Dynamic and static stereochemistry in dimolybdenum and ditungsten compounds containing a central (m ≡ m)6+ unit

Abstract

The chemistry of molybdenum and tungsten in oxidation state + 3 is now dominated by compounds containing a central (M ≡ M)⁶⁺ core, with metal-to-metal distances in the range 2.2–2.3 Å. The molecular orbital configuration of the triple bond is σ²π⁴ arising from the mutual interaction of metal atomic d_z²(σ) and d_xz, d_yz(π) orbitals. In compounds containing the central (M ≡ M)⁶⁺ core, the metal atoms may be bonded to three, four, five or six ligand atoms. Examples of each are given and for a given coordination number the preferred geometry is discussed. Low temperature n.m.r. studies support the view that the structures found in the solid state are also present in solution. Variable temperature n.m.r. studies reveal a wealth of knowledge concerning the dynamic behaviour of these molecules in solution. For example, (i) M₂(NR₂)₆ and M₂(NR₂)₄Y₂ molecules (Y = halide, alkyl or alkoxy group) are molecular propellers; (ii) rotational barriers about metal-to-metal triple bonds in M₂Y₄X₂(M ≡ M) compounds are comparable to those in related Si₂Y₄X₂ and P₂Y₄ compounds; the latter contain a central element–element σ-bond of length 2.2-2.3 Å; (iii) molecules of the type L(RO)₃M ≡ M(OR)₃L′ contain OR groups which are cis and trans to the neutral ligands L and L′. Site exchange between cis and trans groups occurs on the n.m.r. time-scale by a threshold mechanism which does not involve exchange of OR groups between the two metal atoms. This is compared to the fluxional properties associated with MX₅ compounds (D_3h⇌D_4h). Finally, the absence of any complex containing a central M₄¹²⁺ tetrahedral arrangement is noted. The closest approach to such a compound is seen in the tetranuclear complexes Mo₄(µ-F)₄(OBu^t)₈ and Mo₄(µ-F)₃(µ-NMe₂)(OBu^t)₈ which contain a bisphenoid of molybdenum atoms having two short Mo–Mo distances (2.26 Å) and four long Mo–Mo distances (3.75 Å) corresponding to localized triple and non-bonding interactions, respectively.