Monohapto ligands

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... 

The remaining classes oF monohapto organic ligands listed in Table 19.2 are carbene (=CR2), carbyne (=CR), and carbido (C). Stable carbene complexes were first reported in 1964 by E. O. Fischer and A, Maasbol. Initially they OMe... 

In transition metal complexes, two main coordination modes have been described for urea ligands W-monohapto, or N,0-chelates. In the case of molecules containing more than one urea function, the molecules act as Ar,AT-chelates, so that one of the urea functions always behaves as monohapto-ligand. For example, complexes of Co(III) with N-(2-pyridylmethyl)urea and ethylenediamine have been characterised by X-ray crystallographic analysis (Scheme 7). The urea group is coordinated through only one of its N... 

The second important structural feature in concerns the orientation of the n -acyl ligand. The C-0 vector in the present case points in the direction away from the Cl ligand, while in the aforementioned titanium and zirconium compounds, the C-0 vector is oriented toward the non-acyl monohapto ligand (E vs F). 

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. 

The pyrrolyl ligand is interesting in that it can function as both a u-bonded amido group or a yr-bonded multihapto ligand. The pyrrolyl group appears to be a weaker electron donor than the cyclopentadienide anion 90). In the tetrakis-(2,5-dimethylp3u rolyl)uranium(IV) complex a preliminary structure has been revised with a tris(monohapto) (pentahapto) structure now assigned from variable temperature PMR spectra 90). 

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. 

---