and Henzi described the bridge-splitting reactions of (5) (M = Mo, X = Cl) with nitrogen donors such as pyridine or bipy to yield complexes of the type 6. 

The reaction of Cp2Tav(H)3 with IrIX(CO)(dppe) (X = Br, I dppe= l,2-bis(diphenylphos-phino)ethane) was rapid and led to clean formation of /uc-Irm(H)3(CO)(dppe) and Cp2TamX or Cp2TamXL (in the presence of added L = CO, C2H4, PrC=CPr).4 6 The total results were consistent with the formation of hydrido- and halo-bridged intermediates in which transfer of all hydrido and halo ligands occurred before dissociation into the mononuclear products. 

CpTi(NCMe)s]3+ by a crystal structure determination. A significant /raw-influence of the cyclopentadienyl ligand affects Ti-N bond lengths in the complex. Proton NMR studies indicate the presence of intermediate halo-bridged species in solution during successive halide abstractions 1 —> 2 3.498... 

Four chiral homologous complexes were also prepared. None of the palladium complexes showed mesomorphic properties, whereas a monotropic chiral discotic nematic phase was observed for the platinum complex (Table 18). The absence of mesomorphism for the dinuclear palladium complexes may be due to the type of chiral chain used, which differed from that used for the platinum system. All of the complexes form charge-transfer complexes with TNF. A Colh phase was induced for the two halo-bridged palladium complexes and for the platinum complex, as was observed for their non-chiral analogs. Flowever, the chiral nematic phase of the platinum compound was suppressed. At low TNF content, a chiral Nq phase was stabilized for the thiocyanato-bridged compound along with a non-chiral No phase at higher concentration. 

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