Question #44878

Consider the following statements regarding the bonding and properties of transition metal complexes and interstitial compounds at $298\\text{ K}$:\n\nI. In the non-stoichiometric interstitial hydride $PdH_{0.56}$, the trapped hydrogen atoms stabilize the lattice via synergic back-bonding between the filled metal $d$-orbitals and the vacant hydrogen $1s$ orbital.\nII. The $C-O$ stretching frequency in $[Ti(CO)_6]^{2-}$ is lower than that in $[V(CO)_6]^-$ due to a greater extent of metal-to-ligand $d\\pi \\rightarrow \\pi^*$ back-donation.\nIII. Interstitial carbides such as $WC$ are chemically inert and extremely hard because the carbon atoms occupy interstitial voids without disrupting the metallic lattice conductivity, lacking the precise coordinate-covalent geometry of synergic bonds.\nIV. The formation of metal carbonyls requires the transition metal to have vacant $d$-orbitals for $\\sigma$-donation and filled $d$-orbitals for $\\pi$-back-donation, a symmetry condition not met by interstitial hydrogen atoms.\n\nHow many of the above statements are correct?