We have previously reported (J. Am. Chem. Soc. 1995, 117, 12639) the arene complex (η 6 -C 6 H 6 )Mo(TRIPOD) (1), where TRIPOD ) 1,1,1-tris(diphenylphosphinomethyl)ethane, is protonated upon addition of 1 equiv of D + to yield the metal-hydride [(η 6 -C 6 H 5 D)Mo-(TRIPOD)H] + (1H + -d 1 ) via exo addition of D + to the arene ligand followed by migration to the metal of the endo proton of the putative η 5 -cyclohexadienyl complex [(η 5 -C 6 H 6 D)Mo-(TRIPOD)] + (1 ‡ ). The opposite isotopomer is obtained when (η 6 -C 6 D 6 )Mo(TRIPOD) (1-d 6 ) is employed to give [(η 6 -C 6 D 5 H)Mo(TRIPOD)D] + (1D + -d 6 ). Our recent investigation of the electronic structure of 1 and its one-electron oxidized derivative [(η 6 -C 6 H 6 )Mo(TRIPOD)] + (1 + ) suggested the mechanism of protonation is not driven entirely by electronic factors; steric factors are likely important for indirect protonation of 1. Consistent with that conclusion, we report herein that some other phosphine derivatives are protonated directly at the metal center. Thus, under the same reaction conditions the monodentate phosphine derivatives (η 6 -C 6 D 6 )Mo(PR 3 ) 3 , where PR 3 ) PPh 2 Me (3-d 6 ), PPhMe 2 (4-d 6 ), and PMe 3 (5-d 6 ), are protonated directly at the metal 0%, 20%, and 40% of the time, respectively. The remainder of the protonations take place via the aforementioned indirect protonation of the arene ligand. Surprisingly, (η 6 -C 6 D 6 )Mo(TRIPHOS) (2-d 6 ), where TRIPHOS ) bis(2-diphenylphosphinoethyl)phenylphosphine, reacts with H + by direct protonation of the metal center 80% of the time to give a C 1 -symmetric kinetic hydride [(η 6 -C 6 D 6 )Mo(TRIPHOS)H] + (2 K H + -d 6 ). The other 20% of the time 2-d 6 reacts via the arene mechanism to give [(η 6 -C 6 D 5 H)Mo(TRIPHOS)D] + (2 K D + -d 6 ). The two enantiomeric forms of 2 K H + are observed to be in rapid equilibrium (k ) 6.4 × 10 3 s -1 at -87°C). Compound 2 K H + eventually isomerizes (k ) 6.1 × 10 -4 s -1 at -60°C) to the C s -symmetric thermodynamic hydride [(η 6 -C 6 H 6 )Mo-(TRIPHOS)H] + (2 T H + ). We conclude from the crystal structures of 1-4, tracer studies, and electronic structure calculations of the five derivatives 1-5 presented herein that indirect protonation does not represent a general mechanism for this class of compounds. In addition to electronic factors that favor direct protonation of the metal center from an equatorial trajectory, subtle steric demands of the phosphine ligands play an important role in dictating whether a proton initially attacks the arene ligand or the metal of 1-5. The semiempirical PM3(tm) method may be used to map metal-based frontier orbitals of the complexes 1-5 onto plots of their surface electron densities, thereby revealing sterically accessible metal electron density. Such plots provide straightforward models that predict the mechanisms of protonation.

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