  We have studied the electronic structure of 3d transition-metal- MgB_{2}
alloys, Mg_{0.97}TM_{0.03}B_{2}, (TM\equiv Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn) using KKR-CPA method in the atomic-sphere approximation. For unpolarized
calculations, our results for Mg_{0.97}TM_{0.03}B_{2} alloys are similar to
that of 3d impurities in other s and s-p metals. In particular, the local
densities of states (DOS) associated with the 3d impurities are similar to our
earlier work on 3d impurities in bulk Al. For spin-polarized calculations, we
find only the alloys of V, Cr, Mn, Fe and Co with MgB_{2} to be magnetic of all
the 3d elements. We also find that Cr and Mn in MgB_{2} have a relatively large
local magnetic moment of 2.43 \mu_{B} and 2.87 \mu_{B}, respectively. We have
used the unpolarized, self-consistent potentials of Mg_{0.97}TM_{0.03}B_{2}
alloys, obtained within the coherent- potential approximation, to calculate the
electron-phonon coupling constant \lambda using the Gaspari-Georffy formalism
and the superconducting transition temperature T_{c} using the Allen-Dynes
equation. We find that the calculated T_{c} is the lowest for
Mg_{0.97}Cr_{0.03}B_{2} and the highest for Mg_{0.97}Zn_{0.03}B_{2}, in
qualitative agreement with experiment. The calculated trend in variation of
T_{c} from Mn to Zn is also similar to the available experimental data. Our
analysis of the variation in T_{c}, in terms of the DOS and the spectral
function along \Gamma to A direction, shows the variation to be an interplay
between the total DOS at the Fermi energy and the creation/removal of states
along \Gamma to A direction [P. P. Singh, cond-mat/0201093].
