Hypercoordination, Bond Length Alterations, Sub-van der Waals Interatomic DistancesIn an 8-dimethylamino-naphth-l-yl-phosphorane a dative N-P bond forms in spite of the spacer properties of the naphthalene skeleton. Its "normal" length, 213.2 pm, much too short to fill the peri space, forces the Cio skeleton into severe distortion. In the light of these properties peri distances d(N-P/Si) 250 pm in 8-dimethylamino-naphth-l-yl-phosphines, -silanes etc.are recognized as evidence for steric hindrance.für Naturforschung, Tübingen -www.znaturforsch.com K

Enhancement of the basicity of the amino group in (8-dialkylamino-naphth-1-yl)-diphenylphosphines diverts protonation from the P to the N atom. Thus the cation 8-Et2N+(H)-C10H6-PPh2 becomes available whose 1H and 31P NMR spectra provide arguments against dative N→P interactions in the phosphines and their quaternary phosphonium salts. Likewise, the X-ray structure of 8-Et2N-C10H6-PPh2 does not indicate such interactions.

The 31P NMR data of non-quaternary (8-dimethylamino-naphth-1-yl)phosphonium salts, with emphasis on the 1J(31P, 1H) coupling constants, where scrutinized for their potential to yield information about N→P dative interactions. As for δ (29Si) and 1J(29Si, 1H) in the isosteric silanes, the data do not permit conclusions in favour of such interactions. 1J(31P, 1H) of bis(8-dimethylaminonaphth- 1-yl)phosphine in conjunction with the distances d(N···P) invalidates the basic assumption on which the claim of dative N→P/Si bonding in such phosphorus and silicon compounds rests, viz. that N···P/Si distances of ca. 270 pm are evidence for P/Si-hypercoordination. No evidence for hydrogen bonds N···H-P was found.

Under favourable conditions, the phosphorus centres in tetraorganophosphonium cations are sufficiently electrophilic to become hypercoordinate by reaction with strong nucleophiles. However, as a peri substituent at the naphthalene system, such a centre proved to be unable to induce the nitrogen of a peri-bound dialkylamino group to bond formation. The distortion of the naphthalene skeleton revealed by X-ray structure determination of four 8-dialkylaminonaphth- 1-yl phosphonium salts does not exhibit the criteria of N-P bond formation but rather those of intersubstituent repulsion.

The failure of the sum of van der Waals radii axiom to provide information about donor-acceptor interactions in 8-dimethylamino-naphth-1-yl phosphines and their derivatives called for alternative methods. The coupling constants J(C,H) of the Me2N groups proved useful and indicate that in these compounds, N→P interactions are negligible.