AGOSTIC BOND

Metal hydrogen interactions with C-H groups

There is compelling evidence that transition metal complexes may bind to C-H groups on ligands to give M......H-C "hydrogen bonding" type interactions. The term "agostic"has been coined by M.L.H Green for such hydrogen atoms (In Greek agostic means "to clasp" )

Two main types of M........H-C interactions referred to as open or closed are known. Complexes which would otherwise be coordinatively unsaturated can be stabilized by the interaction of C-H bonding pair of electrons with the metal.

1) Open Interaction:

The interaction appears to arise between an electron deficient metal center and a C-H bond of an alkyl group or a phenyl group. Examples of ortho hydrogen atoms on phenyl rings of a phosphene being near to a metal and occupying a coordination site have long been known. An early examples was RuCl2(PPh3)2 (16 electrons) where interaction with C-H would tend to move the complex to an 18 electron configuration.

Another exemple of this agostic is in the structure of W(CO)3(PCy3)2(Cy-cyclo-hexyl) where one ligand distort to permit close approach of the C-H bond to the tungsten and thus increases the electron count from 16 to 18.

2) Closed M........H-C Interactions:

This class differs from the previous one in that it involves allylic or cycloalkenyl systems where the metal atom attempts to achieve an 18 electron configuration by M.......H-C 2e-3C bond formation.
A vacant metal orbital is the necessary but not the sufficient condition for agostic interaction to occur. Many compounds are known where structure and spectra give no evidence of agostic interactions even though there is an empty metal orbital.