2,5-Diphenyl-3,4-bis(trimethylsilyl)-1-phosphacyclopentadienide as a Ligand at Calcium, Strontium, and Tin(II)

The addition reaction of tetrakis(tetrahydrofuran)calcium or -strontium bis[bis(trimethylsilyl)phosphanide] with
diphenylbutadiyne in toluene yields nearly quantitative amounts of slightly yellow tetrakis(tetrahydrofuran)calcium
(1) and -strontium bis[2,5-diphenyl-3,4-bis(trimethylsilyl)phosphacyclopentadienide] (2). Due to the fact that the
alkaline earth metal bis[bis(trimethylsilyl)amides] do not react with alkynes, colorless (tetrahydrofuran)calcium
bis[2,5-bis(tert-butyl)-1-azacyclopentadienide] (3) was prepared by the transmetalation of N-trimethylstannyl-2,5-bis(tert-butyl)-1-azacyclopentadiene with distilled calcium. The metathesis reaction of 1 and 2 with SnCl2
gives yellow tin bis[2,5-diphenyl-3,4-bis(trimethylsilyl)phosphacyclopentadienide] (4).The phosphorus atoms of
1 and 2 are in a planar environment with long M−P distances, whereas in 4 the P atoms comprise a pyramidal
coordination sphere. The tin−carbon distances are larger than observed in stannocenes which could be interpreted
as a stannylene-like structure with σ-bonded ligands. Crystallographic data for 1:  monoclinic, C2/c, a = 2108.9(3) pm, b = 1296.2(2) pm, c = 2735.3(3) pm, β = 93.37(1)°, Z = 4, wR2 = 0.1381 (all data, on F2).
Crystallographic data for 2:  monoclinic, C2/c, a = 2104.7(1) pm, b = 1296.43(6) pm, c = 2717.8(1) pm, β =
93.189(1)°, Z = 4, wR2 = 0.0954 (all data, on F2). Crystallographic data for 4:  triclinic, P1̄, a = 1253.25(4) pm,
b = 1312.58(4) pm, c = 1481.61(5) pm, α = 87.667(1)°, β = 79.438(1)°, γ = 72.684(1)°, Z = 2, wR2 = 0.0692
(all data, on F2).