  We analyze the spin-orbit and Jahn-Teller interactions in $T_d$ symmetry that
are relevant for substitutional transition metal defects in semiconductors. We
apply our theory to the substitutional nickel defect in diamond and compute the
appropriate fine-leve structure and magneto-optical parameters by means of
hybrid density functional theory. Our calculations confirm the intepretations
of previous experimental findings that the 2.56-eV and 2.51-eV optical centres
are associated with this defect. Our analysis of the electronic structure
unravels possible mechanisms behind the observed optical transitions and the
optically detected magnetic resonance signal, too.
