Complexes of the bis(trimethylsilyl)-benzamidinato ligand 'siam':: synthesis and X-ray structures of (siam)2M, (siam)(siamH)MX (M = Ni, Pd), (siam)2MnI and (siam)ReO3, and their reactivity towards CO2

Complexes with the bis(trimethylsilyl)-benzamidinato ligand 'siam' of the type (siam)(2)M (M=Ni (2), Pd (3)), (siam)(siamH)NiBr (4a), (siam)(siamH)PdCl (4b) and (siam)(2)MnI (5) were prepared by reaction of (siam)(2)Mg(thf)(2) (1) with the corresponding metal halides and addition of water or iodine, respectively. The reaction of (siam)(2)Zn with Re2O7 yielded (siam)ReO3 (6). X-ray analysis of 1-6 revealed that siam acts as a bidentate ligand forming four-membered chelate rings in all complexes. In 2 four nitrogen atoms show a distorted tetrahedron; in contrast, 3, 4a and 4b are square planar complexes. In both 4a and 4b siamH is coordinated as a monodentate ligand. The Mn(III) complex 5 exhibits a trigonal bipyramidal coordination of the four amidinato nitrogens and the iodo ligand, which occupies an equatorial position. The Re(VII) complex 6 shows an irregular trigonal bipyramidal coordination. In all compounds the bond lengths and angles in the amidinato ligand are comparable and exhibit typical values. The reaction of 2 with excess CO2 gave a green dimeric complex 7 in which one Ni(II) centre is octahedral and the other is square pyramidal coordinated. X-ray analysis revealed that 5 mol CO2 per 2 mol nickel are inserted in N-Si bonds forming new types of tridentate bridging ligands which contain N-COOSiMe3 groups. (C) 1998 Elsevier Science S.A. All rights reserved.