A study is reported on synthesis and ligand substitution/modification reactions of the syn and anti geometric isomers of the six-coordinate ruthenium(II) complexes [mer-Ru(kappa(2)-O2CX)(CO)(Cyttp)](n) (n=0, X=O (2); n=+1, X=Me (3), Ph (4), OMe (5), OEt (6); Cyttp=PhP(CH2CH2CH2PCy2)(2)) and cis-mer-RuX2(CO)(Cyttp) (X = I (7), Cl (8)) (syn (s) and anti (a) refer to the orientation of the Ph group on the central P atom of Cyttp). Reaction of cis-mer-Ru(BF4)(2)(CO)(Cyttp) (1) in freshly prepared aqueous acetone solution with each of sodium carbonate, sodium acetate, and sodium benzoate affords 2a, 3a(BF4), and 4a(BF4), respectively. The syn isomer of 2a, 2s, was synthesized by prolonged treatment of solid mer-Ru(CO)(2)(Cyttp) with gaseous O-2. Complexes 2a and 2s do not interconvert even on heating in toluene or methanol at reflux temperature for 24 h. Alkylation of 2a and 2s with 1 equiv. of [Me3O]BF4 or [Et3O]PF6 affords the methylcarbonato (5a(BF4) and 5s(BF4), respectively) and ethylcarbonato (6a(PF6) and 6s(PF6), respectively) complexes. Use of >2 equiv. of [Me3O]BF4 still furnishes 5s(BF4) from 2s, but yields 1 instead of 5a(BF4) from 2a. The foregoing reactions represent, to our knowledge, the first example of different reactivity of syn and anti isomers of metal Cyttp complexes. Prolonged treatment of 2a with an excess of MeI at room temperature results in the formation of 7a. A parallel reaction of 2s with MeI to yield 7s requires heating; at ambient temperature, 5s(I) is obtained instead. Each of the complexes 2, 3(BF4)-5(BF4), and 6(PF6) is converted to 8 with retention of the syn or anti configuration by the action of hydrochloric acid. It is concluded from this and previous studies that complexes stable to ligand dissociation do not undergo syn-anti isomerization. in contrast to those that contain weakly coordinated ligands. C-13{H-1} and P-31{H-1} NMR spectroscopic generalizations were developed that enable syn-anti assignment to be made for this class of complexes. The structures of 2a (as 2a.CH2Cl2.2H(2)O), 2s (as 2s.(3/4)MeC(O)Me-2H(2)O). and 5s (as 5s(BF4).C6H6) were determined by single crystal X-ray diffraction analysis. (C) 2004 Elsevier B.V. All rights reserved.