Bonding situation and stability of η1- and η6-bonded heteroarene complexes M(η1-EC5H5)6 and M(η6-EC5H5)2 (M = Cr, Mo, W; E = N, P, As, Sb, Bi)

Density functional calculations at the BP86/TZ2P level are reported for the pseudo-octahedral heteroarene complexes M(η1-EC5H5)6 and for the sandwich complexes M(η6-EC5H5)2 (M = Cr, Mo, W; E = N, P, As, Sb, Bi). The complexes M(CO)6 and M(η6-C6H6)2 have been calculated for comparison. The nature of the metal–ligand interactions was analyzed with the EDA (energy decomposition analysis) method. The calculated bond dissociation energies (BDE) of M(η1-EC5H5)6 have the order for E = P > As > N > Sb ≫ Bi and for M = Cr < Mo < W. All hexaheteroarenes bind more weakly than CO in M(CO)6. Except for pyridine, which is the weakest η6-bonded ligand, the trend in the BDE of the M(η6-EC5H5)2 complexes is opposite to the trend of the M(η1-EC5H5)6 complexes NC5H5 < PC5H5 < AsC5H5 < SbC5H5 < BiC5H5. The opposite trend is explained with the different binding modes in M(η6-EC5H5)2 and M(η1-EC5H5)6. The bonding in the former complexes mainly takes place through the π electrons of the ligand which are delocalized over the ring atoms while the bonding in the latter takes place through the lone-pair electrons of the heteroatoms E. The Lewis basicity of the group-15 heterobenzenes EC5H5 becomes weaker for the heavier elements E. The occupied π orbitals of the heterobenzene ring become gradually more polarized toward the five carbon atoms in the heavier arenes EC5H5 which induces stronger metal-carbon bonds in M(η6-EC5H5)2 and weaker metal-E bonds. The EDA calculations show that the nature of the M-EC5H5 bonding in M(η1-EC5H5)6 is similar to the M–CO bonding in M(CO)6. Both types of bonds have a slightly more covalent than electrostatic character. The π orbital interactions in the chromium and molybdenum complexes of CO and heterobenzene are more important than the σ interactions. This holds true also for the tungsten complexes of CO and the lighter heteroarenes while the σ- and π-bonding in the heavier W(η1-EC5H5)6 species have similar strength. The EDA results also show that the nature of the bonding in the sandwich complexes M(η6-EC5H5)2 is very similar to the bonding in the bisbenzene complexes M(η6-C6H6)2. The orbital interactions contribute for all metals and all arene ligands about 60% of the attractive interactions while the electrostatic attraction contributes about 40%. The largest contribution to the orbital term comes always from the δ orbitals. The calculations predict that the relative stability of the sandwich complexes M(η6-EC5H5)2 over the octahedral species M(η1-EC5H5)6 increases when E becomes heavier and it increases from W to Mo to Cr when E = N, P, As.