Structure and bonding of H2 coordination compounds

The activation of hydrogen by a metal center is one of the most important chemical reactions. The H-H bond is strong (436 kJ mol-1), so that H2 addition to unsaturated organic and other compounds must be mediated by metal centers whose roles constitute the basis of catalytic hydrogenation. In catalytic mechanisms, hydride complexes formed by the cleavage of H2 are regarded as key intermediates. 

The first isolable transition metal complex containing a coordinated H2 molecule is W(CO)3[P(CHMe2)3]2(H2). X-ray and neutron diffraction studies and a variety of spectroscopic methods have confirmed that it possesses a i72-H2 ligand, as shown in Fig. 11.5.1. 

The resulting geometry about the W atom is that of a regular octahedron. The H2 molecule is symmetrically coordinated in an q2 mode with an average W-H distance of 185 pm (X-ray) and 175 pm (neutron) at -100 °C. The H-H distance is 75 pm (X-ray) and 82 pm (neutron), slightly longer than that of the free H2 molecule (74 pm). 

The side-on bonding of H2 to the metal involves the transfer of a bonding electrons of H2 to a vacant metal d orbital (or hybrid orbital), together with the transfer of electrons from a tilled metal d orbital into the empty a orbital of H2, as shown in Fig. 11.5.2. This synergistic (mutually assisting) bonding mode is similar to that of CO and ethylene with metal atoms. The n back donation from 

The structure of /rry v-[ 1 e( r 2-I l2)f 11)fPPh2(TI I2CU l2l Ph2) 1 BPh4 has been determined by neutron diffraction at 20 K. The coordination environment about the Fe atom is shown in Fig. 11.5.3. This is the first conclusive demonstration of the expected difference between a hydride and a H2 molecule in coordination to the same metal center. The H-H bond distance is 81.6 pm and the H-Fe bond length is 161.6 pm, which is longer than the terminal H-Fe distance of 153.5 pm. 

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