Nickel"II#\ palladium"II#\ and platinum"II# complexes
2108
of the "BF3#− anions in 5 by chlorides leads to a shift nature of these CÐH = = = M"d7# interactions ð6Ł"a#\ an
of n"NÐH# to 1706 cm−0 certainly due to a NÐH = = = Cl− attractive weak hydrogen bond can now be ruled out
bridge[
Comparable
to
this\
n"NÐH#
of ð6Ł"b#\ð18Ł[ The distortions could be a result of the
ðPt"dppe#"dppam#ŁCl1 "8# occurs at 1790 cm−0[ The general trend in molecular crystals to produce closest
lack of this shift of n"NÐH# toward lower energies in packings ð29Ł and a repulsive Coulomb interaction
ðNiCl"cis!dppen#"dppam#Ł"BF3# "3# "see above#\ between the positive partial charges on the d7 metals
clearly con_rms the presence of coordinated chloride[ and the aryl protons[
The
compounds
ðM1"CN#3"m!trans!dppen#1Ł
The above given trend of 0[95\ 0[06\ and 0[05 Aý for
"M1Pd "6#\ M1Pt "7## are two further rare examples the covalent radii of square planar d7 Ni\ Pd\ and Pt\
of d7 metal cis!dicyanide complexes[ The dP values of respectively\ is in agreement with the trend of the
05[7 "6# and 06[3 "7# clearly indicate a normal coor! corresponding ionic radii in crystals for the square
dination shift without any ring contribution\ since it planar d7 metals Ni of 9[33\ Pd of 9[53\ and Pt of
is impossible for trans!dppen to chelate[ Together with 9[59 Aý ð20Ł[ A similar tendency for the same d7 metals
the single 20P 0H NMR resonances in both cases\ this is has been found only recently in the X!ray structures
in agreement with the symmetrical binuclear structure of another isomorphous series of complexes com!
type "D# in Scheme 0 containing two trans!dppen parable to 0Ð2 ð4Ł"b#[ Together with a reduction in
bridges[ Furthermore\ the cis!dicyanide structure is radius of as much as 5) from Ag to Au ð4Ł"a#\ this
clearly con_rmed by the 0J"Pt\P# value of 0466 Hz could be a general result of the {relativistic con!
in 7 indicating the strong trans in~uence of cyanide traction| ð21Ł from 3d to 4d metals[ Further work on
ð0Ł"d#\ð11Ł[ Analogous binuclear complexes with dppa this is in progress[
have been described only recently ð5Ł"b#[
Acknowled`ements*We thank the Fonds zur Forderung der
Ã
wissenschaftlichen Forschung\ Austria\ for _nancial support[
T[ S[\ R[ H[ and Ch[ L[ thank the Co[ Swarovski and the
Austrian Society of Industrialists for support[
DISCUSSION
It seems likely\ that the easy formation of the other!
wise rare d7 metal cis!dicyanide complexes in 0Ð2\ 6\
and 7 is a consequence of the unsaturated nature of
the diphosphines cis! and trans!dppen[ In the case of
d7 metal complexes containing both the electron rich
anions of b!diketones and cis!dppen\ the above
described p!bonding between the metals and cis!
dppen stabilizes these compounds by decreasing the
electron density at the metal atoms ð14Ł[ Furthermore\
it has been shown by numerous X!ray structures that
the linkage isomerism in ðPd"SCN#1"cis!dppen#Ł and
ðPd"SCN#"NCS#"cis!dppen#Ł cannot be explained by a
strictly steric model and also concurrent electronic
e}ects have to be considered ð15Ł[ In 0Ð2 containing
the also electron rich cyanides\ the coordination
planes and the ethene bridges are nearly coplanar
clearly showing this p!bonding interaction[ In 6 and 7
p!bonding between the coordination units and the
trans!dppen bridges should also be present\ since sev!
eral X!ray structures of the analogous binuclear com!
plexes with dppa clearly indicate p!bonding ð5Ł"b#\ð16Ł[
Therefore\ the lack of the p!bonding stabilization of
cis!dicyanides leads to the preference of square planar
trans!dicyanides ð1Ł\ _ve!coordinate dicyanides ð2Ł or
binuclear complexes with square planar trans!dicy!
anide units ð3Ł[ Comparable to the cis!dicyanides a co!
operative stabilization e}ect could be present in the
compounds 3Ð5\ since it has been shown\ that like cis!
dppen also dppam is able to accept p!electron density
from a metal centre producing a reduced PÐN bond
length ð01Ł"a#[
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