Pyridine with chromium hexacarbonyl

A -coordinated pyridine was first suspected as the product of the reaction of N-methylpyridinium iodide with chromium hexacarbonyl... 

The reactions of the phosphabenzene system [124] confirm these conclusions. Phosphabenzenes have low basicity towards hard acids. They are not protonated by CF3CO2H nor alkylated by trialkyloxonium salts. However, soft acids attack at phosphorus. For instance, 2,4,6-triphenyl-phosphabenzene forms compounds 4 with the hexacarbonyl derivatives of Cr, W and Mo in which the phosphorus coordinates to the metal, possibly with metal-P back-donation. The complexes 4 rearrange photochemically or thermally affording the 67i-heteroarene complexes 5. Although 2,4,6-triphenyl-pyridine is protonated on nitrogen, it undergoes complex formation with chromium hexacarbonyl exclusively on the phenyl moieties yielding the ri -arene complexes 6 [125]. 

Some important reactions of chromium hexacarbonyl involve partial or total replacements of CO ligands by organic moieties. For example, with pyridine (py) and other organic bases, in the presence of UV hght or heat, it forms various pyridine-carbonyl complexes, such as (py)Cr(CO)5, (py)2Cr(CO)4, (py)3Cr(CO)3, etc. With aromatics (ar), it forms complexes of the type, (ar)Cr(CO)3. Reaction with potassium iodide in diglyme produces a potassium diglyme salt of chromium tetracarbonyl iodide anion. The probable structure of this salt is [K(diglyme)3][Cr(CO)4lj. 

According to our work with nitrogen bases, which was done in Heidelberg and in Stuttgart (1933/34), it was shown that pyridine, o-phenanthroline, and 1,2-ethylenediamine react in a different manner with the hexacarbonyls of chromium, molybdenum, and tungsten with, in each case, substitution of the carbon monoxide ligands (VII, 27). 

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