Transition metals borane complexes

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

The electron counting rules of Wade (S3), Williams (117), and Rudolph (118) can serve as a useful concept to explain structure and bonding in a variety of systems which at first glance are very different Zintl phases, boranes and carboranes, transition metal n complexes and carbonyl clusters, nonclassical carbocations, and also n complexes of main-group elements. According to... 

Transition-metal-boryl complexes contain a covalent bond between a metal and three-coordinate boron. Boryl complexes are a subset of the variety of ligands containing a single boron atom that would encompass borane, boryl, and borene ligands. Of this group, boryl complexes are the most abundant. 

The hydrostannation of a diyne followed by transmetallation using a borane leads to the phenylboratabenzene anion, isoelectronic to biphenyl (the prefix borata means that a benzene CH group is replaced by the isoelectronic BH group). It is equivalent to the cyclopentadienyl anion and gives transition-metal sandwich complexes that are equivalent to metallocenes with a variety of metals (V, Cr, Fe, Co, Ru, Os). - ... 

The striking advances in borane chemistry during the period of 1940-1960 led to the successful development of the new field of metal hydride chemistry within the past 50 years. Especially transition-metal hydride complexes not only aroused academic interest but also established an exceptional position as participants in many important homogeneous catalytic reactions such as C==C and C=0 hydrogenation, C—H activation, and hydroformylation. For example, HCo(CO)4 and HCo(CO)3 which are produced by hydrogenolysis of Co2(CO)g ... 

Cyclic divinyl boranes (bora-2,5-cyclohexadienes) also act as good complex ligands and are available from the corresponding stannacyclohexadienes by treatment with PhBCl2. They react photochemically or thermically with transition-metal complexes, e.g. ... 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

---