Small-bite-angle ligands

X-ray crystal structure determinations of compounds 194, 195 and 196 (Figure 95) revealed that these compounds exist in the solid state as discrete monomers. In 194, the zinc atom is trigonal planar coordinated as a result of the Af,Af-chelate-bonding [Zn—N 2.007(1) and 1.995(1) A]. The N—Zn—N bond angle is rather acute [73.83(6)°], but that is a logical consequence of the small bite angle of the ligand. 

The ligand bis (diphenylphosphino)methane, dppm, has been used to stabilise binuclear complexes. Owing to the small bite angle, this ligand is known to bridge between two transition metal centers in many cases although it can also act as a simple chelate ligand (2). 

Sheer congestion of donor atoms around the metal ion and concomitant inter-donor atom repulsions makes these high coordination numbers difficult to attain. They are often associated with multidentate ligands with a small bite angle such as nitrate that take up little space in the coordination sphere, either alone, as in (Ph4As)2[Eu(N03)5] or in combination with other ligands, as in Ln(bipy)2(N03)3, Ln(terpy)(N03)3(H20) (Ln = Ce-Ho), and crown ether complexes (Section 4.3.7) such as Ln(12-crown )(N03)3(Ln = Nd-Lu). Other crown ether complexes can have 11 and 12 coordination, e.g. Eu(15-crown-5)(N03)3 (Ln = Nd-Lu) and Ln(18-crown-6)(N03)3(Ln = La, Nd). 

Another bidentate ligand whose complexes have been studied in some detail is 1,8-naphthyridine (naph) which, like nitrate, has a small bite angle and is similarly capable of affording high coordination numbers two types of perchlorate complexes have been made, M(naph)6(C104)3 (M = La-Pr) and M(naph)5 (0104)3 (M = Nd-Eu). The former complexes have 12-coordinate lanthanides, confirmed by diffraction methods for the praesodymium complex. The change in stoichiometry is doubtless a manifestation of the lanthanide contraction. 

A variety of chiral phosphine ligands are frequently used for asymmetric Heck reactions. The oxidative insertion is favored by basic ligands whereas bidentate ligands with a small bite angle enable good to excellent chirality transfer (>90 % ee). Some selected ligands which meet these requirements for asymmetric Heck reactions are shown in the margin. 

The stereochemistry of Nb(02CNEt2)4 was essentially dodecahedral, as expected from the small bite angle of 1.01 A for the ligand.509... 

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