No abstract

Complexes of CoCl2, CoBr2, CoI2, and Co(NCS)2 with two and four molecules of 4-methylpyridine and two molecules of 2-methylpyridine have been prepared. Their magnetic properties, conductivities, and reflectance and absorption spectra in the visible region show that all the compounds of the type CoX2B2 are tetrahedral except Co(NCS)2(4-mepy)2, which is an octahedral polymer like its pyridine analogue. Infrared spectra in the 3-15 μ region lend no support to the suggestion that there may be differences in the extent of π-bonding between the metal and the base or the metal and NCS- in tetrahedral and octahedral complexes. It is suggested that the position of the ligands in the spectrochemical series and the relative stabilities of the compounds CoX2B2 with respect to dissociation into CoX2B4 and free base can be explained in terms of repulsion effects between the non-bonding electron pairs on the ligands and metal dє-orbitals on the one hand and the non-bonding electron pairs of neighbouring ligands on the other.

The experiment described in this article takes a kinetic approach to the evaporation of several liquids (such as chloroform, benzene, and a few others that are probably now illegal for classroom use.

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.

Bis-ligand cobalt(II) complexes of 2,4,6-tris(2-pyridyl)-1,3,5-triazine and 2,3,5,6.tetrakis(2-pyridyl)pyrazine have been prepared. The conductivities of the complexes in nitromethane solution correspond to values expected for bi-unielectrolytes. The temperature dependence of their magnetism has shown that in some instances there exists a spin-state equilibrium in the metal atom while in others the metal atom is spin-free and Curie-Weiss laws are obeyed.