Electrophilic 16e [Mn(CO)(R2PC2H4PR2)(2)](+) complexes (R = Et, Ph) are synthesized by metathesis of MnBr(CO)(R2PC2H4PR2)(2) with Na or Li salts of low-coordinating baron or gallium anions (e.g., [B{C6H3(3,5-CF3)(2)}(4)](-) or [Ga(C6F5)(4)](-)). They contain weak polyagostic interactions that are reversibly displaced by H-2, N-2, and SO2 (which is a surprisingly weak ligand here). The agostic aid H-2 complexes, as well as the gallium anions including the new species [{Ga(C6F5)(3)}(2)(mu-Cl)](-), have been characterized by NMR, IR, and X-ray crystallography. The agostic Mn-H distances (e.g., 2.9 Angstrom) are much longer than those found for the single agostic interactions in Mo(CO)(diphosphine)(2) and [Mn(CO)(3)(PCy3)(2)](+). The H-H and also the Mn-H distances have been determined in the H-2 complex by T-1 measurements for both the H-2 and HD isotopomers. IR data and C-O and M-C bond lengths are used to gauge the pi-acceptor strengths of ligands trans to the CO. The agostic C-H bonds are the weakest ligands and also the weakest accepters, but the H-2 ligand is an excellent acceptor as strong as N-2 and ethylene. The variation of nu(CO) on increasing the basicity of the cis-phosphine (dppe versus depe) in trans-M(CO)(diphosphine)(2)(L) is less than expected and far less than that on increasing the charge on the complex (M = Mn+ versus Mo). The H-H bond lengths (0.87-0.90 Angstrom) and J(HD) NMR couplings (32-34 Hz) in [Mn(CO)(R2PC2H4PR2)(2)(H-2)](+) and other cationic H-2 complexes with trans-CO are strikingly similar to their neutral analogues and nearly invariant. Activation of H-H in the more electrophilic cationic systems occurs primarily via increased a donation from H-2 as compared to the more electron-rich neutral analogues where back-bonding dominates. The nature of the ligand trans to H-2 (the strong acceptor CO here) controls the H-H distance more so than all of the cis ligands combined, especially fbr cationic complexes.