Comparative behaviours of phospha-alkynes and alkynes at electron-rich phosphinic metal centres

The coordination chemistry of phospha-alkynes (P drop CR) at electron-rich and sterically hindered {M(diphosphine)(2)} [M = Mo(0), W(0), Re(I), Fe(II) ] centres. in which the P drop CR ligand adopts the very rare and electroni cally unfavoured end-on (eta (1)) coordination mode, as well as at the less sterically demanding {Rh(triphos)}(+) site, are described and compared wit h those exhibited by alkynes at the same metal centres. At the former sites , the eta (1)-P drop CR ligand behaves as a weak pi -acceptor and is typica lly activated to alpha -nucleophilic addition to give phospha-alkene, phosp hine and phosphinidene oxide products, although activation towards electrop hilic (protic) addition has also been recognized upon hydrometalation (inse rtion into a metal-H bond to give a phospha-alkenyl species). At the same m etal sites. the eta (2)-coordination of alkynes is unfavoured (on both elec tronic and steric grounds) and these substrates undergo rearrangements (H-s hifts) towards eta (1)-bonded alkynyl and vinylidene derivatives, or toward s a less sterically demanding eta (2)-allene species. and these products ar e activated towards beta -electrophilic addition (protonation) on account o f their pi -electron withdrawal ability which contrasts with the behaviour of the eta (1)-P-ligated phospha-alkyne and derivatives. The alkyne and pho spha-alkyne insertions into an Fe-H bond are also compared, as well as thei r cycloaddition reactions (cyclotri- and cyclodimerisations, respectively) at the Rh(triphos)}(+). centre which exhibits open coordination sites for e ta (2)-ligation. The above reactions are discussed in terms of both stereoc hemical and electronic effects. (C) 2001 Elsevier Science B.V. All rights r eserved.