Monohapto bonding

In several theoretical papers at different degrees of sophistication attempts have been made to determine the most favorable structure for beryllocene (59-66). Ab initio molecular orbital calculations indicate that in the lowest energy form one pentahapto- and one monohapto-bonded cyclopentadienyl ring are present, as portrayed in structure H. Neither the slip sandwich F nor the off-center double well potential structure G suggested experimentally are indicated to be favorable. Even the symmetrical D5<) sandwich structure... 

Phosphonium ylides, which can be written in the two familiar canonical forms, are available with a wide variety of substituents both at the phosphorus and at the carbon atoms (Scheme 30). In gold complexes, without any exceptions, they function as two-electron donors, as proposed by the dipolar form to give discrete Au-C cr-bonds (771, monohapto). No side-on, 7r-coordination (t 2), as might be expected out of the ylene form, has been observed to date. 

Zn-C bond of the monohapto ring is 2.223(4) A long, while the zinc-carbon bonds of the r -bound Cp ring span the wide range 1.991(4)—2.514(3) A. The short contact is the shortest Zn-G bond yet observed for a 7r-bound ring, while the long contact is barely a bond at all. The asymmetry of 22 is maintained in solution. 

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). 

One other structure of a tricyclopentadienide has appeared (57) and it provides a further demonstration of the correlation between ionic size and coordination. Neodymium tris(methylcyclopentadienide) crystallizes as a tetramer (Fig. 10). The Nd + ion (which is slightly larger than Sm3+) is pentahapto bound to three cyclopentadienyl rings and monohapto bound to a fourth ring. This fourth ring is in turn j -bonded to another Nd + ion, until the tetramer is generated. The distances between tetramers are those expected for van der Waals contact. The crystal and molecular parameters are compared with the other tris cyclopentadienyl complexes in Table 5. 

Note that in a compound such as allyllithium, where a tr bond connects the carbon to the metal, the allyl group is referred to as monohapto or rjx. 

The term a-ir rearrangement is defined as an intramolecular process in which an organic group that is a bonded (monohapto. h ) to a transition metal becomes n bonded (n-hapto, hn) to the metal. The ir-cr rearrangement is the opposite change (hn-h ). 

Hz). Several pieces of spectroscopic evidence lead to the conclusion the Me5C5 ring of V is not bonded in the monohapto manner. First, the 1h and 13c spectra of V indicate that the ring and Me carbons of the Me5C5 moiety are equivalent moreover, the equivalence of the methyl groups persists to -100°C and -80°C in lH and 13c NMR experiments, respectively. Second, the 31p chemical shift of V (111.0 ppm) is 33.8 ppm upfield (i.e. shielded) compared to that of the phosphorus(III) chloride precursor, 2Z 1° aH cases reported to date, phosphenium ion for-... 

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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