Cyanide Metallocenes of Trivalent f-Elements

Addition of NnBu4CN to (C5Me5)2UI(py), (C5Me5)2CeI, or (C5Me5)2M(OTf) (M = U, Ce) in acetonitrile
led to the precipitation of the neutral monocyanide species [(C5Me5)2M(μ-CN)]n [M = U (1), Ce (2)],
which likely have an oligomeric structure, as shown by the trimeric cyanide-bridged complex [(C5Me5)2Ce(μ-CN)(CNtBu)]3 (3) obtained by addition of excess tBuNC into a suspension of 2 in acetonitrile. The
structure of the U(III,IV) mixed valence compound [{(C5Me5)2U}2(μ-CN){(μ-CN)2Na(thf)}2]∞ (4), which
crystallized from a thf solution of (C5Me5)2UI(py) in the presence of excess NaCN, reveals a unique
example of an f-element−(μ-CN)−M interaction (M = main group or d transition metal). The anionic
polycyanides [(C5Me5)2M(CN)3][NnBu4]2 [M = U (5), Ce (6)] were synthesized by treatment of 1 and 2
with 2 equiv or an excess of NnBu4CN in acetonitrile; they were also prepared in a one-pot procedure by
stepwise addition of 1 equiv of KCN and 2 equiv of NnBu4CN to the parent iodides in acetonitrile. The
bent metallocenes 5 and 6 are unique low-valent molecular polycyanide compounds of an f-element that
have been structurally identified, while 5 is the first fully characterized actinide(III) cyanide. Comparison
of the crystal structures of 5 and 6 shows that the M−C(C5Me5) and M−C(CN) distances are 0.02−0.03
Å shorter for M = U than for M = Ce, while the ionic radius of uranium(III) is 0.02 Å larger than that
of cerium(III).