Carbene complexes. Part 9. Electron-rich olefin-derived carbene–molybdenum(0) and amidinium molybdate(0) complexes, and the crystal and molecular structure of cis-tetracarbonylbis(1,3-dimethylimidazolidin-2-ylidene)molybdenum(0), cis-[Mo(CO)4{CN(Me)CH2CH2NMe}2]

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Abstract
Carbene–Moo complexes are obtained by the thermal reaction of an electron-rich olefin [graphic omitted]R]2, LR 2, with (a)[Mo(CO)6]{yielding [Mo(CO)5LR] or cis-[Mo(CO)4(LR)2](R = Me, Et, or PhCH2)(the tricarbene complex is unstable)}, and (b)[Mo(CO)2(C5H5-η)NO]{yielding [Mo(CO)(C5H5-η(LR)NO](R = Me or p-tol)}. By contrast, with [Mo(CO)3(C5H5-η)H] or [{Mo(CO)3(C5H5-η)}2], the olefin reacts as a reducing agent, yielding the amidinium molybdate(0) complexes [HLR]+[Mo(CO)3(C5H5-η)]– or [LR 2]2+[Mo(CO)3(C5H5-η)]– 2(R = Me). The six-membered chelate olefin [C[graphic omitted]Me]2, L′Me 2, behaves qualitatively in a similar fashion, but is less reactive, dicarbene–Moo complexes not being accessible. The cis-dicarbene–Moo complexes are isomerised photochemically to the trans-complexes, but the former are thermodynamically the more stable. Reaction of [Mo(CO)5LEt] with LMe 2 yields cis-[Mo(CO)4(LEt)(LMe)], whereas [Mo(CO)5LMe)] with LEt 2 affords also cis-[Mo(CO)4(LMe)2] and cis-[Mo(CO)4(LEt)2]. The redistribution reaction cis-[Mo(CO)4(LR)2]+[Mo(CO)6] 2[Mo(CO)5LR](R = Me or Et) is reversible. Other reactions of monocarbene complexes give cis-[Mo(CO)4(LR)Q][Q = C5H5N, PPh3, or P(C6H11)3, fac-[Mo(CO)3(LR)Q′2][Q′2={P(OMe)3}2 or diphos], or cis-[Mo(CO)4{C(OMe)Me}LR]. ν(CN2) is at 1 510—1 480 cm–1 for the carbene–Moo complexes, but at 1 700–1 640 cm–1 for the amidinium cations, and ν(CO) values are very low; 1H n.m.r. spectra provide information on conformational aspects; ΔG‡ for Mo–Ccarb rotation in cis-[Mo(CO)4(LR)2] is ca. 10 kcal mol–1. 13C Chemical shifts for Ccarb. and CO are comparable (but are distinguished by 1H decoupling and 1H off-resonance studies). Both are in the range 230–210 p.p.m. upfield from SiMe4, whereas for [HLMe]+ or [LMe 2]2+ there is a further 50–60 p.p.m. upfield shift for δ[Ccarbonium ion]. A single-crystal X-ray analysis of (4)cis-[Mo(CO)4(LMe)2] shows octahedral Mo, with MO–CO mutually-trans bond lengths longer by 0.048(2)Å than MO–CO trans to carbene [2.024(3) and 2.032(3)Å]; Mo–Ccarb[2.293(3)Å] is appropriate for single bonds. The dihedral angles between the carbene ligand planes and the co-ordination planes in which they are involved are ca. 45°, which is sterically the preferred conformation. It is concluded that the carbene ligand is a strong σ-donor but a poor π-acceptor.