A Ruthenium complex represented by the structure of any of formulae A1, A2, A3 or A4: wherein L 1 is selected from the group consisting of phosphine (PR b R c), phosphite P(OR b)(OR c), phosphinite P(OR b)(R c), amine (NR b R c), imine, oxazoline, sulfide (SR b), sulfoxide (S(=O)R b), heteroaryl containing at least one heteroatom selected from nitrogen and sulfur; arsine (AsR b R c), stibine (SbR b R c) and a N-heterocyclic carbene represented by the structures: L 2 is a mono-dentate two-electron donor selected from the group consisting of CO, PR b R c R d, P(OR b)(OR c)(OR d), NO +, AsR b R c R d, SbR b R c R d, SR b R c, nitrile, isonitrile, N 2, PF 3, CS, heteroaryl, tetrahydrothiophene, alkene and alkyne; L 3 is absent or is L 2; Y and Z are each independently H or an anionic ligand selected from the group consisting of H, halogen, OCOR, OCOCF 3, OSO 2 R, OSO 2 CF 3, CN, OR, N 2 and RS; R a is H, alkyl, cycloalkyl, aryl, heterocyclyl, heteroaryl, alkylcycloalkyl, alkylaryl, alkylheterocyclyl or alkylheteroaryl; R b, R c and R d are each independently alkyl, cycloalkyl, aryl, heterocyclyl, heteroaryl, alkylcycloalkyl, alkylaryl, alkylheterocyclyl or alkylheteroaryl; R, R 1, R 2 and R 3 are each independently H, alkyl, cycloalkyl, aryl, heterocyclyl, heteroaryl, alkylcycloalkyl, alkylaryl, alkylheterocyclyl or alkylheteroaryl; X represents zero, one, two or three substituents independently selected from the group consisting of alkyl, cycloalkyl, aryl, heterocyclyl, heteroaryl, alkylcycloalkyl, alkylaryl, alkylheterocyclyl, alkylheteroaryl, halogen, nitro, amide, ester, cyano, alkoxy, alkylamino, arylamino, an inorganic support and a polymeric moiety; anion ⊝ represents a group bearing a single negative charge; and cation ⊕ represents a group bearing a single positive charge. L 1 is selected from the group consisting of phosphine (PR b R c), phosphite P(OR b)(OR c), phosphinite P(OR b)(R c), amine (NR b R c), imine, oxazoline, sulfide (SR b), sulfoxide (S(=O)R b), heteroaryl containing at least one heteroatom selected from nitrogen and sulfur; arsine (AsR b R c), stibine (SbR b R c) and a N-heterocyclic carbene represented by the structures: L 2 is a mono-dentate two-electron donor selected from the group consisting of CO, PR b R c R d, P(OR b)(OR c)(OR d), NO +, AsR b R c R d, SbR b R c R d, SR b R c, nitrile, isonitrile, N 2, PF 3, CS, heteroaryl, tetrahydrothiophene, alkene and alkyne; L 3 is absent or is L 2; Y and Z are each independently H or an anionic ligand selected from the group consisting of H, halogen, OCOR, OCOCF 3, OSO 2 R, OSO 2 CF 3, CN, OR, N 2 and RS; R a is H, alkyl, cycloalkyl, aryl, heterocyclyl, heteroaryl, alkylcycloalkyl, alkylaryl, alkylheterocyclyl or alkylheteroaryl; R b, R c and R d are each independently alkyl, cycloalkyl, aryl, heterocyclyl, heteroaryl, alkylcycloalkyl, alkylaryl, alkylheterocyclyl or alkylheteroaryl; R, R 1, R 2 and R 3 are each independently H, alkyl, cycloalkyl, aryl, heterocyclyl, heteroaryl, alkylcycloalkyl, alkylaryl, alkylheterocyclyl or alkylheteroaryl; X represents zero, one, two or three substituents independently selected from the group consisting of alkyl, cycloalkyl, aryl, heterocyclyl, heteroaryl, alkylcycloalkyl, alkylaryl, alkylheterocyclyl, alkylheteroaryl, halogen, nitro, amide, ester, cyano, alkoxy, alkylamino, arylamino, an inorganic support and a polymeric moiety; anion ⊝ represents a group bearing a single negative charge; and cation ⊕ represents a group bearing a single positive charge.