Early transition metal and lanthanide bis(iminophosphorano)methandiide complexes; 'pincer' and bridging bis(phosphorus) metal carbenes

The spontaneous double deprotonation of bis(iminophosphorano)methane ligand s with alkyl or bis(trimethylsilyl)amido metal precursors yields 'pincer' c arbenes of Group 4 and lanthanide metals. The generality of the system is e xtended with the dilithiumbis(iminophosphorano)methandiide salt which is ob tained by lithiating bis(trimethylsilylimino(diphenylphosphorano))methane w ith two moles of lithium alkyl. This dilithium salt reacts with metal halid es with metathetical elimination of LiCl to access the same types of pincer complexes. Herein the synthesis of a complete triad of Group 4 'pincer' co mplexes is reviewed and their reaction chemistry demonstrating (a) nucleoph ilic alkylation, (b) the Lewis Acidity of the metal, (c) 1,2 addition acros s the M=C carbene bond and (d) [2 + 2] cycloaddition across this M=C bond i s surveyed. The structural data for the 'pincer' complexes suggests strongl y that there is a M=C carbene double bond. The addition reactions also supp ort this interpretation because a M-C bond persists after the addition. The reactivity pattern also strongly suggests that the chemistry of this carbe ne center is similar to the metal alkylidene complexes, and not the Fischer heteroatom carbene complexes. The chemistry extends to many other elements . The extension to Sm as a representative lanthanide is described. This spe cies adopts a structure very similar to the Group 4 'pincers'. Chromium(II) gave a novel bridging carbene dimer instead of the monomeric pincer. This bridging structure is developing as another structural motif in this system . (C) 2001 Published by Elsevier Science B.V. All rights reserved.