string input = "key1:value key2:value key3:value";
Dictionary<string, string> dic = input.Split(' ').Select(x => x.Split(':')).ToDictionary(x => x[0], x => x[1]);

First will produce an array:

"key:value", "key:value"

Then an array of arrays:

{ "key", "value" }, { "key", "value" }

And then a dictionary:

"key" => "value", "key" => "value"

Note, that Dictionary<K,V> doesn't allow duplicated keys, it will raise an exception in such case. If such scenario is possible, use ToLookup()

string input = "key1:value key2:value key3:value";
Dictionary<string, string> dic = input.Split(' ').Select(x => x.Split(':')).ToDictionary(x => x[0], x => x[1]);

The first will produce an array:

"key:value", "key:value"

Then an array of arrays:

{ "key", "value" }, { "key", "value" }

And then a dictionary:

"key" => "value", "key" => "value"

Note, that Dictionary<K,V> doesn't allow duplicated keys, it will raise an exception in such a case. If such a scenario is possible, use ToLookup().