The equilibria between the five-and six-coordinate species of alkylcobinamide and cobalamin (base-off) have been studied using high-pressure spectroscopic techniques. The

Studies of the emission spectra of four Co(III) cobinamides (diaquo-, aquohydroxo-, dihydroxo- and dicyano-) show (1) that the excited states corresponding to the alphabeta and epsilon absorption bands behave like the S(1) and S(2) levels in the non-alternant hydrocarbon azulene (with emission from S(2)>> S(1) in violation of Kasha's rule) and (2) that the excited states include a TICT (twisted intramolecular charge transfer) mechanism, as in the simpler cyanines, but where the TICT state gives rise to dual fluorescence instead of cis-trans isomerisation. Combined with the previously reported dual fluorescence from the S(1) level in synthetic metal corrinoids and in the naturally-occurring metal-free corrin, this provides evidence that the existence of an additional (metastable) ground state with a significantly different vibronic splitting and nuclear configuration is an intrinsic property of the basic corrin ligand (irrespective of the nature of the side-chains and the metal ion or even the absence of a metal) which distinguishes it from porphyrin. The occurrence of hysteresis (and its associated oscillations) in redox reactions of the cobinamides involving both the Co(III/II) and Co(II/I) couples indicates that the corrin ligand also has an intrinsic ability to exist in different conformations or "allosteric" forms with differing redox potential, which further distinguishes it from the porphyrin ligand. Possible links between the existence of an additional metastable ground state and of allosteric changes and the likely reasons for the selection of corrin over a porphyrin for the vitamin B(12)-dependent enzymes are discussed.

Equilibrium constants K for the substitution of co-ordinated H 2 O in [Fe II (CN) 5 (OH 2 )] 3Ϫ by sterically hindered amines (both α and β branching, viz. NH 2 Me-NMe 3 and NH 2 Me-NH 2 Bu t ) and by aniline have been determined in aqueous solution with I = 0.1 mol dm Ϫ3 (NaClO 4 ) at 25 ЊC by UV/VIS spectrophotometry. Comparison with published data on the rates of Fe᎐N bond fission and their further correlation with the energetics of protonation of the free amines shows that the observed variation in log K for aliphatic amines reflects 'solvation' as well as more typically steric effects. Aniline differs from the aliphatic amines both in its smaller crystal-field splitting (shared by other aromatic amines) and a low value of log K comparable to that of NMe 3 .