Reversible Displacement of Polyagostic Interactions in 16e Mn(CO)(R2PC2H4PR2)2} by H2,N2, and so2. Binding and Activation of η2-H2 trans to CO Is Nearly Invariant to Changes in Charge and cis Ligands

Wayne A. King, Brian L. Scott, Juergen Eckert, Gregory J. Kubas

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Abstract

Electrophilic l6e [Mn(CO)(R2PC2}LPR2)2]+ complexes (R = Et, Ph) are synthesized by metathesis of MnBr-(CO)(R2PC2H4PR2)2 with Na or Li salts of low-coordinating boron 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 H2, N2, and SO2 (which is a surprisingly weak ligand here). The agostic and H2 complexes, as well as the gallium anions including the new species [{Ga(C6F5)3}2(μ-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(PCy3)2]+. The H-H and also the Mn-H distances have been determined in the H2 complex by T1 measurements for both the H2 and HD isotopomers. IR data and C-O and M-C bond lengths are used to gauge the π-acceptor streǹgths of ligands trans to the CO. The agostic C-H bonds are the weakest ligands and also the weakest acceptors, but the H2 ligand is an excellent acceptor as strong as N2 and ethylene. The variation of v(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)(R2PC2H4PR2)2(H 2)] and other cationic H2 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 σ donation from H2 as compared to the more electron-rich neutral analogues where back-bonding dominates. The nature of the ligand trans to H2 (the strong acceptor CO here) controls the H-H distance more so than all of the cis ligands combined, especially for cationic complexes.

Original languageEnglish
Pages (from-to)1069-1084
Number of pages16
JournalInorganic Chemistry
Volume38
Issue number6
DOIs
StatePublished - 1999

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