The structures of gas-phase M(CO) (M = Co, Rh, Ir; n = 2-15) ion-molecule complexes have been investigated using a combination of infrared resonance-enhanced photodissociation (IR-REPD) spectroscopy and density functional theory. The results provide insight into fundamental metal ion-CO interactions, highlighting the trends with increasing ligand number and with different group 9 ions. Spectra have been recorded in the region of the CO asymmetric stretch around 2350 cm using the inert messenger technique and their interpretation has been aided by comparison with simulated infrared spectra of calculated low-energy isomeric structures. All vibrational bands in the smaller complexes are blue-shifted relative to the asymmetric stretch in free CO, consistent with direct binding to the metal center dominated by charge-quadrupole interactions. For all three metal ions, a core [M(CO)] structure is identified to which subsequent ligands are less strongly bound. No evidence is observed in this size regime for complete activation or insertion reactions.

SUMMARY1. Intracellular pH (pHi) was measured in crab muscle fibres using pH-sensitive micro-electrodes. The mean stable pHi was 7-19+0-02 (S.E. of mean) and the corresponding mean membrane potential was -64-6+ 04 mV (S.E. of mean) at an external pH of 7-5.2. The effects on pHi of replacing 20 % (100 mM) of the external NaCl by the Na salts of various anions were examined. The anions of weak acids (pK'a > 4 5) caused large internal acidifications. The anions of strong acids (pK'a < 2-6) caused little or no change in pHi. The anions of acids with an intermediate pK' had varied effects on pHi. In particular salicylate (pK' = 2 97) was found to cause a large fall in pHi.3. Increasing the external pH reduced the effects of the anions of weak acids on pHi. It is argued that these effects are the result of the entry and subsequent dissociation of undissociated acid molecules.4. The results with propionate were quantified by comparing them with the effects of 5 % CO2 andwere found to be smaller than expected. It is suggested that this is the result of substantial membrane permeability to the propionate anion.