Complexes have been obtained from cobalt(11) chloride, bromide, iodide, and thiocyanate with eight 4-substituted pyridines, NC5H4R, where R = CH2C6H5, p-CH2C,6H,4Cl, CH2OH, Cl, Br, C02C4H9, COC6H5, and CN, in order of decreasing basicity. Twenty-nine of the compounds prepared are of the type CoX2B2, thirteen of the type CoX2B4, three CoX2B33 two CoX2B5, two CoX2B2(H2O)2, and one CoX2B(CH3OH). The stereochemistry of the cobalt atom has been assigned in each case on the basis of magnetic properties and reflectance spectra. In all cases but one the cobalt appears to exhibit octahedral or tetrahedral stereochemistry and to have the high-spin configuration; diiodotris(p-chloropyridine)cobalt(11) is probably an example of a low-spin 5-coordinate compound. The stereochemistry is largely determined by the halogen, the preference for octahedral stereochemistry increasing in the order I � Br < C1 � NCS. There is also a small effect due to the base, octahedral stereochemistry being favoured by the weaker bases. The behaviour of the compounds in nitrobenzene solution has also been studied.

Complexes of zinc halides with heterocyclic bases of the types ZnX2B2, ZnX2B3, and ZnX2B4 have been prepared, where X = Cl, Br, I, or NCS and B = pyridine, 4-methylpyridine, or 2-methylpyridine. Infrared spectra and isomorphous incorporation of cobalt(II) indicate that in the solid state these compounds are tetrahedral, 5-coordinate, and octahedral respectively. Molecular weights and conductivities in nitrobenzene solution show that the compounds ZnX2B3 and ZnX2B4 dissociate in solution to ZnX2B4 and free base.

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Free energies, enthalpies, and entropies of formation of adducts of pyridine and some o o p p e r (~~) complexes of 3-substituted p-diketones have been determined by thermometric titration in benzene solution. The results, which show that adduot stability is determined by the enthalpy, are discussed in terms of the relationships of extrathermodynamios.