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The extraplanar ligand-exchange dynamics of (tetraphenylporphinato)zinc(II) with pyridine and /V-methylimidazole have been studied at 21 °C in CDC13. For the ZnTPP-py system we find kon = 4.90 X 10s M'1 s'1, ko[f = 1.98 X 105 6s'1, and K = 2300 ± 400 M'1. For the ZnTPP-/V-MeIm system we find kon = 1.67 X 108 M'1 s'1, koñ = 4.78 X 104 s'1, and K = 10 200 ± 600 M"1. An analysis of the steric limitations of ZnTPP and the ligands (pyridine and /V-methylimidazole) suggests that kon is close to, if not at, the diffusion-controlled rate limit. Since the ZnTPP-L complexes are known to have the Zn(II) displaced some 0.3 Á out of the mean plane of the porphyrin ring, a diffusion-controlled kon requires that the unligated ZnTPP also be nonplanar.

The title complex, [Co(NH3)5(N02Im)]Cl2, was prepared by the action of a standard HN03-H2S04 nitrating mixture on Co(NH3)5(ImH)3+. The complex has been characterized by NMR and UV-vis spectroscopy and a single-crystal X-ray structure determination. The pH dependence of the NMR spectra has been determined, and the pXa of the coordinated N02lm is 1.66. Crystal data for [Co(NH3)5(N02Im)]Cl2 is as follows: space group Pljn•, Z = 4; a = 9.434(2), b = 13.679 (3), c = 10.457 (3) A; 0 = 93.65 (2)°; V-1346.7 (5) A3; R = 4.5% for 2135 reflections.

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