Some electron donor-acceptor complexes of dihalogens

6 Some electron donor-acceptor complexes of dihalogens 

and some interhalogen compounds XX form complexes with ammonia that are sufficiently stable to allow the structures to be determined in the gas phase. The structure of the complex H3NICI is shown in Fig. 18.6 [4]. The ICl molecule is situated on the threefold symmetry axis of the NH3 molecule, i.e. in the direction of the elecfron lone pair on the N atom. The N-I distance is 271 pm, 71 pm longer than the estimated single, covalent bond distance obtained from the MSS rule, but still 82 pm shorter than the sum of the van der Waals radii of N and 1. The experimental information needed for calculation of the dissociation energy of the complex is unfortunately not available, quantum chemical calculations have yielded dissociation energies ranging from 42 to 52 kJ mol [5,6]. 

The structure of H3NICI is consistent with the formation of a weak dative bond N I. This description implies that the iodine atom in the complex is surrounded by one covalent bond pair, one dative bond pair, and three lone pairs. According to the VSEPR model the five electron pairs will be directed towards the corners of a trigonal bipyramid, and since the three non-bonding electron pairs require more space, they will occupy equatorial positions. 

The structure of the complex HFCIF (see Fig. 18.6) is consistent with the formation of an dative bond corresponding to HF CLF [7]. Quantum chemical calculations have yielded dissociation energies ranging from 6 to 10 kJmol [8]. The dative F- C1 bond is nearly 

The conclusions reached on the basis of the two complexes H3NICI and HFCIF may be generalized if a monovalent halogen atom acts as an electron donor, its coordination geometry is angular if the halogen atom acts as an electron acceptor, the coordination geometry is linear [9]. 

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SOME ELECTRON DONOR-ACCEPTOR COMPLEXES OF DIHALOGENS... 

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