Geometric and electronic structures of metal-substituted fullerenes C59M (M=Fe, Co, Ni, and Rh) have been studied using the local spin density formalism. The clusters are found to be stable in the cage structure, while the deformation of the fullerene network due to the substituted metal atom is larger than that due to the substituted boron or nitrogen atom. The electronic structure of C59M varies with the M, and can be described in terms of defect levels in the host fullerene. Deep defect states due to the metal atom appear in the carbon-derived energy gap, and high chemical reactivity is predicted for the clusters. The magnetic moment of the M atom in C59M is found to be almost completely quenched due to the strong hybridization between the orbitals of the M atom and of their neighboring carbons. The vertical ionization potentials and electron affinities are predicted for all the clusters.