Transition metal complex trigonal planar

Note Axial chiral transition metal complexes may yield racemic products owing to a transition state with unsuitable metal geometry (e.g. trigonal planar or square bipyramidal). 

On the other hand, the least shielded Sn nuclei known so far belong to transition metal complexes containing trigonal-planar coordinated tin atoms in the formal oxidation state 0 (48) or —2 (49 ... 

These carbonylation reactions can be viewed as substitution of an equatorial ligand PMe3 by CO in a d trigonal bipyramid and thus can be related to the well-known substitution reaction in planar (P transition metal complexes (13), the trigonal bipyramid being the ground state and the square planar the transition state or intermediate. 

The common structures having CN = 4 for transition-metal complexes are tetrahedral and square planar. However, these are not the only conceivable structures. Examples of main group compounds having seesaw structures are known, and trigonal-pyramidal structures may also be possible in some cases. 

Complexation of triynes to Pd(0) has been reported to give homoleptic palladium alkyne complexes that show a trigonal-planar arrangement with all of the alkyne carbons and Pd in the same plane. Complex 73 is a macrocyclic complex synthesized by reaction of the triyne with Pd(PPh3)4- Due to coordination to the metal, the alkyne carbons are shifted to the center of the cycle and their substituents deviate from linearity by about 22°. Complex 74 undergoes clean intramolecular cyclization at room temperature upon addition of PPh3 (Equation (24)). No intermediate complexes were detected in the course of this reaction, which is an example of the important cycloisomerization of alkynes and enynes catalyzed, among other transition metal complexes, by Pd(0) derivatives. 

MM calculations with the POS method have been used to obtain square planar, square prismatic, trigonal bipyrami-dal, octahedral, trigonal prismatic, capped trigonal prismatic, capped octahedral, and pentagonal bipyramidal metal complexes as well as a number of geometries that occur with higher coordination numbers. The POS method has been applied to transition metal complexes, a variety of lanthanide complexes,and complexes of the alkali and alkaline earth metal ions. ... 

An interesting example of dioxygen adducts of transition-metal complexes is the trans-planar [IrCl(CO)(PPh3)2] (Vaska s salt), which binds dioxygen reversibly to form a trigonal bipyramidal complex. The 0-0 distance of this compound (1.30 A) is close to that of the O2 ion (1.28 A) its v(02) (857 cm" ) is typical of peroxo adducts. 

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