The dihalide capped trinuclear nickel clusters Ni-3(mu(3)-X)(2)(mu(2)-dppm)(3) (X = I (1a), Br (1b); dppm = Ph(2)PCH(2)PPh(2)) were synthesized and converted to their respective monocations [Ni-3(mu(3)-X)(2)(mu(2)-dppm)(3)](+) (X = I (1a(+)), Br (1b(+))) via a single electron oxidation. Clusters 1a and 1a(+) were characterized by X-ray crystallography. Displacement of a triply-bridging iodide ligand in Ni-3(mu(3)-I)(2)(mu(2)-dppm)(3) (1a) by pi-acceptor ligands produces a class of 48-electron trinuclear nickel clusters of the general formula [Ni-3(mu(3)-L)(mu(3)-I)(mu(2)-dppm)(3)](+) (L = CO (2a); CNR, R = CH3 (3a), 2,6-(CH3)(2)C6H3 (4a), i-C3H7 (5), C6H11 (6), t-C4H9 (7), CH2C6H5 (8), C6H5 (9), p-C6H4I (10), p-C6H4Br (11), p-C6H4Cl (12), p-C6H4F (13), p-C6H4CH3 (14), p-C6H4CF3 (15), p-C6H4OCH3 (16), and p-C6H4CN (17)). A similar but less extensive study was conducted with the dibromide capped cluster 1b. The X-ray crystal structure of cluster 2a, as the PF6- salt, was also obtained. Clusters 2-17 possess strikingly similar spectroscopic and electrochemical properties. This is ascribed to the lack of interaction between the a(2) LUMO of clusters 2-17 and the molecular orbitals of the capping pi-acceptor ligands. The unexpected appearance of two nu(C=N) bands in the FT-IR spectra of clusters 3-17 was demonstrated to be the result of a Fermi resonance involving the nu(C=N) fundamental and the first overtone of the nu(N-C(alkyl)) fundamental of the capping isocyanide. In addition, molecular orbital calculations on 1-17 provide insights into the differences in the physical properties and reactivities of clusters of this class capped by pi-donor (1) or pi-acceptor ligands (2-17).