This review article deals with the influence exerted by the ligands in catalytic processes promoted by substituted metallocene complexes. Here we present basically the findings of our group over the years on the syntheses, structures and reactions of substituted cyclopentadienyl (Cp^*) group 4 transition metal complexes, especially titanocenes. Application of the complexes as reagents and catalysts to the isomerization, hydrogenation and polymerization of olefins, such as 1,5 hexadiene and ethylene is discussed. Also reviewed are ring opening metathesis polymerization (ROMP) of norbonene & dicyclopentadiene, and the syndiotactic polymerization of styrene (s-PS). The synthesis and application of substituted group 3 rare-earth metallocenes to the polymerization of methyl methacrylate (MMA) is also explored. The levels of catalytic activity and conversion recorded (up to 100% in some cases) and strict selectivity (up to 98% syn) are analyzed in the context of the role played by the substitution-bearing cyclopentadienyl ligands, i.e. a structure-property correlation.