The authors report the synthesis and structure and discuss in substantial detail the bonding of a remarkable family of compounds spanning most of the transition series. All members of the MTSi (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni; T = Co, Ni) series have now been prepared, and their crystal structures have been determined accurately using X-ray diffraction and, for MnNiSi and FeNiSi, neutron diffraction. Each of these compounds, four of which were previously unknown, crystallizes in the TiNiSi structure type. In contrast, for T = Cu, one has only representatives with M = Sc and Ti. A simple Zintl picture, which works so well for the three-dimensional four-connected indium net of the related BaIn{sub 2} structure, is not applicable to these intermetallics; there is substantial M-T and M-Si bonding, and the extra electrons in the MTSi structure enter orbitals essentially nonbonding within the TSi network. A structural change observed for M = Fe and higher d-electron counts, distorting the six-rings in the structure, is traced to the formation of M-M bonds. The instability of the corresponding Cu phases is discussed in terms of Miedema`s ideas about electronegativity and electron density.