ELSEVIER Inorganica Chimica Acta 243 (1996) 177-183 d='=l Molecular structure of [W(N)(O-2,6-C6H3ipr2)3]2: a metal-nitrido compound with a cyclic diyne structure ' Timothy P. Pollagi, Joseph Manna, Steven J. Geib, Michael D. Hopkins* Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA Abstract The compound [W(N)(O-2,6-C6H3iprz)3]2has been studied by single-crystal X-ray diffraction (triclinic, Pi, a = 12.880(2)A, b = 14.103(4) A, c = 20.317(4) A, a = 90.63(2) °, fl = 90.72(2) °, ? = 113.82(2)% Z= 2). The structure exhibits the first example of an asymmetric, cyclic MzN2 core, with bond orders that alternate between ca. 3 for the short W-N linkage (d(W-N) = 1.77 A) and ca. 1 for the longer one (d(W-N)= 2.02 A). The IH NMR spectrum of the compound suggests that the asymmetric, cyclic structure is maintained in solution. This structure represents the fourth distinct type identified for [W(N)(OR)3] x complexes; in the present case, steric factors appear to play a significant role in favoring the observed structure. Keywords: Crystal structure; Tungsten complex; Alkoxo complex; Nitrido complex 1. Introduction Metal-nitrido complexes are among the few classes of transition-metal compounds that number among its mem- bers structural and electronic analogues of conjugated organic compounds and polymers [1]. Examples of these analogues are the cyclic compounds with symmetric MzN 2 (i) [2] and M3N 3 (ii) [3] cores and asymmetric M3N3 (iii) [4] and M4N4 (iv) [5] cores, and one- dimensional [M--N]~ polymers (v) [6,7] in which the bond order alternates along the backbone. Group VI L n ,,~ L L. III Lr~M/ "~" MLn LnM ( ) MLn III I N ~M .~ N~M~hl III Ln Ln M n N~MLn i ii iii iv --'M~N --['--" M-~-- N--"]---M ~ N'-- Ln [Ln J~ Ln V * Corresponding author. I Dedicated by M.D.H. to his friend and mentor Harry Gray on the occasion of his 60th birthday. 0020-1693/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved PII: S0020-1693(96)04905-5 metal-nitrido compounds of the general form [M(-N)- (OR)3]x (M = Mo, W; R = alkyl, fluorinated alkyl) are particularly interesting in this regard because the struc- tures they adopt are a sensitive function of the nature of the metal and the alkoxide R-group. Crystallographic studies by Chisholm and co-workers have revealed that W(N)(OtBu)3 is a linear polymer of type v [7b], as are related molybdenum derivatives [7c,8], while W(N)- (OCMe2CF3)3 is a cyclic trimer of type iii [4a]. Although it is not a structural type of interest in the context of RO OR )v_-._ I_ w-. /// \ ..o. R~RO"(~ RO _J - RO--,,W~N~W~--OR " .,/oi. R = t-Bu d(W~N) = 1.740(15)'A d(W-N) = 2.661(16) A R = CMe2CF3 d(W-=N) = 1.72(2) A d(W-N) = 2.14(2) A conjugation, these workers have also reported NMR- spectroscopic evidence that W(N)(OSiMe2tBu)3 adopts an alkoxide-bridged, tail-to-tail structure in solution [4a]. The electronic structures of oligomeric metal-nitrido