Complexes, dihapto

Re(Tp)(CO)( BuNC)(2,3-ri2-5-R-furan)] (R = H, Me), obtained as a mixture of coordination diastereomers, reacts with aldehydes RC(=0)H (R = Me, Ph, 3-furyl) in the presence of BF3 OEt2 to yield dihydrofuran complexes. Dihapto-coordinated trans-2-alkyl-3-acyl-2,3-dihydrofuran, tr[Pg.131]

The solvated sulfenamides [Li2( BuNSC6H4Me-4)2(THF)n] (n = 2,4) have dimeric structures with a central Li2N2 ring. The coordination mode is determined by the extent of solvation of the Li" ions monosolvation allows for rj -N,S coordination whereas disolvation restricts the coordination mode to // -M Variable temperature NMR studies indicated that a dynamic exchange between these two structural types occurs in THF solution (Scheme 10.10). The dihapto coordination mode is observed exclusively in transition-metal complexes and the... 

Stable U ansition-metal complexes of dihapto-dihydrogen (ij -H2) discovered by G. Kubas. 

The crucial experiment suggesting that the H2 molecule might act as a dihapto ligand to transition metals was the dramatic observation that toluene solutions of the deep purple coordinatively unsaturated 16-electron complexes [Mo(CO)3(PCy3)2] and [W(CO)3-(PCy3)2l (where Cy = cyclohexyl) react readily and cleanly with Ha (I atm) at low temperatures to precipitate yellow crystals of [M(CO)3H2(PCy3)2] in 85-95% yield. The... 

The carbonate ion, CO3-, by contrast, is a classic Werner ligand whieh forms innumerable complexes as a monohapto, dihapto or bridging donor. Examples of this latter mode... 

In the dihapto mode the pyridine ring can be protonated intermolecularly at nitrogen, or even intramolecularly deprotonated at carbon. The first evidence for metal C—N insertion is the reaction of the metallaaziridine complex (111) with homogeneity LiHBEt3 in THF at low temperature that yields (112) (Scheme 49).251-254 Experiments with carbon nucleophiles (RMgCl, MeLi) in place of LiHBEt3 have provided valuable information to allow discrimination between... 

The centrosymmetrical complex 154, formed by interaction of the quinolyl ligand with HgCl2, features intramolecular coordination of one of the cyclopentadiene C=C bonds to the mercury center. The resulting Hg-C(sp2) distances of 2.81 and 2.88 A are longer than typical Hg-C c-bonds but remain much shorter than the sum of the van der Waals radii. This dihapto interaction does not result in a significant lengthening of the coordinated C=C... 

In view of the fact that early transition metal alkyls insert CO under very mild conditions (2, 5.) we chose to examine the reactions of electron-rich metal hydrides ( ) with the resultant dihapto acyl complexes. Such acyls obviously benefit from reduction of the CO bond order from three (in OO) to two. More significantly, the dihapto binding mode will significantly enhance the electrophilic character of the acyl carbon. 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

In summary, the detailed electronic character of dihapto metal-acetylene complexes depends strongly on the Lewis-acceptor capacity of the metal. Formal two-versus four-electron rp ligation to a transition metal can lead to breaking of one or both 7T bonds, dramatically altering the structure and reactivity of the alkynyl... 

Experimental evidence pointing to intermolecular methanolysis, or at least, not excluding this termination path has been reported as well. An in situ NMR investigation showed in fact that the Pd" acyl complexes [Pd(C(0)Me)(NCMe)(dppf)]OTs and [Pd(C(0)Me)(dppomf)]OTs, containing dihapto and trihapto ligands, respectively, undergo fast methanolysis at room temperature (dppomf = l,T-bis(diphe-... 

Whereas the reaction of Cjq with RhH(CO)(PPh3)3 results in complex formation in a dihapto fashion, a hydrometalation is possible with the stronger nucleophilic reagent Cp2Zr(H)Cl (Cp = r -C5H5) [81] (Scheme 7.14) [82], Upon treatment of... 

The first organometallic compound of the transition metals to be characterized (1827) was Zeise s salt, K[(C2H4)PtCl3]-H20 (Fig. 18.1). It forms when K2[PtCl4] in aqueous ethanol is exposed to ethylene (ethene) a dimeric Pt—C2H4 complex with Cl bridges is also formed. In both species, the ethylene is bonded sideways to the platinum(II) center so that the two carbon atoms are equidistant from the metal. This is called the dihapto-or T]2 mode. A ligand such as an allyl radical with three adjacent carbons directly bonded to a metal atom would be trihapto- or t 3, and so on. 

A thermally robust dihapto C02 complex was easily obtained according to equation (62).631 Complex (46) is the first authentic (X-ray diffraction) C02 complex of a hard oxophilic early transition element. The C02 (C—O bond lengths 1.283(8) and 1.216(8) A) lies close to the alkyl group, but its orientation is such that its insertion into the metal-alkyl bond would produce the unfavored metallacarboxylate ester instead of a carboxylato complex. 

Heterocyclic complexes of pentaammineosmium(II) have also been reported for pyridinium (85), 2,6-lutidine (85), 2,6-lutidinium (85), pyrrole (179), furan (179), and thiophene (179), in which the organic ligand is dihapto coordinated via a C==C bond (90). These ligands are thought to rearrange upon oxidation to coordinate through the heteroatom. 

Dihapto 7>ligands, in copper complexes, 2, 174 Dihydride iridium complexes, preparation, 7, 396 Dihydrido clusters, with decarutheniums, 6, 1036 Dihydrobenzofuran, carbene C-H insertions, 10, 193 Dihydrobenzopyran, carbene C-H insertions, 10, 193 ring-closing diene metathesis,... 

Several complexes with acetyl ligands bonded in a dihapto fashion have now been reported 120-122). It cannot be decided whether the chiral intermediates C5H5Mn(NO)(COCgH8) contain monohapto or dihapto acyl ligands, because they cannot be observed spectroscopically. However, dihapto bonding would not change the stereochemical outcome of all the results described. 

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