钛,锆和铪羰基化合物M(CO)_n(M=Ti,Zr,Hf;n=4～7)的理论研究

Theoretical study of M(CO)_n(M=Ti,Zr,Hf;n=4～7)

利用密度泛函方法对标题化合物的平衡几何、热化学及振动频率进行了理论预测,发现这3种金属原子都有相似的M(CO)n(n=4～7)结构.全局最低构型对M(CO)7都是单态C3v戴帽八面体7S-1,对M(CO)6都是三重态D3d畸变八面体6T(而对应的单重态M(CO)5仅比它低不到21kJ·mol-1).对M(CO)n(n=5,4)都是三重态6S-1,其构型分别为从6T中移去1个或2个CO基的衍生物5T和4T.此外,五重态的D3h的三角双锥M(CO)5和单态的Td四面体M(CO)4以及能量更高的含有C和O同时与金属成键的独特配位CO基的M(CO)6和M(CO)3也被发现.最后,给出M(CO)7→M(CO)6+CO反应的离解能.并讨论了金属18价电子的Ti(CO)7存在的可能性.

The equilibrium geometries and therrnochemistry of the homoleptic metal-carbonyls of the group 4 elements,M（CO）n （M = Ti,Zr, Hf; n = 4 - 7） were predicted using density functional theory. Analogous M（CO）n structures were found for all three metals. The global minima for the 18-electron M（CO）7 molecules are all singlet C3v capped octahedra. The global minima for the 16-electron M（CO）6 species are triplet M（ CO）6 structures distorted from Oh symmetry to D3a symmetry.However, the corresponding singlet M（CO）6 structures lie within 21 kJ/mol of the triplet global minima.The global minima for M（CO）n（n = 5,4） are triplet structures derived from the D3d distotted octahedral structures of M （ CO）6 by removal of one or two CO groups, respectively. Quintet D3h trigonal bipyramidal structures for M（CO）5 and singlet Td tetrahedral structures for M（CO）4 are also found, as well as higher energy structures for M（ CO）6 and M（CO）7 containing a unique CO group bonded to the metal atom through both M-C and M-O bonds. The dissociation energies M（ CO）7→M（CO）6 ＋ CO are stthstantial, indicating no ftmdamental problem in bonding seven CO groups to a single metal atom.