Bonding, dihapto/monohapto

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. 

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. 

The simplest binary borane species is the tetrahydroborate anion, BH4", which is isoelectronic with CH4 and NH4" . Many salts of this anion, such as LiBH4 and NaBH4, are essentially ionic and have been used for more than 50 years as versatile reducing agents (7, 27). However, BH4" can also react by ligand displacement to form covalently bonded complexes in which it acts as a monohapto, dihapto, or trihapto... 

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