Structures and Strengths ofM-H-C Interaction

In 1965 Ibers13 and Mason14 independently provided the first evidence for close approach of C-H bonds to Ru and Pd metal centers as in 1. The d were long (e.g., 

Multiple agostic interactions are common, in which case each interaction may represent fractional donation of electron pairs and is correspondingly weaker (longer M-H and M-C distances). In some cases hydrogen bonding between M and the proton may predominate, as found by Brammer in [R3N-H+ Co(CO)4], where Co-H is 2.613(2) A by neutron diffraction.21 In this case and others (see Section 

2) it is clear that the 18e Co(CO)T ion cannot accept an electron pair from the NH bond, and the interaction is a 3c-4c bond as in a conventional hydrogen bond except that M is now the weak-base bond acceptor. 

The C-H bond initially approaches M end-on as for incipient M-H2 interaction, and then rotates to bring the C closer to M as the C-H bond weakens 

IrX(PH3)2 (X = H, Cl)29 where a two-stage mechanism occurs for the latter. In the early part of the reaction (electrophilic stage), which closely parallels agostic interactions, a donation to M is important. Thus a more strongly bound CH4 adduct 

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