Transition metal complexes multicenter bonding

Metallaborane and borylene complexes are related to some extent because both types possess direct metal-boron bonds however, the nature of these interactions is varied. In metallaborane clusters, the framework is made up by nonclassical, electron-deficient, multicenter two-electron bonds, while the borylene ligand (BR) is related to one or more transition metal centers by classicrJ, electron-precise, two-center two-electron (2c-2e) bonds [280 -283]. Since the discovery of various bridged and terminal borylene complexes by Braunschweig and Aldrige, the chemistry of this subarea of transition metal complexes of boron has received significant attention from both structure/bonding and... 

Theoretical investigations of the molecule P4 show that the valence shell electron density in this species should be concentrated inside and on the faces of the tetrahedron taking part in multicenter molecular bonding. Although after such a description little or no nucleophilic properties of P4 can be expected, there are some metallic compounds in which the unit participates as a whole. Examples of some transition metal complexes containing the unit P4 are shown in Table 4.15. 

In a first discussion of the possible mechanism (Schrauzer, e< al., 32, 33) it was assumed that the products are formed within tt complexes in a concerted fashion ( it complex multicenter reaction ). Norbom-adiene forms many complexes with transition metals in which it is symmetrically coordinated. Some pertinent examples are (XII) (1, 2), (XIII) (35), and (XIV) (36). Ni(CO)4 reacts with acrylonitrile to produce bisacrylonitrile nickel, Ni(CH2=CHCN)2 which adds phosphines forming 1 1 and 1 2 adducts (37, 38). Acrylonitrile is a relatively strong TT-bonding ligand and may back-accept charge from the metal via d -p bonding. In this regard it is related to duroquinone (tetra-methylquinone), which is known to form Ni(0) it complexes of the type Ni(Dqu)2 or (XIV). The most likely intermediate in the reaction... 

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