Sensitizers monodentate

Zirconium(IV) and hafnium(IV) halides form adducts with a variety of monodentate and polydentate amines. Typical complexes are listed in Table 3. In general, these compounds are air- and moisture-sensitive white solids (yellow when the halide is iodide), thermally stable and insoluble in most organic solvents. 

Apart from a comparison of the cis and trans dichloro complexes and a comparison of dichloro with chloroaquo species, little has been done to study the effect of the non-amine, monodentate non-participating ligands upon the lability of Rum complexes. Where studied, cis-dichlorotetra-(amine) complexes are considerably more labile than their trans isomers but there are insufficient data at this time to say whether there is any great sensitivity of lability to the nature of the tetraamine ligand in such systems. The extremely low reactivity of irans-[Ru(cyclam)Cl2]+ (the rate constant for Cl- exchange is reported293 to be 3 x 10 7s 1 at 102 °C) seems to be an exception that is, as yet, unexplained. Some relevant data are collected in Table 11. 

The passage of allene into a benzene solution of dicobalt octacarbonyl at room temperature produces air-sensitive yellow crystals of composition [(C3H4)Co(CO)3]2 (428). The spectral data indicate a 77-allyl complex in which a CO grouping has been transferred to the 2-position of each allylic moiety. Although Co2(CO)g does not form a 77 complex with tetraphenylallene, cyclopentadienyldicarbonylcobalt does react under reflux in isooctane to yield the product [C3(C(,H5)4]Co(CO)(C5H6) in which the olefin appears to be a monodentate ligand (434). 

EXAFS data. However, XAFS spectroscopy is not very sensitive to coordination numbers, so independent verification is needed. The As(V) species with the characteristic As-Fe distance of 3.5-3.6 A can be matched by a geometry in which only one bond is formed between an As(V) tetrahedron and an Fe(III) octahedron (i.e., a monodentate coordination geometry, Fendorf et al 1997 Manceau, 1995 Waychunas et al., 1995 Waychunas et al 1996 Waychunas et al., 1993, table 4). 

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