cis-[Ru(NO)Cl(pyca)(2)] (pyca = 2-pyridinecarboxylato), in which the two pyridyl nitrogen atoms of the two pyca ligands coordinate at the trans position to each other and the two carboxylic oxygen atoms at the trans position to the nitrosyl ligand and the chloro ligand, respectively (type I shown as in Chart 1), reacted with NaOCH3 to generate cis-[Ru(NO)(OCH3)(pyca)(2)] (type I). The geometry of this complex was confirmed to be the same as the starting complex by X-ray crystallography: C13.5H13N3O6.5Ru; monoclinic, P2(1)/n; a = 8.120(1), b = 16.650(1), c = 11.510(1) Angstrom; beta = 99.07(1)degrees; V = 1536.7(2) Angstrom(3); Z = 4. The cis-trans geometrical change reaction occurred in the reactions of cis-[Ru(NO)(OCH3)(pyca)(2)] (type I) in water and alcohol (ROH, R = CH3, C2H5) to form [{trans-Ru-(NO)(pyca)(2)}(2)(H3O2)](+) (type V) and trans-[Ru(NO)(OR)(pyca)(2)] (type V). The reactions of the trans-form complexes, trans-[Ru(NO)(H2O)(pyca)(2)](+) (type V) and trans-[Ru(NO)(OCH3)(pyca)(2)] (type V), with Cl- in hydrochloric acid solution afforded the cis-form complex, cis-[Ru(NO)Cl(pyca)(2)] (type I). The favorable geometry of [Ru(NO)X(pyca)(2)](n+) depended on the nature of the coexisting ligand X. This conclusion was confirmed by theoretical, synthetic, and structural studies. The mono-pyca-containing nitrosylruthenium complex (C2H5)(4)N[Ru(NO)Cl(3)pyca)] was synthesized by the reaction of [Ru(NO)Cl-5](2-) with Hpyca and characterized by X-ray structural analysis: C14H24N3O3Cl3Ru; triclinic, P (1) over bar, a = 7.631 (1), b = 9.669(1), c = 13.627(1) Angstrom; alpha = 83.05(2), beta = 82.23(1), gamma = 81.94(1)degrees; V = 981.1(1) Angstrom(3); Z = 2. The type II complex of cis-[Ru(NO)Cl(pyca)(2)] was synthesized by the reaction of [Ru(NO)Cl-3(pyca)(2)](-) or [Ru(NO)CI5]2- with Hpyca and isolated by column chromatography. The structure was determined by X-ray structural analysis: C12H8N3O5ClRu; monoclinic, P2(1)/n; a = 10.010(1), b = 13.280(1), c = 11.335(1) Angstrom; beta = 113.45(1)degrees; V = 1382.4(2) Angstrom(3); Z = 4.