A STUDY OF POLARITIES, POLARIZABILITIES AND INFRARED-SPECTRA OF LM(CO)5 COMPLEXES (M = CR, MO OR W - L = TRIMETHYLAMINE, QUINUCLIDINE, PYRIDINE, PARA-METHYLPYRIDINE OR PARA-TERT-BUTYLPYRIDINE) AND OF THE X-RAY CRYSTAL-STRUCTURE OF QUINUCLIDINE-CR(CO)5 - EVIDENCE FOR PI-ACCEPTOR BEHAVIOR OF COORDINATED PYRIDINE

Carbonyl infrared frequencies. electric dipole moments and optical polarisability anisotropies of pyridine and of p-methyl and p-t-butyl-pyridine-M(CO)5 complexes (M = Cr, Mo or W) are compared with the corresponding parameters for the analogous trimethylamine and quinuclidine-M(CO)5 complexes. For the same ligand L in LM(CO)5, the sequence of nu-(CO)A1 is Cr < W less-than-or-equal-to Mo. The carbonyl bands are only slightly affected by variation in L or M in the complexes studied. Trends in the molecular dipole moments of the LM(CO)5 complexes suggest a degree of pi-acceptor capability for the bonded pyridine system but the evidence is not conclusive. Anisotropic molecular polarisabilities derived from experiment for the pyridine and p-substituted pyridine-M(CO)5 complexes, when compared with parameters calculated for a purely sigma-bonded N-M model for such systems, show a strong enhancement of polarisability specifically along the pyridine-M-trans(CO) axis. This is related to conjugative electron delocalisation between the pyridine and M(CO)5 moieties and indicates a degree of pi-bond character in the connecting N-M bond. The crystal structure of quinuclidine-Cr(CO)5 is reported. Comparison of bond dimensions of this complex with those of pyridine-Cr(CO)5 yields positive though qualified support for a pi-component in the N-Cr bond of pyridine-Cr(CO)5.