Halogeno-Metal Complexes

Kruck and co-workers (174) have shown that reductive fluorophos-phination of transition metal halides with high pressures of PF3 in the 

The dihalogenotetrakis(trifluorophosphine)iron complexes can readily be obtained by direct oxidation of [Fe(PF3)5] with the appropriate halogen under mild conditions (method D). NMR studies establish that the halogens lie in cis-positions of the octahedron. 

Other synthetic routes reported involve the interactions of trifluoro-phosphine metallates and iodine (method E), displacement of carbon monoxide, alkenes, etc. by PF3 from the corresponding halide complexes (method F), addition of PF3 to dinuclear halogeno-bridged PF3 complexes at low temperatures (method G), and treatment of a metal hydrido- PF3 complex with iodoform (method H). 

At low temperatures the intermediate /3-allylrhodium(III) hydride PF3 complex was detected by 1H and 19 F NMR spectroscopy and provided supportive evidence for the isomerisation of alkenes via an allyl-metal-hydride mechanism (285, 293). 

The dinuclear complexes [MX(PF3)2]2 (M = Rh, Ir X = Cl, Br, I) are sufficiently volatile for their He(I) and He(II) photoelectron spectra to be recorded (288, 289) and the data have assisted in the assignments of bands in the photoelectron spectra of the corresponding carbonyl complexes for which transition state SCF-Xa-SW calculations of ionization energies have been made. 

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Gupta has attempted to rationalize rates of electron-transfer reactions involving halogeno-metal complexes in terms of Sanderson s concept of partial charge. For reactions of the type Cr + or Eu + with CrX +, a linear free-energy relationship is found ... 

It is the only example of a free, persistent phosphirenylium ion, and also, only one stable transition-metal complex of this species was published [78,79]. Quantum chemical calculations [80,81] indicated that in the halogeno-phosphirenes the P-X bonds already possesses a high ionic character and can be described as interactions between phosphirenylium and halide ions. The aromatic character of the phosphirenylium ion was shown to be based on a three-centre two-electron bond of 7i-type and the resonance energy was assessed by calculation to 38 kcal/mol. Before the generation of 32, substituted phosphirenylium ions were... 

The reaction of [LnM-SiR3]" with transition metal halides or halogeno complexes L mM Xx gives silyl-substituted heteronuclear complexes [Ln(R3Si)M]xM L m [2,5,18]. Stable complexes are usually only obtained if the metal complex fragment M L m is not too sterically demanding and if the resulting M-M bond is not too polar. A few examples are shown in Scheme 3. 

Finally, a few cyclizations of unsaturated side chains on o-halogeno-anilines or -benzenes have been catalyzed by transition metal complexes. Cyclization of the cinnamylbenzylamine (245) by palladium gives some 4-benzylisoquinoline and some of compound (246) (77TL1037). Acryloylanilines (247) and (248) can be cyclized by a nickel complex (75MI20800) or by a palladium complex (79JA5281). The mechanism for the latter reaction is given in equation (50). 

Other synthetic routes to alkene-metal complexes reported in the literature involve bridge cleavage reactions of halogeno complexes (method F), facile intermolecular scrambling reactions involving metal-alkene and metal-PF3 complexes (18, 81) (method G) (Scheme 4), and the coupling together of two alkyne units to afford a metallo-cyclopentadiene derivative (method H). 

The imino complexes are prepared by (i) insertion into complexes containing a metal-carbon bond, (ii) oxidative addition of RX to zero-valent isonitrile complexes and subsequent insertion of the coordinated isonitrile, (iii) nucleophilic reaction of R with halogeno-metal isonitrile complexes and (iv) reaction of anionic isonitrile complexes with alkyl halides. 

The transmetalation causes partial or total replacement of halogeno ligands in the starting transition metal complexes with organic ligands. 

The reaction of halogeno transition metal complexes with [Cu(CsCR)] produces heterobimetallic complexes that have a bridging alkynyl ligand bonded to Cu in j7 -fashion and to the other transition metal in J7 -fashion (Scheme 5.34) [176-178]. The structure of the products suggests initial coordination of alkynyl ligand to transition metal to form the intermediate dinuclear complex with a or )7 )7 -coordinated alkynyl ligand. It is soon transformed into the product... 

Apparently, halogeno-transition-metal complexes supported on phosphine-modified silica carriers are more active than their homogeneous counterparts by a factor of 2—4 orders of magnitude, as measured by their effectiveness on the hydrogenation of cyclohexene. ... 

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