COORDINATION BOND PROPERTIES IN PHTHALOCYANINATOMETAL(II) COMPLEXES .1. STEREOCHEMISTRY AND BOND PROPERTIES IN BIS(4-METHYLPYRIDINE)-PHTHALOCYANINATOMETAL(II)-4-METHYLPYRIDINE (1/2) (METAL = COBALT OR IRON)

A comparison between the vibrational properties and stereochemistry of [Co(pc)(4Me-py)2] and [Fe(pc)(4Me-py)2] [pc = phthalocyaninato(2 -), 4Me-py = 4-methylpyridine] is carried out by using far-i.r. spectroscopy and X-ray diffractometry. The crystals of the two complexes, based on the packing of [M(pc)(4Me-py)2] adducts and solvated 4Me-py molecules, are isostructural and their orthorhombic unit cells (space group Pbca) have the following parameters: M = Co, a = 10.395(1), b = 25.069(3), c = 17.992(2) Å, 1 503 unique observed reflections, final R 0.061; M = Fe, a = 10.315(1), b = 25.006(3), c = 17.876(2) Å; 2 950 unique observed reflections, final R 0.063. All the findings lead to the conclusion that structural results and bond properties are strictly related in these complexes. The vibrational M-N(pc) parameters are much more affected than the corresponding structural parameters by differences in the electronic structures of cobalt and iron. In the case of axial M-N(4Me-py), however, the bond length depends strongly on the metal and its value (2.322 Å for M = Co and 2.040 Å for Fe) is a clear indication of the localization of the unpaired electron in the dz2 orbital of the cobalt complex. The importance of dπ→π*(pc) back donation is shown by the significant difference between the average values of the 'internal' and' external C-N bond lengths in the macrocycle.