Reversible Displacement of Polyagostic Interactions in 16e [Mn(CO)(R(2)PC(2)H(4)PR(2))(2)](+) by H(2), N(2), and SO(2). Binding and Activation of eta(2)-H(2) trans to CO Is Nearly Invariant to Changes in Charge and cis Ligands.

Electrophilic 16e [Mn(CO)(R(2)PC(2)H(4)PR(2))(2)](+) complexes (R = Et, Ph) are synthesized by metathesis of MnBr(CO)(R(2)PC(2)H(4)PR(2))(2) with Na or Li salts of low-coordinating boron or gallium anions (e.g., [B{C(6)H(3)(3,5-CF(3))(2)}(4)](-) or [Ga(C(6)F(5))(4)] (-)). They contain weak polyagostic interactions that are reversibly displaced by H(2), N(2), and SO(2) (which is a surprisingly weak ligand here). The agostic and H(2) complexes, as well as the gallium anions including the new species [{Ga(C(6)F(5))(3)}(2)(&mgr;-Cl)](-), have been characterized by NMR, IR, and X-ray crystallography. The agostic Mn-H distances (e.g., 2.9 Å) are much longer than those found for the single agostic interactions in Mo(CO)(diphosphine)(2) and [Mn(CO)(3)(PCy(3))(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 acceptors, 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 Å) and J(HD) NMR couplings (32-34 Hz) in [Mn(CO)(R(2)PC(2)H(4)PR(2))(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 sigma 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 for cationic complexes.